The Committee to Nuke Elysium

About 12,000 years ago, humans began farming, living in denser settlements and burying their dead, so skeletons younger than that are plentiful, said Stanley Ambrose, an African archaeologist and paleoanthropologist at the University of Illinois at Urbana-Champaign, who was not involved in the study.

But relatively little is known about the people who came before them. Only a handful of human burials around the world date from about 12,000 to 30,000 years ago, Ambrose said.

To learn more about this lost period of human history, Tryon and his colleagues took a second look at specimens that were sitting in the collections of the National Museums of Kenya in Nairobi. The artifacts were unearthed in the 1970s at rock shelters at Lukenya Hill, a granite promontory that overlooks the savanna in Kenya.

Among the finds was the top portion of an ancient skull. The team took several measurements of the skull, then compared it with skulls from Neanderthals, several other fossil human skulls from the same time and other periods, as well as those of modern-day humans.

Though the skull clearly belonged to a Homo sapien who was anatomically modern, its dimensions were markedly different from those of both the European skull and the African skulls from the same time. In addition, the skull was thickened, either from damage, nutritional stress or a highly active childhood. (There is not enough evidence to say the fossil represents a subspecies of Homo sapien, Tryon said.)

By measuring the ratio of radioactive isotopes of carbon (or carbon atoms with different numbers of neutrons), the team concluded that the skull was about 22,000 years old. That means the ancient human would have lived during the height of the last ice age.

Modern-day Africans have greater genetic diversity than other populations. But the new findings suggest that during this early period of human history, Africa may have supported even greater human diversity, with small, offshoot lineages that no longer exist today, Tryon said.