for National Geographic News
A pair of fossils recently discovered in Kenya is challenging the straight-line story of human evolution.
Traditional evolutionary theories of the genus Homo suggest a successive progression: Homo habilis gave rise to Homo erectus, which then begat modern humans, Homo sapiens.
H. erectus is commonly seen as the most similar ancestor to modern humans, differing mostly by having a brain about three-quarters the size.
But the newly found upper jawbone and skull, which come from two separate skeletons, suggest that H. habilis was not a direct ancestor of modern humans and that H. erectus was less modern than previously thought.
The fossils, described today in the science journal Nature, were discovered by the Koobi Fora Research Project—an international group of scientists led by the mother-daughter team of Meave and Louise Leakey and affiliated with the National Museums of Kenya.
(Meave and Louise Leakey are National Geographic Society Explorers-in-Residence. National Geographic News is part of the National Geographic Society, which also funded this research.)
The jawbone is attributed to H. habilis and was dated to 1.44 million years ago—meaning its far younger than previously known H. habilis fossils and dates to well after the emergence of H. erectus.
The finding indicates the two species lived side-by-side for half a million years in eastern Africa, according to study lead author Fred Spoor, a professor of evolutionary anatomy at University College London.
"I'm very cautious saying this, [but] it has the potential to remove Homo habilis from the direct ancestral line to us modern humans," he said.
Instead, H. habilis and H. erectus may have had a sister relationship that originated sometime between two and three million years ago, which is a well-known gap in the fossil record.
The two species likely occupied different ecological niches, allowing them to co-exist without competing against each other for limited resources, Spoor noted.
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