Currently, there are as many as seven recognized groups or branches to the Hominid tree, which encompass the entire human lineage. Ayala, who received the 2002 National Medal of Science for advances in evolutionary genetics, believes that paring down the Hominid family tree to four well-defined branches, plus one tentative branch, and redefining the species groups will better illustrate the course of human evolution.
The report appears in the early edition online site of the Proceedings of the National Academy of Sciences (http://www.pnas.org).
"An unnecessary multiplication of categories for classifying our Hominid ancestors distorts our evolutionary biology," Ayala said. "Our proposal provides a clearer overview of human evolution, so that the trees do not obscure the forest or -- a better analogy in this case -- so that a multiplicity of branches does not obscure the growth of the tree trunk."
For most of the 20th century, scientists have traced our human lineage through three Hominid branches -- Australopithecus, Paranthropus and Homo. During the past eight years, however, fossil findings have led researchers to propose adding four new branches to the Hominid tree -- Ardipithecus, Orrorin, Kenyanthropus and Sahelanthropus.
Under closer examination, however, Ayala and co-author Camilo J. Cela-Conde of the Universitat de les Illes Baleares question whether sufficient data has been unearthed to warrant these new branches. They also have doubts whether species in one of these new branches, Sahelanthropus, could walk upright -- an important factor in deciding whether they fit in the Hominid family.
Ayala and Cela-Conde propose trimming the currently recognized human evolutionary tree by eliminating as branches Paranthropus, Orrorin and Kenyanthropus. Instead, Ayala and Cela-Conde would keep three existing branches -- Australopithecus, Homo and Ardipithecus -- then add one new branch called Praeanthropus.
"Specimens that existed in the trimmed branches can be incorporated into this simpler family tree," explained Ayala. "For example, Praeanthropus would encompass the very earliest hominids at the time they split from what would evolve into the chimpanzee family."
THE NEW HUMAN EVOLUTIONARY TREE
-- Praeanthropus. This branch, or genus, would encompass the earliest Hominids – estimated to have split from the chimpanzee family about 7 million years ago. Species in this branch have a developing ability to walk upright. Currently, Praeanthropus is considered a subgenus that connects the Australopithecus and Orrorin branches. In the new tree, Orrorin loses its status as a branch and becomes a subgenus of Praeanthropus.
-- Ardipithecus. This new branch would split off from Praeanthropus about 4.5 million years ago. Its species are notable for distinct differences from the Hominids that evolved into human beings, such as having thin molar enamel, similar to that of the African Great Apes.
-- Australopithecus. Already on the evolutionary tree, this branch would now split off from Praeanthropus about 3.5 million years ago. Species in this branch have a developing jaw and teeth sturdy enough to eat hard vegetables. In the new tree, Paranthropus loses its status as a branch and becomes a subgenus of Australopithecus.
-- Homo. Already on the evolutionary tree, this branch would now split off from Praeanthropus around 3.5 million years ago. Homo species are characterized by larger brains and increased bipedal activity. Homo sapiens, or modern human beings, are part of the Homo branch. In the new tree, Kenyanthropus loses its status as a branch and becomes a subgenus of Homo.
Finally, Ayala and Cela-Conde propose making Sahelanthropus a tentative branch separate from the Hominid tree until more evidence can be found proving this species had bipedal traits.
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Journal
Proceedings of the National Academy of Sciences