vladdimpailer
May 12 2006, 03:52 AM
IINTRODUCTION Adaptation word used by biologists in two different senses, both of which imply the accommodation of a living organism to its environment. One form of adaptation, called physiological adaptation, involves the acclimatization of an individual organism to a sudden change in environment. The other kind of adaptation, discussed here, occurs during the slow course of evolution and hence is called evolutionary adaptation.
IIMECHANISMS OF ADAPTATION Evolutionary adaptations are the result of the competition among individuals of a particular species over many generations in response to an ever-changing environment, including other animals and plants. Certain traits are culled by natural selection (see Evolution), favoring those individual organisms that produce the most offspring. This is such a broad concept that, theoretically, all the features of any animal or plant could be considered adaptive. For example, the leaves, trunk, and roots of a tree all arose by selection and help the individual tree in its competition for space, soil, and sunlight.
Biologists have been accused of assuming adaptiveness for all such features of a species, but few cases have actually been demonstrated. Indeed, biologists find it difficult to be certain whether any particular structure of an organism arose by selection and hence can be called adaptive or whether it arose by chance and is selectively neutral. The best example of an evolutionary development with evidence for adaptation is mimicry. Biologists can show experimentally that some organisms escape predators by trying to be inconspicuous and blend into their environment and that other organisms imitate the coloration of species distasteful to predators. These tested cases are only a handful, however, and many supposed cases of adaptation are simply assumed.
On the contrary, it is possible that some features of an organism may be retained because they are adaptive for special, limited reasons, even though they may be maladaptive on the whole. The large antlers of an elk or moose, for example, may be effective in sexual selection for mating but could well be maladaptive at all other times of the year. In addition, a species feature that now has one adaptive significance may have been produced as an adaptation to quite different circumstances. For example, lungs probably evolved in adaptation to life in water that sometimes ran low on oxygen. Fish with lungs were then "preadapted" in a way that accidentally allowed their descendants to become terrestrial.
IIIADAPTIVE RADIATION Because the environment exerts such control over the adaptations that arise by natural selection-including the coadaptations of different species evolving together, such as flowers and pollinators-the kind of organism that would fill a particular environmental niche ought to be predictable in general terms. An example of this process of adaptative radiation, or filling out of environmental niches by the development of new species, is provided by Australia. When Australia became a separate continent some 60 million years ago, only monotremes and marsupials lived there, with no competition from the placental mammals that were emerging on other continents. Although only two living monotremes are found in Australia today, the marsupials have filled most of the niches open to terrestrial mammals on that continent. Because Australian habitats resemble those in other parts of the world, marsupial equivalents can be found to the major placental herbivores, carnivores, and even rodents and moles.
This pattern can be observed on a restricted scale as well. In some sparsely populated islands, for example, one species of bird might enter the region, find little or no competition, and evolve rapidly into a number of species adapted to the available niches. A well-known instance of such adaptive radiation was discovered by Charles Darwin in the Galápagos Islands. He presumed, probably correctly, that one species of finch colonized the islands thousands of years ago and gave rise to the 14 species of finchlike birds that exist there now. Thus, one finch behaves like a warbler, another like a woodpecker, and so on. The greatest differences in their appearance lie in the shapes of the bills, adapted to the types of food each species eats.
IVANALOGY AND HOMOLOGY When different species are compared, some adaptive features can be described as analogous or homologous. For example, flight requires certain rigid aeronautical principles of design; yet birds, bats, and insects have all conquered the air. In this case the flight structures are said to be analogous-that is, they have different embryological origins but perform the same function. By contrast, structures that arise from the same structures in the embryo but are used in entirely different kinds of functions, such as the forelimb of a rat and the wing of a bat, are said to be homologous.
Contributed By:
John Tyler Bonner
"Adaptation," Microsoft® Encarta® Encyclopedia 2000. © 1993-1999 Microsoft Corporation. All rights reserved.
IINTRODUCTION Human Evolution, lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people evolved over a period of at least 5 million years.
One of the earliest defining human traits, bipedalism-the ability to walk on two legs-evolved over 4 million years ago. Other important human characteristics-such as a large and complex brain, the ability to make and use tools, and the capacity for language-developed more recently. Many advanced traits-including complex symbolic expression, such as art, and elaborate cultural diversity-emerged mainly during the past 100,000 years.
Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the so-called great apes (large apes) of Africa-chimpanzees (including bonobos, or so-called pygmy chimpanzees) and gorillas-share a common ancestor that lived between 8 million and 5 million years ago. Humans first evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 5 million and 2 million years ago come entirely from Africa.
Most scientists distinguish among 10 to 15 different species of early humans. Scientists do not all agree, however, about how the species are related or which ones simply died out. Many early human species-probably the majority of them-left no descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.
Early humans first migrated out of Africa into Asia probably between 2 million and 1.6 million years ago. They entered Europe somewhat later, generally within the past million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years, and to the Americas within the past 35,000 years. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years.
The scientific study of human evolution is called paleoanthropology. Paleoanthropology is a subfield of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behavior. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, paleoanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, as well as the fundamental connections between humans and other living organisms on earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world came to be. Yet many people have come to reconcile such beliefs with the scientific evidence.
IITHE PROCESS OF EVOLUTION All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring-that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, two-part scientific name. In this system, modern humans are classified as Homo sapiens.
The mechanism for evolutionary change resides in genes-the basic units of heredity. Genes affect how the body and behavior of an organism develop during its life. The information contained in genes can change-a process known as mutation. The way particular genes are expressed-how they affect the body or behavior of an organism-can also change. Over time, genetic change can alter a species' overall way of life, such as what it eats, how it grows, and where it can live.
Genetic changes can improve the ability of organisms to survive, reproduce, and, in animals, raise offspring. This process is called adaptation. Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population-a group of organisms of the same species that share a particular local habitat. Many factors can favor new adaptations, but changes in the environment often play a role. Ancestral human species adapted to new environments as their genes changed, altering their anatomy (physical body structure), physiology (bodily functions, such as digestion), and behavior. Over long periods, evolution dramatically transformed humans and their ways of life.
Scientists estimate that the human line began to diverge from that of the African apes between 8 million and 5 million years ago. This figure comes from comparing differences in the genetic makeup of humans and apes, and then calculating how long it probably took for those differences to develop. Using similar techniques and comparing the genetic variations among human populations around the world, scientists have calculated that all people may share common genetic ancestors that lived sometime between 290,000 and 130,000 years ago.
IIICHARACTERISTICS, CLASSIFICATION, AND EVOLUTION OF THE PRIMATES Humans belong to the scientific order named Primates, a group of over 230 species of mammals that also includes lemurs, lorises, tarsiers, monkeys, and apes. Modern humans, early humans, and other species of primates all have many similarities as well as some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits, as well as the significance of each step in human evolution.
All primates, including humans, share at least part of a set of common characteristics that distinguish them from other mammals. Many of these characteristics evolved as adaptations for life in the trees, the environment in which earlier primates evolved. These include more reliance on sight than smell; overlapping fields of vision, allowing stereoscopic (three-dimensional) sight; limbs and hands adapted for clinging on, leaping from, and swinging on tree trunks and branches; the ability to grasp and manipulate small objects (using fingers with nails instead of claws); large brains in relation to body size; and complex social lives.
The scientific classification of primates reflects evolutionary relationships among individual species and groups of species. Strepsirhine (meaning "wet nosed") primates-of which the living representatives include lemurs, lorises, and other groups of species all commonly known as prosimians-evolved earliest. The earliest monkeys and apes evolved from ancestral haplorhine (meaning "dry nosed") primates, of which the most primitive living representative is the tarsier. Humans evolved from ape ancestors.
Tarsiers have traditionally been grouped with prosimians, but many scientists now recognize that tarsiers, monkeys, and apes share some distinct traits, and group the three together. Monkeys, apes, and humans-who share many traits not found in other primates-together make up the suborder Anthropoidea. Apes and humans together make up the superfamily Hominoidea, a grouping that emphasizes the close relationship among the species of these two groups.
AStrepsirhines Strepsirhines are the most primitive types of living primates. The last common ancestors of strepsirhines and other mammals-creatures similar to tree shrews and classified as Plesiadapiformes-evolved at least 65 million years ago. The earliest primates evolved by about 55 million years ago, and fossil species similar to lemurs evolved during the Eocene Epoch (about 54 million to 34 million years ago). Strepsirhines share all of the basic characteristics of primates, although their brains are not particularly large nor complex and they have a more elaborate and sensitive olfactory system (sense of smell) than do other primates.
BHaplorhines B1Tarsiers Tarsiers are the only living representatives of a primitive group of primates that ultimately led to monkeys, apes, and humans. Fossil species called omomyids, with some traits similar to those of tarsiers, evolved near the beginning of the Eocene, followed by early tarsier-like primates. While the omomyids and tarsiers are separate evolutionary branches (and there are no living omomyids), they both share features having to do with a reduction in the olfactory system, a trait shared by all haplorhine primates, including humans.
B2Anthropoids The anthropoid primates are divided into New World (South America, Central America, and the Caribbean Islands) and Old World (Africa and Asia) groups. New World monkeys-such as marmosets, capuchins, and spider monkeys-belong to the infraorder of platyrrhine (broad-nosed) anthropoids. Old World monkeys and apes belong to the infraorder of catarrhine (downward-nosed) anthropoids. Since humans and apes together make up the hominoids, humans are also catarrhine anthropoids.
B2aThe First Catarrhine Primates The first catarrhine primates evolved between 50 million and 33 million years ago. Most primate fossils from this period have been found in a region of northern Egypt known as Al Fayyûm (or the Fayum). A primate group known as Propliopithecus, one lineage of which is sometimes called Aegyptopithecus, had primitive catarrhine features-that is, it had many of the basic features that Old World monkeys, apes, and humans share today. Scientists believe, therefore, that Propliopithecus resembles the common ancestor of all later Old World monkeys and apes. Thus, Propliopithecus may also be considered an ancestor or a close relative of an ancestor of humans.
B2bHominoids Hominoids evolved during the Miocene Epoch (24 million to 5 million years ago). Among the oldest known hominoids is a group of primates known by its genus name, Proconsul. Species of Proconsul had features that suggest a close link to the common ancestor of apes and humans-for example, the lack of a tail. The species Proconsul heseloni lived in the trees of dense forests in eastern Africa about 20 million years ago. An agile climber, it had the flexible backbone and narrow chest characteristic of monkeys, but also a wide range of movement in the hip and thumb, traits characteristic of apes and humans. See also Plate Tectonics: Continental Drift
Large ape species had originated in Africa by 23 million or 22 million years ago. By 15 million years ago, some of these species had migrated to Asia and Europe over a land bridge formed between the Africa-Arabian and Eurasian continents, which had previously been separated.
Early in their evolution, the large apes underwent several radiations-periods when new and diverse species branched off from common ancestors. Following Proconsul, the ape genus Afropithecus evolved about 18 million years ago in Arabia and Africa and diversified into several species. Soon afterwards, three other ape genera evolved-Kenyapithecus of Africa and the similar Griphopithecus of western Asia, both about 15 million years ago; and Dryopithecus of Europe about 12 million years ago. Scientists have not yet determined which of these groups of apes may have given rise to the common ancestor of modern African apes and humans.
Scientists do not all agree about the appropriate classification of hominoids. They group the living hominoids into either two or three families: Hylobatidae, Hominidae, and sometimes Pongidae. Hylobatidae consists of the small or so-called lesser apes of Southeast Asia, commonly known as gibbons and siamangs. The Hominidae (hominids) include humans and, according to some scientists, the great apes. For those who include only humans among the Hominidae, all of the great apes, including the orangutans of Southeast Asia, belong to the family Pongidae.
In the past only humans were considered to belong to the family Hominidae, and the term hominid referred only to species of humans. Today, however, genetic studies support placing all of the great apes and humans together in this family and the placing of African apes-chimpanzees and gorillas-together with humans at an even lower level, or subfamily.
According to this reasoning, the evolutionary branch of Asian apes leading to orangutans, which separated from the other hominid branches by about 13 million years ago, belongs to the subfamily Ponginae. The ancestral and living representatives of the African ape and human branches together belong to the subfamily Homininae (hominines). Lastly, the line of early and modern humans belongs to the tribe (classificatory level above genus) Hominini, or hominins.
This order of classification corresponds with the genetic relationships among ape and human species. It groups humans and the African apes together at the same level in which scientists group together, for example, all types of foxes, all buffalo, or all flying squirrels. Within each of these groups, the species are very closely related. However, in the classification of apes and humans the similarities among the names hominoid, hominid, hominine, and hominin can be confusing. In this article the term early human refers to all species of the human family tree since the divergence from a common ancestor with the African apes. Popular writing often still uses the term hominid to mean the same thing.
CHumans as Primates About 98 percent of the genes in people and chimpanzees are identical, making chimps the closest living biological relatives of humans. This does not mean that humans evolved from chimpanzees, but it does indicate that both species evolved from a common ape ancestor. Orangutans, the great apes of Southeast Asia, differ much more from humans genetically, indicating a more distant evolutionary relationship.
Modern humans have a number of physical characteristics reflective of an ape ancestry. For instance, people have shoulders with a wide range of movement and fingers capable of strong grasping. In apes, these characteristics are highly developed as adaptations for brachiation-swinging from branch to branch in trees. Although humans do not brachiate, the general anatomy from that earlier adaptation remains. Both people and apes also have larger brains and greater cognitive abilities than do most other mammals.
Human social life, too, shares similarities with that of African apes and other primates-such as baboons and rhesus monkeys-that live in large and complex social groups. Group behavior among chimpanzees, in particular, strongly resembles that of humans. For instance, chimps form long-lasting attachments with each other; participate in social bonding activities, such as grooming, feeding, and hunting; and form strategic coalitions with each other in order to increase their status and power. Early humans also probably had this kind of elaborate social life.
However, modern humans fundamentally differ from apes in many significant ways. For example, as intelligent as apes are, people's brains are much larger and more complex, and people have a unique intellectual capacity and elaborate forms of culture and communication. In addition, only people habitually walk upright, can precisely manipulate very small objects, and have a throat structure that makes speech possible.
IVTHE FIRST HUMANS: AUSTRALOPITHECINES By around 5 million years ago in Africa, an apelike species had evolved with two important traits that distinguished it from apes: (1) small canine, or eye, teeth (teeth next to the four incisors, or front teeth) and (2) bipedalism-that is, the ability to walk on two legs. Scientists refer to these earliest human species as australopithecines, or australopiths for short. The earliest australopith species known today belongs to the genus Ardipithecus. Other species belong to the genus Australopithecus and, by some classifications, Paranthropus. The name australopithecine translates literally as "southern ape," in reference to South Africa, where the first known australopith fossils were found.
The Great Rift Valley, a region in eastern Africa in which past movements in the earth's crust have exposed ancient deposits of fossils, has become famous for its australopith finds. Countries in which scientists have found australopith fossils include Ethiopia, Tanzania, Kenya, South Africa, and Chad. Thus, australopiths ranged widely over the African continent.
AFrom Ape to Human Fossils from several different early australopith species that lived between 4 million and 2 million years ago clearly show a variety of adaptations that mark the transition from ape to human. The very early period of this transition, prior to 4 million years ago, remains poorly documented in the fossil record, but those fossils that do exist show the most primitive combinations of ape and human features.
Fossils reveal much about the physical build and activities of early australopiths, but not everything about outward physical features such as the color and texture of skin and hair, or about certain behaviors, such as methods of obtaining food or patterns of social interaction. For these reasons, scientists study the living great apes-particularly the African apes-to better understand how early australopiths might have looked and behaved, and how the transition from ape to human might have occurred.
For example, australopiths probably resembled the great apes in characteristics such as the shape of the face and the amount of hair on the body. Australopiths also had brains roughly equal in size to those of the great apes, so they probably had apelike mental abilities. Their social life probably resembled that of chimpanzees.
BAustralopith Characteristics Most of the distinctly human physical qualities in australopiths related to their bipedal stance. Before australopiths, no mammal had ever evolved an anatomy for habitual upright walking. Australopiths also had small canine teeth, as compared with long canines found in almost all other catarrhine primates.
Other characteristics of australopiths reflected their ape ancestry. They had a low cranium behind a projecting face, and a brain size of 390 to 550 cubic cm (24 to 34 cubic in)-in the range of an ape's brain. The body weight of australopiths, as estimated from their bones, ranged from 27 to 49 kg (60 to 108 lb), and they stood 1.1 to 1.5 m (3.5 to 5 ft) tall. Their weight and height compare closely to those of chimpanzees (chimp height measured standing). Some australopith species had a large degree of sexual dimorphism-males were much larger than females-a trait also found in gorillas, orangutans, and some other primates.
Australopiths also had curved fingers and long thumbs with a wide range of movement. In comparison, the fingers of apes are longer, more powerful, and more curved, making them extremely well adapted for hanging and swinging from branches. Apes also have very short thumbs, which limits their ability to manipulate small objects. Paleoanthropologists speculate as to whether the long and dexterous thumbs of australopiths allowed them to use tools more efficiently than do apes.
B1Bipedalism The anatomy of australopiths shows a number of adaptations for bipedalism, in both the upper and lower body. Adaptations in the lower body included the following: The australopith ilium, or pelvic bone, which rises above the hip joint, was much shorter and broader than it is in apes. This shape enabled the hip muscles to steady the body during each step. The australopith pelvis also had a bowl-like shape, which supported the internal organs in an upright stance. The upper legs angled inward from the hip joints, which positioned the knees to better support the body during upright walking. The legs of apes, on the other hand, are positioned almost straight down from the hip, so that when an ape walks upright for a short distance, its body sways from side to side. Australopiths also had shorter and less flexible toes than do apes. The toes worked as rigid levers for pushing off the ground during each bipedal step.
Other adaptations occurred above the pelvis. The australopith spine had an S-shaped curve, which shortened the overall length of the torso and gave it rigidity and balance when standing. By contrast, apes have a relatively straight spine. The australopith skull also had an important adaptation related to bipedalism. The opening at the bottom of the skull through which the spinal cord attaches to the brain, called the foramen magnum, was positioned more forward than it is in apes. This position set the head in balance over the upright spine.
Australopiths clearly walked upright on the ground, but paleoanthropologists debate whether the earliest humans also spent a significant amount of time in the trees. Certain physical features indicate that they spent at least some of their time climbing in trees. Such features include their curved and elongated fingers and elongated arms. However, their fingers, unlike those of apes, may not have been long enough to allow them to brachiate through the treetops.
B2Small Canine Teeth Compared with apes, humans have very small canine teeth. Apes-particularly males-have thick, projecting, sharp canines that they use for displays of aggression and as weapons to defend themselves. By 4 million years ago australopiths had developed the human characteristic of having smaller, flatter canines. Canine reduction might have related to an increase in social cooperation among humans and an accompanying decrease in the need for males to make aggressive displays.
The australopiths can be divided into an early group of species, known as gracile australopiths, which arose prior to 3 million years ago; and a later group, known as robust australopiths, which evolved after 3 million years ago. The gracile australopiths-of which several species evolved between 4.5 million and 3 million years ago-generally had smaller teeth and jaws. The later-evolving robusts had larger faces with large jaws and molars (cheek teeth). These traits indicate powerful and prolonged chewing of food, and analyses of wear on the chewing surface of robust australopith molar teeth support this idea. Some fossils of early australopiths have features resembling those of the later species, suggesting that the robusts evolved from one or more gracile ancestors. A 5-million-year-old jaw fragment with one molar tooth and another 4.5 million-year old jaw with two molars, both from Kenya, may be the oldest australopith fossils.
CEarly Australopiths Paleoanthropologists recognize at least four species of early australopiths. These include the earliest established species, which belongs to the genus Ardipithecus, and three species of the genus Australopithecus.
C1Ardipithecus ramidus An Ethiopian member of a research team led by American paleoanthropologist Tim White discovered the earliest known australopith species in Ethiopia in 1994. These recognizably human fossils were estimated to be about 4.4 million years old. White and his colleagues gave their discovery the name Ardipithecus ramidus. Ramid means "root" in the Afar language of Ethiopia and refers to the closeness of this new species to the roots of humanity. At the time of this discovery, the genus Australopithecus was scientifically well-established. White devised the genus name Ardipithecus to distinguish this new species from other australopiths because its fossils had a very ancient combination of apelike and humanlike traits.
The teeth of Ardipithecus ramidus had a thin outer layer of enamel-a trait also seen in the African apes but not in other australopith species or most older fossil apes. This trait suggests a fairly close relationship with an ancestor of the African apes. In addition, the skeleton shows strong similarities to that of a chimpanzee but has slightly reduced canine teeth and adaptations for bipedalism.
C2Australopithecus anamensis In 1965 a research team from Harvard University discovered a single arm bone of an early human at the site of Kanapoi in northern Kenya. The researchers estimated this bone to be 4 million years old, but could not identify the species to which it belonged or return at the time to look for related fossils. It was not until 1994 that a research team, led by British-born Kenyan paleoanthropologist Meave Leakey, found numerous teeth and fragments of bone at the site that could be linked to the previously discovered fossil. Leakey and her colleagues determined that the fossils were those of a very primitive species of australopith, which was given the name Australopithecus anamensis. Researchers have since found other A. anamensis fossils at nearby sites, dating between about 4.2 million and 3.9 million years old. The skull of this species appears apelike, while its enlarged tibia (lower leg bone) indicates that it supported its full body weight on one leg at a time, as in regular bipedal walking.
C3Australopithecus afarensis Australopithecus anamensis was quite similar to another, much better-known species, A. afarensis, a gracile australopith that thrived in eastern Africa between about 3.9 million and 3 million years ago. The most celebrated fossil of this species, known as Lucy, is a partial skeleton of a female discovered by American paleoanthropologist Donald Johanson in 1974 at Hadar, Ethiopia. Lucy lived 3.2 million years ago. Scientists have identified several hundred fossils of A. afarensis from Hadar, including a collection representing at least 13 individuals of both sexes and various ages, all from a single site.
Researchers working in northern Tanzania have also found fossilized bones of A. afarensis at Laetoli. This site, dated at 3.6 million years old, is best known for its spectacular trails of bipedal human footprints. Preserved in hardened volcanic ash, these footprints were discovered in 1978 by a research team led by British paleoanthropologist Mary Leakey. They provide irrefutable evidence that australopiths regularly walked bipedally.
Paleoanthropologists have debated interpretations of the characteristics of A. afarensis and its place in the human family tree. One controversy centers on the Laetoli footprints, which some scientists believe show that the foot anatomy and gait of A. afarensis did not exactly match those of modern humans. This observation may indicate that early australopiths did not live primarily on the ground or at least spent a significant amount of time in the trees. The skeleton of Lucy also indicates that A. afarensis had longer, more powerful arms than most later human species, suggesting that this species was adept at climbing trees.
Another controversy has to do with the scientific classification of the A. afarensis fossils. Compared with Lucy, who stood only 1.1 m (3.5 ft) tall, other fossils identified as A. afarensis from Hadar and Laetoli came from individuals who stood up to 1.5 m (5 ft) tall. This great difference in size leads some scientists to suggest that the entire set of fossils now classified as A. afarensis actually represents two species. Most scientists, however, believe the fossils represent one highly dimorphic species-that is, a species that has two distinct forms (in this case, two sizes). Supporters of this view note that both large (presumably male) and small (presumably female) adults occur together in one site at Hadar.
A third controversy arises from the claim that A. afarensis was the common ancestor of both later australopiths and the modern human genus, Homo. While this idea remains a strong possibility, the similarity between this and another australopith species-one from southern Africa, named Australopithecus africanus-makes it difficult to decide which of the two species gave rise to the genus Homo.
C4Australopithecus africanus Australopithecus africanus thrived in the Transvaal region of what is now South Africa between about 3.5 million and 2.5 million years ago. Australian-born anatomist Raymond Dart discovered this species-the first known australopith-in 1924 at Taung, South Africa. The specimen, that of a young child, came to be known as the Taung Child. For decades after this discovery, almost no one in the scientific community believed Dart's claim that the skull came from an ancestral human. In the late 1930s teams led by Scottish-born South African paleontologist Robert Broom unearthed many more A. africanus skulls and other bones from the Transvaal sites of Sterkfontein.
A. africanus generally had a more globular braincase and less primitive-looking face and teeth than did A. afarensis. Thus, some scientists consider the southern species of early australopith to be a likely ancestor of the genus Homo. According to other scientists, however, certain heavily built facial and cranial features of A. africanus from Sterkfontein identify it as an ancestor of the robust australopiths that lived later in the same region. In 1998 a research team led by South African paleoanthropologist Ronald Clarke unearthed an almost complete early australopith skeleton at Sterkfontein. This important find may resolve some of the questions about where A. africanus fits in the story of human evolution.
DLate Australopiths By 2.7 million years ago the later, robust australopiths had evolved. These species had what scientists refer to as megadont cheek teeth-wide molars and premolars coated with thick enamel. Their incisors, by contrast, were small. The robusts also had an expanded, flattened, and more vertical face than did gracile australopiths. This face shape helped to absorb the stresses of strong chewing. On the top of the head, robust australopiths had a sagittal crest (ridge of bone along the top of the skull from front to back) to which thick jaw muscles attached. The zygomatic arches (which extend back from the cheek bones to the ears), curved out wide from the side of the face and cranium, forming very large openings for the massive chewing muscles to pass through near their attachment to the lower jaw. Altogether, these traits indicate that the robust australopiths chewed their food powerfully and for long periods.
Other ancient animal species that specialized in eating plants, such as some types of wild pigs, had similar adaptations in their facial, dental, and cranial anatomy. Thus, scientists think that the robust australopiths had a diet consisting partly of tough, fibrous plant foods, such as seed pods and underground tubers. Analyses of microscopic wear on the teeth of some robust australopith specimens appear to support the idea of a vegetarian diet, although chemical studies of fossils suggest that the southern robust species may also have eaten meat.
Scientists originally used the word robust to refer to the late australopiths out of the belief that they had much larger bodies than did the early, gracile australopiths. However, further research has revealed that the robust australopiths stood about the same height and weighed roughly the same amount as Australopithecus afarensis and A. africanus.
D1Australopithecus aethiopicus The earliest known robust species, Australopithecus aethiopicus, lived in eastern Africa by 2.7 million years ago. In 1985 at West Turkana, Kenya, American paleoanthropologist Alan Walker discovered a 2.5-million-year-old fossil skull that helped to define this species. It became known as the "black skull" because of the color it had absorbed from minerals in the ground. The skull had a tall sagittal crest toward the back of its cranium and a face that projected far outward from the forehead. A. aethiopicus shared some primitive features with A. afarensis-that is, features that originated in the earlier East African australopith. This may indicate that A. aethiopicus evolved from A. afarensis.
D2Australopithecus boisei Australopithecus boisei, the other well-known East African robust australopith, lived over a long period of time, between about 2.3 million and 1.2 million years ago. In 1959 Mary Leakey discovered the original fossil of this species-a nearly complete skull-at the site of Olduvai Gorge in Tanzania. Kenyan-born paleoanthropologist Louis Leakey, husband of Mary, originally named the new species Zinjanthropus boisei (Zinjanthropus translates as "East African man"). This skull-dating from 1.8 million years ago-has the most specialized features of all the robust species. It has a massive, wide and dished-in face capable of withstanding extreme chewing forces, and molars four times the size of those in modern humans. Since the discovery of Zinjanthropus, now recognized as an australopith, scientists have found great numbers of A. boisei fossils in Tanzania, Kenya, and Ethiopia.
D3Australopithecus robustus The southern robust species, called Australopithecus robustus, lived between about 1.8 million and 1.3 million years ago in the Transvaal, the same region that was home to A. africanus. In 1938 Robert Broom, who had found many A. africanus fossils, bought a fossil jaw and molar that looked distinctly different from those in A. africanus. After finding the site of Kromdraai, from which the fossil had come, Broom collected many more bones and teeth that together convinced him to name a new species, which he called Paranthropus robustus (Paranthropus meaning "beside man"). Later scientists dated this skull at about 1.5 million years old. In the late 1940s and 1950 Broom discovered many more fossils of this species at the Transvaal site of Swartkrans.
D4The Origins and Fate of Late Australopiths Many scientists believe that robust australopiths represent a distinct evolutionary group of early humans because these species share features associated with heavy chewing. According to this view, Australopithecus aethiopicus diverged from other australopiths and later gave rise to A. boisei and A. robustus. Paleoanthropologists who strongly support this view think that the robusts should be classified in the genus Paranthropus, the original name given to the southern species. Thus, these three species are sometimes referred to as P. aethiopicus, P. boisei, and P. robustus.
Other paleoanthropologists believe that the eastern robust species, A. aethiopicus and A. boisei, may have evolved from an early australopith of the same region, perhaps A. afarensis. According to this view, A. africanus gave rise only to the southern species, A. robustus. Scientists refer to such a case-in which two or more independent species evolve similar characteristics in different places or at different times-as parallel evolution. If parallel evolution occurred in australopiths, the robust species would make up two separate branches of the human family tree.
The last robust australopiths died out about 1.2 million years ago. At about this time, climate patterns around the world entered a period of fluctuation, and these changes may have reduced the food supply on which robusts depended. Interaction with larger-brained members of the genus Homo, such as Homo erectus, may also have contributed to the decline of late australopiths, although no compelling evidence exists of such direct contact. Competition with several other species of plant-eating monkeys and pigs, which thrived in Africa at the time, may have been an even more important factor. But the reasons why the robust australopiths became extinct after flourishing for such a long time are not yet known for sure.
EWhy Did Humans Evolve? Scientists have several ideas about why australopiths first split off from the apes, initiating the course of human evolution. Virtually all hypotheses suggest that environmental change was an important factor, specifically in influencing the evolution of bipedalism. Some well-established ideas about why humans first evolved include (1) the savanna hypothesis, (2) the woodland-mosaic hypothesis, and (3) the variability hypothesis.
The global climate cooled and became drier between 8 million and 5 million years ago, near the end of the Miocene Epoch. According to the savanna hypothesis, this climate change broke up and reduced the area of African forests. As the forests shrunk, an ape population in eastern Africa became separated from other populations of apes in the more heavily forested areas of western Africa. The eastern population had to adapt to its drier environment, which contained larger areas of grassy savanna.
The expansion of dry terrain favored the evolution of terrestrial living, and made it more difficult to survive by living in trees. Terrestrial apes might have formed large social groups in order to improve their ability to find and collect food and to fend off predators-activities that also may have required the ability to communicate well. The challenges of savanna life might also have promoted the rise of tool use, for purposes such as scavenging meat from the kills of predators. These important evolutionary changes would have depended on increased mental abilities and, therefore, may have correlated with the development of larger brains in early humans.
Critics of the savanna hypothesis argue against it on several grounds, but particularly for two reasons. First, the 1994 discovery by a French scientific team of australopith fossils in Chad, in central Africa, suggests that the environments of East Africa may not have been fully separated from those farther west. Second, recent research suggests that open savannas were not prominent in Africa until sometime after 2 million years ago.
Criticism of the savanna hypothesis has spawned alternative ideas about early human evolution. The woodland-mosaic hypothesis proposes that the early australopiths evolved in patchily wooded areas-a mosaic of woodland and grassland-that offered opportunities for feeding both on the ground and in the trees, and that ground feeding favored bipedalism.
The variability hypothesis suggests that early australopiths experienced many changes in environment and ended up living in a range of habitats, including forests, open-canopy woodlands, and savannas. In response, their populations became adapted to a variety of surroundings. Scientists have found that this range of habitats existed at the time when the early australopiths evolved. So the development of new anatomical characteristics-particularly bipedalism-combined with an ability to climb trees, may have given early humans the versatility to live in a variety of habitats.
Scientists also have many ideas about which benefits of bipedalism may have influenced its evolution. Ideas about the benefits of regular bipedalism include that it freed the hands, making it easier to carry food and tools; allowed early humans to see over tall grass to watch for predators; reduced exposure of the body to hot sun and increased exposure to cooling winds; improved the ability to hunt or use weapons, which became easier with an upright posture; and made extensive feeding from bushes and low branches easier than it would have been for a quadruped. Scientists do not overwhelmingly support any one of these ideas. Recent studies of chimpanzees suggest, though, that the ability to feed more easily might have particular relevance. Chimps move on two legs most often when they feed from the ground on the leaves and fruits of bushes and low branches. Chimps cannot, however, walk in this way over long distances.
Bipedalism in early humans would have enabled them to travel efficiently over long distances, giving them an advantage over quadrupedal apes in moving across barren open terrain between groves of trees. In addition, the earliest humans continued to have the advantage from their ape ancestry of being able to escape into the trees to avoid predators. The benefits of both bipedalism and agility in the trees may explain the unique anatomy of australopiths. Their long, powerful arms and curved fingers probably made them good climbers, while their pelvis and lower limb structure was reshaped for upright walking.
VTHE GENUS HOMO People belong to the genus Homo, which first evolved at least 2.3 million to 2.5 million years ago. The earliest members of this genus differed from the australopiths in at least one important respect-they had larger brains than did their predecessors.
The evolution of the modern human genus can be divided roughly into three periods: early, middle, and late. Species of early Homo resembled gracile australopiths in many ways. Some early Homo species lived until possibly 1.6 million years ago. The period of middle Homo began perhaps between 2 million and 1.8 million years ago, overlapping with the end of early Homo. Species of middle Homo evolved an anatomy much more similar to that of modern humans but had comparatively small brains. The transition from middle to late Homo probably occurred sometime around 200,000 years ago. Species of late Homo evolved large and complex brains and eventually language. Culture also became an increasingly important part of human life during the most recent period of evolution.
AOrigins The origin of the genus Homo has long intrigued paleoanthropologists and prompted much debate. One of several known species of australopiths, or one not yet discovered, could have given rise to the first species of Homo. Scientists also do not know exactly what factors favored the evolution of a larger and more complex brain-the defining physical trait of modern humans.
Louis Leakey originally argued that the origin of Homo related directly to the development of toolmaking-specifically, the making of stone tools. Toolmaking requires certain mental skills and fine hand manipulation that may exist only in members of our own genus. Indeed, the name Homo habilis (meaning "handy man") refers directly to the making and use of tools.
However, several species of australopiths lived at the same time as early Homo, making it unclear which species produced the earliest stone tools. Recent studies of australopith hand bones have suggested that at least one of the robust species, Australopithecus robustus, could have made tools. In addition, during the 1960s and 1970s researchers first observed that some nonhuman primates, such as chimpanzees, make and use tools, suggesting that australopiths and the apes that preceded them probably also made some kinds of tools.
According to some scientists, however, early Homo probably did make the first stone tools. The ability to cut and pound foods would have been most useful to these smaller-toothed humans, whereas the robust australopiths could chew even very tough foods. Furthermore, early humans continued to make stone tools similar to the oldest known kinds for a time long after the gracile australopiths died out.
Some scientists think that a period of environmental cooling and drying in Africa set the stage for the evolution of Homo. According to this idea, many types of animals suited to the challenges of a drier environment originated during the period between about 2.8 million and 2.4 million years ago, including the first species of Homo. A toolmaking human might have had an advantage in obtaining alternative food sources as vegetation became sparse in increasingly dry environments. The new foods might have included underground roots and tubers, as well as meat obtained through scavenging or hunting. However, some scientists disagree with this idea, arguing that the period during which Homo evolved fluctuated between drier and wetter conditions, rather than just becoming dry. In this case, the making and use of stone tools and an expansion of the diet in early Homo-as well as an increase in brain size-may all have been adaptations to unpredictable and fluctuating environments. In either case, more scientific documentation is necessary to strongly support or refute the idea that early Homo arose as part of a larger trend of rapid species extinction and the evolution of many new species during a period of environmental change.
BEarly Homo Paleoanthropologists generally recognize two species of early Homo-Homo habilis and H. rudolfensis (although other species may also have existed). The record is unclear because most of the early fossils that scientists have identified as species of Homo-rather than robust australopiths who lived at the same time-occur as isolated fragments. In many places, only teeth, jawbones, and pieces of skull-without any other skeletal remains-indicate that new species of smaller-toothed humans had evolved as early as 2.5 million years ago. Scientists cannot always tell whether these fossils belong to late-surviving gracile australopiths or early representatives of Homo. The two groups resemble each other because Homo likely descended directly from a species of gracile australopith.
B1Homo habilis In the early 1960s, at Olduvai Gorge, Tanzania, Louis Leakey, British primate researcher John Napier, and South African paleoanthropologist Philip Tobias discovered a group of early human fossils that showed a cranial capacity from 590 to 690 cu cm (36 to 42 cu in). Based on this brain size, which was completely above the range of that in known australopiths, the scientists argued that a new genus, Homo, and a new species, Homo habilis, should be recognized. Other scientists questioned whether this amount of brain enlargement was sufficient for defining a new genus, and even whether H. habilis was different from Australopithecus africanus, as the teeth of the two species look similar. However, scientists now widely accept both the genus and species names designated by the Olduvai team.
H. habilis lived in eastern and possibly southern Africa between about 1.9 million and 1.6 million years ago, and maybe as early as 2.4 million years ago. Although the fossils of this species somewhat resemble those of australopiths, H. habilis had smaller and narrower molar teeth, premolar teeth, and jaws than did its predecessors and contemporary robust australopiths.
A fragmented skeleton of a female from Olduvai shows that she stood only about 1 m (3.3 ft) tall, and the ratio of the length of her arms to her legs was greater than that in the australopith Lucy. At least in the case of this individual, therefore, H. habilis had very apelike body proportions. However, H. habilis had more modern-looking feet and hands capable of producing tools. Some of the earliest stone tools from Olduvai have been found with H. habilis fossils, suggesting that this species made and used the tools at this site.
Scientists began to notice a high degree of variability in body size as they discovered more early Homo fossils. This could have indicated that H. habilis had a large amount of sexual dimorphism. For instance, the Olduvai female skeleton was dwarfed in comparison with some other fossils-exemplified by a sizable early Homo cranium from East Turkana in northern Kenya. However, the differences in size actually exceeded those expected between males and females of the same species, and this finding later helped convince scientists that another species of early Homo had lived in eastern Africa.
B2Homo rudolfensis This second species of early Homo was given the name Homo rudolfensis, after Lake Rudolf (now Lake Turkana). The best-known fossils of H. rudolfensis come from the area surrounding this lake and date from about 1.9 million years ago. Paleoanthropologists have not determined the entire time range during which H. rudolfensis may have lived.
This species had a larger face and body than did H. habilis. The cranial capacity of H. rudolfensis averaged about 750 cu cm (46 cu in). Scientists need more evidence to know whether the brain of H. rudolfensis in relation to its body size was larger than that proportion in H. habilis. A larger brain-to-body-size ratio can indicate increased mental abilities. H. rudolfensis also had fairly large teeth, approaching the size of those in robust australopiths. The discovery of even a partial fossil skeleton would reveal whether this larger form of early Homo had apelike or more modern body proportions. Scientists have found several modern-looking thighbones that date from between 2 million and 1.8 million years ago and may belong to H. rudolfensis. These bones suggest a body size of 1.5 m (5 ft) and 52 kg (114 lb).
CMiddle Homo By about 1.9 million years ago, the period of middle Homo had begun in Africa. Until recently, paleoanthropologists recognized one species in this period, Homo erectus. Many now recognize three species of middle Homo: H. ergaster, H. erectus, and H. heidelbergensis. However, some still think H. ergaster is an early African form of H. erectus, or that H. heidelbergensis is a late form of H. erectus.
The skulls and teeth of early African populations of middle Homo differed subtly from those of later H. erectus populations from China and the island of Java in Indonesia. H. ergaster makes a better candidate for an ancestor of the modern human line because Asian H. erectus has some specialized features not seen in some later humans, including our own species. H. heidelbergensis has similarities to both H. erectus and the later species H. neanderthalensis, although it may have been a transitional species between middle Homo and the line to which modern humans belong.
C1Homo ergaster Homo ergaster probably first evolved in Africa around 2 million years ago. This species had a rounded cranium with a brain size of 800 to 850 cu cm (49 to 52 cu in), a prominent brow ridge (bony, protruding ridge above the eyes), small teeth, and many other features that it shared with the later H. erectus. Many paleoanthropologists consider H. ergaster a good candidate for an ancestor of modern humans because it had several modern skull features, including relatively thin cranial bones. Most H. ergaster fossils come from the time range of 1.8 million to 1.6 million years ago.
The most important fossil of this species yet found is a nearly complete skeleton of a young male from West Turkana, Kenya, which dates from about 1.55 million years ago. Scientists determined the sex of the skeleton from the shape of its pelvis. They also determined from patterns of tooth eruption and bone growth that the boy had died when he was between 9 and 12 years old.
The Turkana boy, as the skeleton is known, had elongated leg bones and arm, leg, and trunk proportions that essentially match those of a modern human, in sharp contrast with the apelike proportions of H. habilis and Australopithecus afarensis. He appears to have been quite tall and slender. Scientists estimate that, had he grown into adulthood, the boy would have reached a height of 1.8 m (6 ft) and a weight of 68 kg (150 lb). The anatomy of the Turkana boy indicates that H. ergaster was particularly well adapted for walking and perhaps for running long distances in a hot environment (a tall and slender body dissipates heat well) but not for any significant amount of tree climbing.
H. ergaster, H. rudolfensis, and H. habilis, in addition to possibly two robust australopiths, all might have coexisted in Africa around 1.9 million years ago. This finding goes against a traditional paleoanthropological view that human evolution consisted of a single line that evolved progressively over time-an australopith species followed by early Homo, then middle Homo, and finally H. sapiens. It appears that periods of species diversity and extinction have been common during human evolution, and that modern H. sapiens has the rare distinction of being the only living human species today.
Although H. ergaster appears to have coexisted with several other human species, they probably did not interbreed. Mating rarely succeeds between two species with significant skeletal differences, such as H. ergaster and H. habilis. Many paleoanthropologists now believe that H. ergaster descended from an earlier population of Homo-perhaps one of the two known species of early Homo-and that the modern human line descended from H. ergaster.
C2Homo erectus Paleoanthropologists now know that humans first evolved in Africa and lived only on that continent for a few million years. The earliest human species known to have spread in large numbers beyond the African continent was first discovered in Southeast Asia. In 1891 Dutch physician Eugène Dubois found the cranium of an early human on the Indonesian island of Java. He named this early human Pithecanthropus erectus, or "erect ape-man." Today paleoanthropologists refer to this species as Homo erectus.
H. erectus appears to have evolved in Africa from earlier populations of H. ergaster, and then spread to Asia between 1.8 million and 1.5 million years ago. The youngest known fossils of this species, from the Solo River in Java, may date from about 50,000 years ago (although that dating is controversial). So H. erectus was a very successful species-both widespread, having lived in Africa and much of Asia, and long-lived, having survived for possibly more than 1.5 million years.
H. erectus had a low and rounded braincase that was elongated from front to back, a prominent brow ridge, and an adult cranial capacity of 800 to 1,250 cu cm (50 to 80 cu in), an average twice that of the australopiths. Its bones, including the cranium, were thicker than those of earlier species. Prominent muscle markings and thick, reinforced areas on the bones of H. erectus indicate that its body could withstand powerful movements and stresses. Although it had much smaller teeth than did the australopiths, it had a heavy and strong jaw.
In the 1920s and 1930s German anatomist and physical anthropologist Franz Weidenreich excavated the most famous collections of H. erectus fossils from a cave at the site of Zhoukoudian (Chou-k'ou-tien), China, near Beijing (Peking). Scientists dubbed these fossil humans Sinanthropus pekinensis, or Peking Man, but others later reclassified them as H. erectus. The Zhoukoudian cave yielded the fragmentary remains of over 30 individuals, ranging from about 500,000 to 250,000 years old. These fossils were lost near the outbreak of World War II, but Weidenreich had made excellent casts of his finds. Further studies at the cave site have yielded more H. erectus remains.
Other important fossil sites for this species in China include Lantian, Yuanmou, Yunxian, and Hexian. Researchers have also recovered many tools made by H. erectus in China at sites such as Nihewan and Bose, and other sites of similar age (at least 1 million to 250,000 years old).
Ever since the discovery of H. erectus, scientists have debated whether this species was a direct ancestor of later humans, including H. sapiens. The last populations of H. erectus-such as those from the Solo River in Java-may have lived as recently as 50,000 years ago, at the same time as did populations of H. sapiens. Modern humans could not have evolved from these late populations of H. erectus, a much more primitive type of human. However, earlier East Asian populations could have given rise to H. sapiens.
C3Homo heidelbergensis Many paleoanthropologists believe that early humans migrated into Europe by 800,000 years ago, and that these populations were not Homo erectus. A growing number of scientists refer to these early migrants into Europe-who predated both Neandertals and H. sapiens in the region-as H. heidelbergensis. The species name comes from a 500,000-year-old jaw found near Heidelberg, Germany.
Scientists have found few human fossils in Africa for the period between 1.2 million and 600,000 years ago, during which H. heidelbergensis or their ancestors first migrated into Europe. Populations of H. ergaster (or possibly H. erectus) appear to have lived until at least 800,000 years ago in Africa, and possibly until 500,000 years ago in northern Africa. When these populations disappeared, other massive-boned and larger-brained humans-possibly H. heidelbergensis-appear to have replaced them. Scientists have found fossils of these stockier humans at sites in Bodo, Ethiopia; Saldanha (also known as Elandsfontein), South Africa; Ndutu, Tanzania; and Kabwe, Zimbabwe.
Scientists have come up with at least three different interpretations of these African fossils. Some scientists place the fossils in the species H. heidelbergensis and think that this species gave rise to both the Neandertals (in Europe) and H. sapiens (in Africa). Others think that the European and African fossils belong to two distinct species, and that the African populations-which, in this view, were not H. heidelbergensis but a separate species-gave rise to H. sapiens. Yet other scientists advocate a long-held view that H. erectus and H. sapiens belong to a single evolving lineage, and that the African fossils belong in the category of archaic H. sapiens (archaic meaning not fully anatomically modern).
The fossil evidence does not clearly favor any of these three interpretations over another. A growing number of fossils from Asia, Africa, and Europe have features that are intermediate between early H. ergaster and H. sapiens. This kind of variation makes it hard to decide how to identify distinct species and to determine which group of fossils represents the most likely ancestor of later humans.
C4Why Did Humans Spread Out of Africa? Humans evolved in Africa and lived only there for as long as 3 million years or more, so scientists wonder what finally triggered the first human migration out of Africa (a movement that coincided with the spread of early human populations throughout the African continent). The answer to this question depends, in part, on knowing exactly when that first migration occurred. Some studies claim that sites in Asia and Europe contain crude stone tools and fossilized fragments of humanlike teeth that date from more than 1.8 million years ago. Although these claims remain unconfirmed, small populations of humans may have entered Asia prior to 1.6 million years ago, followed by a more substantial spread between 1.6 million and 1 million years ago. The first major habitation of central and western Europe, on the other hand, does not appear to have occurred until between 1 million and 500,000 years ago.
Scientists once thought that advances in stone tools could have enabled early humans such as Homo erectus to move into Asia and Europe, perhaps by helping them to obtain new kinds of food, such as the meat of large mammals. If African human populations had developed tools that allowed them to hunt large game effectively, they would have had a reliable source of food wherever they went. In this view, humans first migrated into Eurasia based on a unique cultural adaptation.
By 1.6 million years ago, early humans had begun to make new kinds of tools, which scientists call Acheulean. Common Acheulean tools included large handaxes and cleavers. While these new tools might have helped early humans to hunt, the first known Acheulean tools in Africa date from later than the earliest known human presence in Asia. Also, most East Asian sites over 200,000 years old contain only simply shaped cobble and flake tools . In contrast, Acheulean tools were more finely crafted, larger, and more symmetrical. Thus, the earliest settlers of Eurasia did not have a true Acheulean technology, and advances in toolmaking alone cannot explain the spread out of Africa.
antiaging
May 14 2006, 02:52 AM
QUOTE(Big cheese @ May 2 2006, 08:02 AM) [snapback]1171451[/snapback]
Time and time again I see statements such as why are there still monkeys
We evolved from monkeys ect...Made as an argument against Evolution .To make such a statement shows a lack of understanding of evolution as a working process
So I would like to know is this doubt due to lack of education in this field or has real evidence been found to discredit it
I would like to know why some believers discount evolution
What do you base your doubt on?
What evidence have you found out side of your religious teaching to make you doubt it as a working biological process?
The laws of probability will tell you that this universe with all of its ordered complexity, could not have come into being by chance. To have that much order and complexity, the universe had to be designed by an intelligent creator. There is enough coded information in one human chromosome to
fill a small library of books. This had to be designed by an
intelligent creator.
The probability against that happening by chance is very
very high. It's like giving a chimpanzee a typewriter and letting him hit the keys at
random. The probability against his being able to type a small library full of books by hitting keys at random is so high that for all
practical purposes you can consider it impossible.
Because of this, there are some scientists and mathematicians who are forced to
believe in the existence of God by logic alone.
In order for a single cell to live, all of the parts of the cell must be assembled before life starts. This involves 60,000 proteins that are assembled in roughly 100 different combinations. The probability that these complex groupings of proteins could have happened just by chance is extremely small. It is about 1 chance in 10 to the 4,478,296 power. The probability of a living cell being assembled just by chance is so small, that you may as well consider it to be impossible. This means that the probability that the living cell is created by an intelligent creator, that designed it, is extremely large. The probability that God created the living cell is 10 to the 4,478,296 power to 1.
Example: 10 to the 6th power is one million, 10 to the 7th power is 10 million, 10 to the 8th power is 100 million, 10 to the 9th power is a billion; each time the power goes up by one, the number goes up by ten times as much. 10 to the 4,478,296 power, is a tremendously large number.
[The probability of this was calculated by Fred Hoyle, famous astronomer and mathematician.]
Natural selection will weed out inferior members of a species according to environmental requirements. But, this only leads to a species changing to another variety of the same species known as a subspecies; that is all that is observed in nature. [Crickets in dark caves become white with no eyes; also fish in caves.] But natural selection has not been observed to cause one species to change into another new species. Fish do not change into amphibians; amphibians do not change into reptiles; reptiles do not change into mammals. Natural selection cannot account for the origin of the different species. There are a million missing links in the fossil record as it has been found. The intermediate stages that would be necessary for fish to become amphibians, and reptiles to become mammals, have not been found in the fossils. The fossils show evidence that all of the species were originally created by God and they did not evolve into one another.
"Biochemical systems are exceedingly complex, so much so that the chance
of their being formed through random shufflings of simple organic
molecules is exceedingly minute, to a point indeed where it is
insensibly different from zero"
- Hoyle and Wickramasinghe, p.3
"No matter how large the environment one considers, lfe cannot have had
a random beginning. Troops of monkeys thundering away at random on
typewriters could not produce the works of Shakespeare, for the
practical reason that the whole observable universe is not large enough
to contain the necessary monkey hordes, the necessary typewriters, and
certainly the waste paper baskets required for the deposition of wrong
attempts. The same is true for living material"
Ibid., p.148
"The trouble is that there are about two thousand enzymes, and the
chance of obtaining them all in a random trial is one one part in
(10^20)^2000 = 10^40000, an outrageously small probability that could
not be faced even if the whole universe consisted of organic soup. If
one is not prejudiced either by social beliefs or by a scientific
training into the conviction that life originated on the Earth [by
chance or natural processes], this simple calculation wipes the idea
entirely out of court"
Ibid., p.24
"Any theory with a probability of being correct that is larger than one
part in 10^40000 must be judged superior to random shuffling. The
theory that life was assembled by an intelligence has, we believe, a
probability vastly higher than one part in 10^40000 of being the correct
explaination of the many curious facts discussed in previous chapters.
Indeed, such a theory is so obvious that one wonders why it is not
widely accepted as being self-evident. The reasons are psychological
rather than scientific."
Ibid., p.130
"All point mutations that have been studied on the molecular level turn
out to reduce the genetic information and not to increase it."
- Lee Spetner, "Not by Chance"(Brooklyn, New York: The Judaica
Press,Inc.) p.138
"It appears that the neo-darwinism hypothesis is insufficient to explain
some of the observations that were not available at the time the
paradigm took shape. ...One might ask why the neo-darwinian paradigm
does not weaken or disappear if it is at odds with critical factual
information. The reasons are not necessarily scientific ones but rather
may be rooted in human nature"
- Christian Schwabe "On the Validity of Molecular Evolution", Trends in
Biochemical Sciences, July 1986, p.282
"The really significant finding that comes to light from comparing the
proteins' amino acid sequences is that it is impossible to arrange them
in any sort of evolutionary series" - Ibid. p.289
"Thousands of different sequences, protein, and nucleic acid, have now
been compared in hundreds of different species but never has any
sequnces been found to be in any sense the lineal descendant or ancestor
of any other sequence." - Ibid. pp. 289-290
"Each class at a molecular level is unique, isolated and unlinked by
intermediates. Thus molecules, like fossils, have failed to provide the
elusive intermediates so long sought by evolutionary biology." - Ibid
p.290
"There is little doubt that if this molecular evidence had been
available one century ago it would have been seized upon with
devastating effect by the opponents of evolution theory like Agassiz and
Owen, and the idea of organic evolution might never have been
accepted." - Ibid pp.290-291
"In terms of their biochemistry, none of the species deemed
'intermediate', 'ancestral' or 'primitive' by generations of
evolutionary biologists, and alluded to as evidence of sequence in
nature, show any sign of their supposed intermediate status" - Ibid
p.293
Duane T. Gish, The Origin of Mammals : If this view of evolution is true, the fossil record should produce an enormous number of transitional forms. Natural history museums should be overflowing with undoubted intermediate forms. About 250,000 fossil species have been collected and classified?Applying evolution theory and the laws of probability, most of these 250,000 species should represent transitional forms.
Dr. Walt Brown, In the Beginning: Compelling Evidence for Creation and the Flood, page 10: Fossil links are missing between numerous plants, between single-celled forms of life and invertebrates, between invertebrates and vertebrates, between fish and amphibians, between amphibians and reptiles, between reptiles and mammals, between reptiles and birds, between primates and other mammals, and between apes and other primates. The fossil record has been studied so thoroughly that it is safe to conclude that these gaps are real; they will never be filled. ---
Charles Darwin, The Origin of Species:
the number of intermediate varieties, which have formerly existed [must] truly be enormous. Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely-graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against the theory [of evolution].
Dr. Niles Eldredge, paleontologist at the American Museum of Natural History, "Missing, Believed Nonexistent", Manchester Guardian, 26 November 1978:?
"The search for 'missing links' between various living creatures, like humans and apes, is probably fruitless?because they probably never existed as distinct transitional types...But no one has yet found any evidence of such transitional creatures?If it is not the fossil record which is incomplete then it must be the theory."
Lyall Watson, "The Water People", Science Digest, May 1982:
"Modern apes, for instance, seem to have sprung out of nowhere. They have no yesterday, no fossil record. And the true origin of modern humans?of upright, naked, toolmaking, big-brained beings?is, if we are to be honest with ourselves, an equally mysterious matter."
Dr. Collin Patterson, a paleontologist at the Natural History Museum in Britain, when asked why he hadn't included any illustrations of transitional forms in his book, Evolution, he replied in a letter: "I fully agree with your comments on the lack of direct illustration of evolutionary transitions in my book. If I knew of any, fossil or living, I would certainly have included them?I will lay it on the line?there is not one such fossil for which one could make a watertight argument."
"The absence of fossil evidence for intermediary stages between major transitions in the organic design, indeed our inability, even in our imagination, to construct functional intermediates in many cases, has been a persistent and nagging problem for gradualistic accounts of evolution." S.J.Gould. "Evolution Now: A Century After Darwin", 1982, p. 140
Prigogine, a Nobel Prize winning thermodynami
And they say linkage is a myth. 
The Catholics? Didn't I read someone say elsewhere that most Christian's believe in a 6000-year-old earth? I guess we don't after all.......
EXACTLY!!
