Earth's atmosphere

[Yengo68]

Not sure if I have the correct sub forum here.

I was interested in finding out when approx the Earth's atmosphere was started to become breathable for Humans? For instance, when Dinosaurs were around say, the 100 mya mark, would Humans have needed any breathing devices?

I've tried to find out but no luck.

[Ashigaru]

Earths atmosphere has probably changed many times. There have been at least 7 major climatic shifts in the earths history, these would be impossible without some change in the atmosphere.

[Leonardo]

Link to origin of Earth's atmosphere.

Doesn't contain all the info you asked for but the atmospheric composition would have been suitable for our needs (in that O₂ levels were sufficient) as early as 400m ya. If there were significant differences in the other gases composing our current atmosphere some breathing aids may have been required but it's possible our physiology could have adapted (within limits).

Other questions like surface temperature, radiation etc would require additional searching.

Edited February 15, 2007 by Leonardo

[Trinitrotoluene]

The history of the Earth's atmosphere prior to one billion years ago is poorly understood, but the following presents a plausible sequence of events. This remains an active area of research.

The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current chemical composition from two notably different previous compositions. The original atmosphere was primarily helium and hydrogen. Heat from the still-molten crust, and the sun, plus a probably enhanced solar wind, dissipated this atmosphere.

About 4.4 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes which released steam, carbon dioxide, and ammonia. This led to the early "second atmosphere", which was primarily carbon dioxide and water vapor, with some nitrogen but virtually no oxygen. This second atmosphere had approximately 100 times as much gas as the current atmosphere, but as it cooled much of the carbon dioxide was dissolved in the seas and precipitated out as carbonates. The later "second atmosphere" contained nitrogen, carbon dioxide, and very recent simulations run at the University of Waterloo and University of Colorado in 2005 suggest that it may have had up to 40% hydrogen[5]. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide and methane, kept the Earth from freezing. In fact temperatures were probably very high, over 70 degrees C (158 degrees F), until some 2.7 billion years ago.

One of the earliest types of bacteria were the cyanobacteria. Fossil evidence indicates that bacteria shaped like these existed approximately 3.3 billion years ago and were the first oxygen-producing evolving phototropic organisms. They were responsible for the initial conversion of the earth's atmosphere from an anoxic state to an oxic state (that is, from a state without oxygen to a state with oxygen) during the period 2.7 to 2.2 billion years ago. Being the first to carry out oxygenic photosynthesis, they were able to convert carbon dioxide into oxygen, playing a major role in oxygenating the atmosphere.

Photosynthesising plants would later evolve and convert more carbon dioxide into oxygen. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to release nitrogen; in addition, bacteria would also convert ammonia into nitrogen. But most of the modern day level of nitrogen are due mostly to sunlight-powered photolysis of ammonia released steadily over the aeons from volcanoes.

As more plants appeared, the levels of oxygen increased significantly, while carbon dioxide levels dropped. At first the oxygen combined with various elements (such as iron), but eventually oxygen accumulated in the atmosphere, resulting in mass extinctions and further evolution. With the appearance of an ozone layer (ozone is an allotrope of oxygen) lifeforms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere". 200 - 250 million years ago, up to 35 percent of the atmosphere was oxygen (bubbles of ancient atmosphere were found in an amber).

This modern atmosphere has a composition which is enforced by oceanic blue-green algae as well as geological processes. O2 does not remain naturally free in an atmosphere, but tends to be consumed (by inorganic chemical reactions, as well as by animals, bacteria, and even land plants at night), while CO2 tends to be produced by respiration and decomposition and oxidation of organic matter. Oxygen would vanish within a few million years due to chemical reactions and CO2 dissolves easily in water and would be gone in millennia if not replaced. Both are maintained by biological productivity and geological forces seemingly working hand-in-hand to maintain reasonably steady levels over millions of years.

Source: http://en.wikipedia.org/wiki/Earth%27s_atmosphere

[Ashigaru]

The earths atmosphere has probably been pretty stable since a few thousand years, maybe less, after the last ice age.

[laveticus666]

damn i tried looking it up agian to get you the exact date. I just read this last night. Oxygen levels where at about 17% about 400 myo compared to 21% nowadays. I think that would be profiecent for our needs.