Skull find defies old theories

Fossil discovery is closest yet to 'missing link' – but complicates mankind's family tree.

By , Staff writer of The Christian Science Monitor

At the southern fringes of the Saharan sand dunes, a team of French scientists has come closer than ever before to finding the holy grail of anthropology: the missing link between humans and their ape forebears.

In one of the most inhospitable places on the planet, buffeted by sand storms and seared by average high temperatures well over 100 degrees F. in summer, a 10-year mission has unearthed the complete skull of what is believed to be the oldest human ancestor yet found – between 6 million and 7 million years old.

It is one of the most significant discoveries in the history of anthropology.

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The skull sheds light on the crucial, yet largely unknown period 6 million to 10 million years ago, when the human lineage is thought to have branched off from apes. Already, its characteristics and location are forcing anthropologists to rethink their most basic tenets – from where the human line originated to how and when it developed.

The result, say scientists, will likely be one of the most fecund periods of paleoanthropology, as researchers seek similar fossils across Africa in an attempt to understand how this peculiar cranium fits into the ever more complicated story of human evolution.

"This is the first time that we've been able to take a glimpse of the world that connected us to the tree of life," says Bernard Wood, an anthropologist at George Washington University in Washington. "That's a pretty big deal."

Until now, that epoch had been an almost complete mystery. Although it held the secrets of mankind's beginnings, all the hominid fossils found from that time couldn't fill a locker at the YMCA.

Lacking a fossil record to look at, many scientists held to the traditional idea of human development: that human ancestors originated in eastern Africa and – at least in the earliest years – could be traced along a single ancestral line to today's Homo sapiens.

The ancient skull, reported in this week's issue of the journal Nature, emphatically refutes those notions.

For one, it is unlike anything scientists could have imagined, with a strange mixture of a chimp-like brain case and a more human face. The combination of features point to a diversity of hominids, even at that earliest stage of development, with perhaps a half dozen or so species all emerging at once.

"There was a lot of variation out there," says Daniel Lieberman, an anthropologist at Harvard University who has seen the skull. "We've been connecting the dots when most of the dots have been missing."

What's more, it was found along the shores of a dry lake in the country of Chad, 1,500 miles west of the east African rift valleys often called "the cradle of humankind."

For years, lead researcher Michel Brunet has tilted mostly unsuccessfully against the long-held theory that hominids emerged from the Great Rift Valley around Kenya then spread westward across Africa and into the broader world. Now, in the hominid he has named Toumai, or "hope of life" in the local language, he has proof that the earliest prehumans covered a larger area.

Indeed, the skull, startling in its quality and completeness, has opened up the entire continent to exploration. "It's almost a challenge to the rest of the community," says Dr. Wood. "There is really good evidence out here, now we just have to find it."

In some ways, Toumai is merely the continuation of a trend. During the past decade, scientists' understanding of more-recent periods of human evolution – from 1 to 4 million years ago – have undergone a renaissance.

Findings in China and the former Soviet republic of Georgia have shifted the timeline of when hominids spread out from Africa. A discovery in Kenya last year suggested that many different species of hominids lived in eastern Africa 3.5 million years ago – showing that hominids developed in fits and starts, thus debunking the theory that humans followed one uninterrupted line of evolution from prehistory to the present.

Today's report merely moves that diversity further into the past.

Yet that revelation is both tantalizing and troublesome. True, scientists now have a closer glimpse of what the "missing link" might have looked like. But if several species of hominids split from chimpanzees at the same time across Africa, how can anthropologists know which line is the "true" line – the one that gave rise to today's Homo sapiens – as opposed to all the others?

The simple answer is that, for now, they can't. Toumai may be one of humanity's direct ancestors. He may not be. Few scientists will speculate. Some have even proclaimed the search for the "missing link" all but dead, believing it impossible to determine which species is mankind's evolutionary precursor.

The nature of science, however, is to answer questions, and for that reason many researchers say Toumai could mark the beginning of a new era.

"We're just about to enter a period of chaos in paleoanthropology," says Henry Gee, senior editor at Nature. "We're going to discover all sorts of weird fossils from places we've never looked at before – fossils that will confuse more than enlighten," before a new, clearer picture emerges a few decades from now.

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