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Skeletal aging of New World settlers.

Skeletal aging of New World settlers

About two dozen human bones recently found on gravelly bars of the Kansas River may be the oldest human skeletal remains yet found in the Americas, according to scientists at the annual meeting of the American Association of Physical Anthropologists, held in Kansas City last week.

"Although we don't have a firm, undeniable date, all the analytic tests conducted so far on the chemistry of the bones suggest that one bone fragment is older than any other previously found in the New World," says geologist Wakefield Dort of the University of Kansas in Lawrence, who directed the project with University of Kansas anthropologist Lawrence D. Martin. A preliminary estimate, based on a technique called electron spin resonance (ESR), is that the piece of skull is 15,400 years old.

Archaeologists generally believe that humans entered the New World about 12,000 years ago, although some controversial sites have led scientists to extend that estimate as far back as 100,000 years ago (SN: 10/31/87, p.284). Most of these sites do not contain human bones. Investigators usually uncover tools, such as sharpened stone flakes and spear points, that in some cases lie among the bones of extinct animals, or mounds of debris left by early settlers (SN: 3/12/88, p.164).

Erosion along the Kansas River is uncovering bones of a wide range of extinct animals, including mastodon, mammoth, musk ox and giant beaver, mixed with those of humans, says Martin. The human remains -- found near Bonner Springs, about 15 miles west of Kansas City, Kan. -- consist of leg and arm bones, several skull fragments and most of the top half of one skull. No teeth or jaws have been found.

The bones are not embedded in soil, indicating that they have been moved some distance by the river, says Dort. But evidence of minimal abrasion from sand in the river and the near-perfect preservation of delicate bone features suggest the fossils traveled no more than a few hundred yards. Radiocarbon dating of sediment layers lining the river is proving to be a complex task, notes Dort, but the fossils are believed to have come from soil lower than a layer dated at 10,430 years old.

There appear to be two populations of different size represented by the bones, says Martin. The bones of smaller individuals are darker, more mineralized and denser. Cut marks on some bones may have resulted from burial practices, he says.

The shape of the human skull fragments and limb bones is similar to that of the oldest known Central Plains settlers, who lived up to 7,000 years ago, but Martin says radiocarbon dating of the Bonner Springs fossils ran into problems. A human bone burned for radiocarbon dating was found to contain no material suitable for the technique.

The investigators then turned to ESR dating, which has been used with several archaeological samples over the last decade. The technique depends on measuring the density of trapped electrons that accumulate in bone and other organic material as a result of environmental radiation after the material is buried. The natural radioactivity of the material is determined, as well as the sources of the radioactivity, such as uranium and thorium. The remains are then exposed to standardized doses of high-energy radiation. The intensity of natural and laboratory ESR signals are compared and, using an estimate of the annual radiation dose for a specimen, researchers calculate its age.

Initial ESR dating was performed by Don Robins of the University of London, England, a pioneer in its use with archaeological remains. According to Dort, this work established that the human skull fragment was older than an associated mammoth bone, giving the human specimen a minimum age of 12,000 years. The age of 15,400 years is the result of further ESR study by physicists at the University of Kansas.

The antiquity of the Bonner Springs site needs to be further explored, says Dort, by digging deeper into the river banks and, possible, blocking off a portion of the river to expose lower layers of soil. "Then," he says, "we can see if human bones and those of extinct animals occur together." -- B. Bower
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Title Annotation:ESR dating used on human skeletal remains found in Kansas
Author:Bower, Bruce
Publication:Science News
Date:Apr 2, 1988
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