Skull Find Defies Old Theories ; Fossil Discovery Is Closest Yet to 'Missing Link' - but Complicates Mankind's Family Tree

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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.

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. …