2 Human Ancestors Probably Co-Existed
African discovery shakes up Homo sapiens' evolutionary tree
WEDNESDAY, Aug. 8, 2007 (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.
The geologists have also gained a keen understanding of the environment at the time a sediment was laid down, "to know if the fossil died near a big river or lake," for example, Gathogo added.
In all, he and Brown used five different methods to help Meave Leakey and her daughter, Louise Leakey, date the fossils.
Meave Leakey said in a prepared statement that the fossils' co-existence "makes it unlikely that Homo erectus evolved from Homo habilis." She speculated that the two groups may have originated separately more than 2 million or 3 million years ago -- a time for which the fossil record is relatively sparse.
"The fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding each other," Leakey said. In other words, for more than half a million years, the two groups may have foraged or otherwise gathered food in very different ways, with neither needing to "outcompete" the other.
Not every specialist was surprised by the findings, however.
"The general idea that Homo habilis and Homo erectus overlapped in time has been known in the fossil record for 15 or 20 years," said Erik Trinkaus, an anthropology professor at Washington University in St. Louis.
"Twenty years ago, we had Homo erectus clearly documented in East Africa by 1.8 million years," he said. "And most people would agree that Homo habilis have a time range of somewhere around 2.5 million years down to about 1.6 million. So, we've had a couple hundred thousand years of [documented] overlap since sometime in the 1980s."
Trinkaus also believes the Leakey discovery does not rule out the traditional notion that habilis gave rise to erectus.
"If you have one species that gives rise to another species, the ancestral species doesn't necessarily go extinct," he said. "Maybe in one part of its geographical range it carries on as it always has, and in another part of its range, it evolves into a new species." Later on -- as may have happened in the Turkana Basin -- these two branches might have met up once again, Trinkaus said.
Homo habilis and Homo erectus might have even interbred, Trinkaus added, since studies suggest closely related mammal species can and often do, given the right conditions.
Still, the Nature report does appear to extend the time at which these two groups of human ancestors co-existed -- to at least 500,000 years, Trinkaus said.
And what of the erectus specimen's surprisingly tiny skull size?
According to the Leakeys, that could point to "sexual dimorphism" -- a species trait whereby males and females differ significantly in body size. Sexual dimorphism, in turn, often points to a more polygamous mating pattern, with larger males breeding with a succession of smaller females. Male and female gorillas, for example, vary greatly in size, and this breeding pattern is the norm for that species.
The finding -- if proven -- would be new for Homo erectus, the researchers said, and move the group a little further away, behaviorally speaking, from Homo sapiens.
Still, Laitman cautioned that -- unlike fossils -- anthropological theory is seldom fixed in stone.
"These fossils don't come with name tags on them, and this is tough stuff to try and pinpoint," he said.
However, he added, the Kenya findings do suggest that humans' ancestors had a cozier relationship than was once believed.
"If you envision our species Homo sapiens as being allowed to sit around the dinner table, then the genus Homo is our 'house,' " he said. "Now, we have two of our intelligent ancestors sitting in the house at the same time. It's quite exciting."
Find out more about human evolution at Minnesota State University.