New fossil skull from Africa reveals tangled roots at base of the human family tree

August 8, 2012

A 2 million-year-old flat-faced skull pulled from the sandstones of East Africa has shored up claims that at least three species of early humans once coexisted in an “evolutionary experiment” that saw an explosive increase in brain size paired with radically different faces, teeth and jaws.

While the new partial skull and two newly found jawbones look radically different from modern humans, they match an enigmatic, nearly complete skull found 40 years ago that paleoanthropologists have long struggled to fit into the human family tree.

Together, the new finds and the puzzling skull describe a species of early humans clearly distinct from two others known from fossils from the same period, said Meave Leakey, the 70-year-old paleoanthropologist who led the team that discovered the fossils.

The “base of the human lineage was indeed diverse,” Leakey said from her longtime home at the Turkana Basin Institute in northern Kenya. Her colleagues made the finds near there.

Long thought to be the seat of human origins, East Africa was once “quite a crowded place with multiple species,” said Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, a co-author of a report describing the finds.

Leakey and her colleagues stop short of assigning the fossils a species name. But 20 years ago, others scientists classified the 40-year-old mystery skull as Homo rudolfensis.

An associate of Leakey’s noticed a jawbone sticking out of a block of sandstone in the arid region in 2007. After hauling the block to their laboratory, the team whittled away with dental drills and revealed a face, its right cheek and upper jaw intact. The small fossil likely came from an adolescent, Leakey’s team reports in this week’s issue of the journal Nature.

Nearby, the team also found two partial jawbones that match both the new skull and the mystery skull, Spoor said. All of the fossils date between 1.78 and 1.95 million years old.

At that time, East Africa was a roiling hotbed of human evolution. Other fossil finds show that the long-lived species thought to be the direct ancestor of modern humans, Homo erectus, thrived in the region, which was undergoing rapid changes in plant cover, rainfall and, in all likelihood, availability of various foods.

Meanwhile, another group of early human fossils from the region has been classified as Homo habilis, which means “handy man,” as these creatures were thought to create primitive stone blades.

Yet another, more primitive hominid species, called Paranthropus bosei, also lived in the region at the time. Stout-bodied and with giant molars, these beings more closely resembled the more ape-like creatures known as the Australopitecines and are not thought to be human ancestors. Instead of evolving, they died out.

But the new finds — and the mystery skull — clearly don’t belong to any of these groups, said Leakey, who is also a National Geographic explorer-in-residence. The face is too flat, falling like a cliff from brow to chin. The front teeth line up straight as a ruler instead of arcing forward as in the other human species. The molars are also quite large, and, as the mystery skull shows, the brain was bigger — though still only roughly half the size of that of a modern human.

The mystery skull — known by only a number, KNM-ER 1470 — was discovered in 1972 and shown to the patriarch of the Leakey family, Louis Leakey, just days before he died. An associate had pulled it out of the ground and didn’t know what to make of it. Louis Leakey didn’t either, calling it “indeterminate Homo.”

In the 40 years since, despite combing the arid sandstone surrounding Lake Turkana, three generations of the Leakey family found no other fossils resembling 1470 until now. And so controversy raged in the famously contentious circle of researchers studying human origins on whether this outlier belonged to a separate species or was a deformed member of one of the other groups.

“There just hasn’t been any evidence one way or another,” said Susan Anton of New York University, who collaborated with Meave Leaky in describing the new fossils.

Rick Potts, director of the Smithsonian Institution’s human origins program, said he’s convinced the new fossils do represent a distinct species. Potts, who did not participate in the new work, added, “There will still be controversy over what to call these things.”

Whatever name the fossils eventually receive, one thing is certain, said Potts: The old picture of human evolution heading in a straight line — where an early species gave rise to a more advanced species and so on, until finally reaching modern humans — is all but defunct.

Instead, with each new find, human origins appear more and more complex.

“It does look like these are a lot of experiments in how to be a Homo species, doing slightly different things and looking a little different,” Anton said.

As researchers pull more early human fossils from the sedimentary rocks of East Africa, Potts expects to see more evidence of “evolutionary chaos” at the base of the human family tree. “That experimentation is a brilliant part of the evolutionary process.”

All of these groups likely made simple stone tools, Potts said. And yet, they left only a fragmentary picture of who they were — let alone what they ate or how they behaved.

“At the moment, all we’re doing is classifying heads,” said Bernard Wood, a paleoanthropologist at George Washington University who studied the mystery skull in the early 1990s and declared it likely represented a new species. “It will be a different ballgame when we can match heads with limbs. There are limb bones, but with no heads.”

Spoor cautioned that it’s still unclear if the different species of early humans interacted or interbred. “It’s not that we can say for sure that in one given month in one particular year that these species could meet each other and shake hands at the lake margin.”

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