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Fossil jaw offers clues to human ancestry.

Investigators have uncovered a lower jaw in Africa that represents one of the earliest known fossils of a direct human ancestor, or member of the genus Homo. The specimen, assigned a preliminary age of about 2.4 million years by its discovers, turned up in fossil-poor sediments near Lake Malawi, roughly half-way between sites in eastern and southern Africa that have yielded the bulk of ancestral human remains.

Another Homo fossil may also date to 2.4 million yearse ago (SN: 2/29/92, p.134).

The Homo lineage apparently originated in the tropics of eastern Africa around 2.5 million years ago, following a gradual shrinkage of habitable land to the south and north caused by global cooling, assert Friedemann Schrenk, a pale-ontologist at Hessisches Landesmuseum in Darmstadt, Germany, and his colleagues. The species they assign to the new find, H., rudolfensis, eventually migrated as far south as Lake Malawi, they state. Anther species, H. habilis, traveled even farther south between 1.8 million and 1.5 million years ago as temperatures climbed, they argue in the Oct. 28 NATURE.

"Many scenarios of human evolution are still possible," contends Timothy G. Bromage, an anthropologist at Hunter College in New York City who participated in the Malawi excavations. "But this new fossil looks like the large-toothed, large-jawed specimens from eastern Africa that some researchers call Homo rudolfensis."

Microscopic study of tooth enamel shows a pattern of dental development in the jaw similar to that of proposed H. rudolfensis fossils, Broomage adds.

The leading proponent of H. rudolfensis as a bona fide species, anthropologist Bernard Wood of the University of Liverpool in England, supports Bromage's classification of the fossil jaw. Given the small number of early Homo fossils and disagreements over how to group them (SN: 6/20/92, p.408), other scientists decline to assign the jaw to a specific species.

Schrenk has directed the Homind Corridor Research Project at sites in south-eastern Africa since 1983. These excavations have attempted to clarify how animals and hominids, or members of the human evolutionary family, evolved and moved through that part of the continent.

The Lake Malawi site does not contain an abundant trove of fossils. Beach sand propelled by powerful waves on the lake has scoured out much fossil-bearing sediment, Bromage notes. In the past decade, field workers have unearthed about 600 animal bones, mainly from pigs and antelopes. Last year, two joining parts of the hominid's lower jaw appeared. Many teeth remained in their sockets.

For now, Bromage considers the age estimate of 2.4 million years, based on previously known ages of animal bones found near the jaw, to be a best guess. Investigators have not found any datable volcanic ash in the hominid-bearing sediment, but measurements of magnetic orientations in the deposits may eventually produce a more accurate estimate, he says.

The jaw indicates that H. rudolfensis evolved a face specialized for chewing, much like that of Paranthropus, a smaller-brained hominid linkage that lived at the same time, Wood argues. The find also supports the view that hominids and other mammals at the Malawi site maintained stronger links to eastern, rather than southern, Africa, he asserts.

Tim D. White, an anthropologist at the University of California, Berkeley, says the jaw may represent either a large male H. habilis or a H. rudolfensis of undermined sex. Early Homo species may have expected large fluctuations in body size over time, making it difficult to identify them from fossil remains, White contends.

He calls the age assigned to the new specimen "speculative." Still, the jaw shows that early Homo species extended south about 1,000 miles from eastern Africa and bore "stroking similarities" in anatomy, White holds.
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Title Annotation:jaw of member of genus Homo found in Africa
Author:Bower, Bruce
Publication:Science News
Date:Oct 30, 1993
Words:616
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