Fossil hints at hominids' European stall.

Excavations at an archaeological site in the nation of Georgia have yielded a fossil jaw that, according to its discoverers, represents the earliest known evidence of human ancestors in that region of Asia -- and perhaps anywhere outside Africa.

The well-preserved lower jaw, complete with all of its teeth, belonged to a Homo erectus individual who lived between about 1.8 million and 1.6 million years ago, assert Leo Gabunia and A. Vekua, both anthropologists at the Georgian Academy of Sciences in Tblisi.

If this age estimate holds up, it suggests that a substantial delay occurred before human ancestors moved from western Asia into Europe. Prior studies place the human occupation of Europe at no earlier than 500,000 years ago (SN: 10/8/94, p.235).

"[This] suggests that humans either waited outside the `gates to Europe' for more than 1 million years or inhabited the subcontinent at a very low density during that interval," Gabunia and Vekua conclude in the Feb. 9 NATURE.

The new fossil came to light at Dmanisi, a settlement that achieved regional prominence between 1,200 and 800 years ago. After excavations began in 1981, researchers noted that the bones of now-extinct animals dot the walls of deep pits once used for grain storage. In 1991, the exploration of one such pit turned up this jaw of a hominid, or member of the human evolutionary family.

The specimen contains small teeth attached to relatively thick bone, the scientists say. The jaw shows several similarities to H. erectus fossils from Africa as well as to some Chinese finds, they contend.

A basalt deposit adjacent to the hominid-bearing soil dates to 1.8 million years ago. This estimate relies on a comparison of the amount of radioactive potassium in the basalt to the amount of radioactive argon, into which potassium decays. Measurements of magnetic reversals in the soil and analysis of associated animal bones further narrowed the age range for the ancient jaw.

Some scientists argue that Indonesian H. erectus specimens are about as old as the Dmanisi jaw (SN: 3/5/94, p.150).

Researchers don't know why human ancestors in western Asia waited for roughly a million years before trekking west, write David Dean of Case Western Reserve University in Cleveland and Eric Delson of the City University of New York in an accompanying comment.

Frigid temperatures and dangerous carnivores in Europe may have kept H. erectus at bay until the species developed stone weapons and social systems sufficient for survival under such inhospitable conditions, they propose.

Bernard Wood of the University of Liverpool in England, who has briefly examined a cast of the Dmanisi jaw, says it may belong to H. erectus, H. ergaster (which he considers the direct ancestor of modern humans), or archaic H. sapiens.