Modern humans take a spin back in time.

Modern humans take a spin back in time

Scientists using a recently developed dating technique say anatomically modern humans apparently inhabited an Israeli cave known as Skhul about 100,000 years ago. Their conclusion, described in the April 27 NATURE, is consistent with reports that early modern humans were living at the nearby Qafzeh cave more than 90,000 years ago (SN: 2/27/88, p.138).

Nonetheless, the accumulating evidence for an early human presence in the Near East, more than twice as long ago as had been previously assumed, does not quell an ongoing dispute over the evolution of modern Homo sapiens. Some investigators say anatomically modern humans originated in Africa and spread throughout the world, replacing groups such as the Neanderthals, while others contend there was at least some interbreeding between early modern humans and Neanderthals.

Researchers had roughly dated Skhul at about 40,000 years old by comparing stone tools and the remains of animals and humans at the site with those from nearby caves.

The new age for Skhul was obtained by Christopher B. Stringer of the British Museum in London and his colleagues. They analyzed two teeth from an ancestral form of the cow or ox excavated more than 50 years ago and housed in a British Museum collection. The teeth came from sediment containing the remains of at least 10 early modern humans. Stringer's group calculated the age estimate through electron spin resonance (ESR) dating, a technique used with several archaeological samples over the last decade.

ESR measures the density of trapped electrons that gather in bone and other organic material as a result of environmental radiation after the material is buried. The technique gauges a specimen's natural radioactivity as well as sources of the radiation, such as uranium and thorium. Natural electron density is then compared with that produced by standardized doses of high-energy radiation. From this, researchers can calculate the specimen's annual radiation dose and an estimated age.

Assuming the uranium in the animal teeth was absorbed soon after burial, the researchers place Skhul's minimum age at approximately 81,000 years. A better estimate, based on the assumption that the uranium was taken up more gradually, is around 101,000 years, they contend.

These dates are nearly the same as those previously obtained at Qafzeh with ESR and another technique, thermoluminescence dating.

Anatomically modern humans appear to have inhabited the Near East about 50,000 years before entering Europe, the investigators maintain. But modern humans' relationship, if any, to early Neanderthals who lived near Skhul and Qafzeh remains unknown, Stringer and his co-workers assert. To solve that puzzle, researchers need better dates for the Neanderthal sites, which themselves have yielded some important remains (SN: 4/15/89, p.229).

Early modern humans at Skhul and Qafzeh may have briefly inhabited the area as they migrated from Africa to Asia and much later to Europe, the scientists suggest. On the other hand, modern humans and Neanderthals may have alternated or overlapped in their occupation of the Near East for 60,000 years.