2 Human Ancestors Probably Co-Existed

Mar. 23 -- WEDNESDAY, Aug. 8 (HealthDay News) -- A fragment of upper jaw and a skull found in Africa are helping rewrite the textbook on how mankind came to be millions of years ago.

Gone is the step-wise theory of one ancient species, Homo habilis, dying off as another, Homo erectus, takes over -- to give rise later to modern-day Homo sapiens.

In its place are two fossils uncovered in Kenya that appear to show habilis and erectus lived together in close proximity for more than half a million years, about 1.5 million years ago.

The findings, from a paleontology team led by Meave Leakey, wife of renowned paleontologist Richard Leakey, were published Wednesday in the Aug. 9 issue of the journal Nature.

"If they are correct, that simple kind of tree -- where you went from habilis and erectus takes over -- clearly is no longer the case," said Jeffrey Laitman, director of the Center for Anatomy and Functional Morphology at the Mount Sinai School of Medicine, in New York City.

The study also adds some intriguing new wrinkles to what scientists knew about habilis and erectus. For example, the 1.55-million-year-old fossil skull of erectus, found in the Ileret region near Lake Turkana in northern Kenya, is the smallest yet ever found for this group.

Not so far away, the researchers uncovered an upper jawbone from the supposedly "earlier" group, Homo habilis. However, a variety of dating methods pinpointed the fossil's deposition at just 1.44 million years ago -- slightly younger than the time the erectus fossil was laid to rest.

Study co-author Patrick Gathogo, a doctoral student in geology and geophysics at the University of Utah, worked on dating the sediments in which the fossils were found, along with University of Utah geology and geophysics professor Francis Brown.

Gathogo said the unique history of Kenya's Turkana Basin made for relatively precise dating of the two remains.

"It's a very unique basin, because we've had lots of volcanic eruptions over the last 4 million years, so we can date those volcanic ashes that confine the fossils and get a very good estimate of the age," he said.