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Hominid evolution: a tale of two trees.

Hominid evolution: A tale of two trees

Paleoanthropologists are getting closer to a general agreement on the early branchings of the hominid family tree, says Eric Delson of the City University of New York. Several contending evolutionary trees have been whittled down to two main alternatives, he reports in the June 25 NATURE.

The two hypotheses emerged from a recent workshop on robust australopithecines, a group of hominids, or human-like creatures, that evolved at the same time as the lineage that led to modern humans, but became extinct around 1 million years ago. A volume of scientific papers from the workshop, which was attended by researchers from around the world (SN: 4/11/87, p.229), will be published next year.

Both evolutionary schemes maintain that fossils found at Hadar and Laetoli in Africa represent one species, Australopithecus afarensis, which dates to between 3.5 million and 3.1 million years ago and was near the common ancestor of all later hominids, says Delson. The "least contested" evolutioary tree holds that A. afarensis led in one direction to A. aethiopicus, a species that includes the recently discovered WT 17000 or "black skull" (SN: 1/24/87, p.58) and was either related to or a direct ancestor of the later robust australopithecines, A. robustus and A. boisei. In another directon, A. afarensis led to A. africanus, a species that has been found only in souther Africa, and then to the genus Homo.

The second hypothesis proposes that A. boisei includes specimens that have been labeled. A. aethiopicus; that A. africanus led only to A. robustus; and that the ancestry of the Homo line is unclear.

One example of the emerging consensus is Todd R. Olson of City University of New York Medical School, who before the workshop held that A. afarensis was actually two species, a "robust" for leading to later robust australopithecines and a "gracile" form, inclqding the partial skeleton called Lucy, which was on the Homo line. Olson says he nows subscribes to the first hypothesis.

Proponents of the "two-species" interpretation are, however, not extinct. One paper from the australopithecine workshop, by Dean Falk of Purdue University in West Lafayette, Inc., suggests that Hadar hominids made up two species, while Laetoli specimens may have been still anoter species. Robust australopithecines in southern and eastern Africa were, in Falk's view, different races of the same species.

A key unresolved question, according to Delson, i the evolutionary role played by A. africanus, which "remains pivotal after more than 60 years of controversy."
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Author:Bower, Bruce
Publication:Science News
Date:Jul 4, 1987
Words:422
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