Dan Smail, a medievalist who arrived last month at Harvard's history department, is a time revolutionary. Historians, Smail says, are in thrall to a chronology of the human race that is, by now, embarrassingly out of date. He wants to move the start date of introductory history courses back, oh, 100,000 years or so.If you have taken the first part of a two-semester, college-level history survey class, you know how it usually starts: a few desultory comments about ''prehistory" and then a pronouncement that civilization as we know it had its first stirrings in the Fertile Crescent, around 4,000 to 6,000 BC. But as Smail points out in an article in the latest issue of the American Historical Review, when you consider recent (and not-so-recent) discoveries in archeology, anthropology, and biology -- the finding that all humankind traces to Africa, for example, or that humans were on the march out of that continent by roughly 100,000 BC, not to mention good guesses for when language, hunting, and farming arose -- the fixation on a start date of 4,000 to 6,000 BC begins to seem awfully arbitrary.And yet, as Smail goes on to argue in his essay, suggestively titled ''In the Grip of Sacred History," this chronological tick has a very interesting back-story. ''Every history curriculum in secondary schools and colleges that tacitly accepts a Near Eastern origin around 6,000 years ago," Smail writes, ''contains the unintended echo of the Judeo-Christian mythology of the special creation of man in the Garden of Eden."Through the 18th century and well into the 19th, Western historians, almost all of them Christian, thought that humankind (and Earth) dated to roughly 4,000 to 7,000 BC. (One especially influential estimate pinpointed 4,004 BC.) And many thought that the Garden of Eden could be traced to the Fertile Crescent. Smail's theory is that, in the 19th century, as the biblical timeline lost credibility and the staggering age of the Earth began to be glimpsed, historians reflexively clung to as much of the traditional timeline as they could. A true reckoning with the long timelines envisioned by Darwin never occurred.