An ancient skull pulled from the arid highlands of northern Kenya is forcing scientists to radically rethink some of the most cherished theories of human evolution.
For generations, many anthropologists have believed that humans' path from prehistory to the present began as a straight line: Our ancestors separated from the apes 4 million to 6 million years ago, then were simply refined, leading to what we are today.
As evidence, there was "Lucy." The 3.2 million-year-old hominid skeleton was discovered in 1974 by the legendary Louis Leakey and was thought to be proof that all human ancestry could be traced to a single primitive lineage.
Now, that model is crumbling. The new find, made by a team including Dr. Leakey's daughter-in-law, suggests that numerous species of hominids roamed the earth throughout prehistory. The discovery is not only casting doubt on who our true ancestors are, but it is presenting a far less romantic view of human evolution, with hominids evolving in fits and starts before one species finally emerged to give rise to today's humans.
"This finding will be a waypoint," says Ian Tattersall of the American Museum of Natural History in New York. "Human evolution was not a single linear structure from primitive to perfection. It's a story of experimentation, with new species being spawned and others becoming extinct."
Today, the notion that there were several species of prehumans might seem a bit peculiar. After all, Homo sapiens is the only human that exists today. But in recent years, scientists have confirmed that several different species of hominids lived during the period from 1 million to 3 million years ago.
The new report published in today's issue of the journal Nature shows that such diversity also existed in Lucy's time - farther back in history than some expected. The 3.5 million-year-old skull is so different from Lucy in so many ways that the project scientists have claimed that it's not just a new species, but a whole new genus - or category, of hominid.
None of its facial features are unique - the flat face, small cranium, and low brow ridge, for example, have all been seen in other hominids. But the combination of all these aspects into one creature is unprecedented.
"What's most exciting is that it's weird," says Daniel Lieberman, a paleontologist at George Washington University in Washington. "It throws a monkey wrench into what most people thought was a pretty clean picture of what human evolution was at that point in history."
As with almost all the landmark discoveries about early man, this one was made in Africa's Great Rift Valley, a massive trench formed by the movements of the African and Arabian tectonic plates over 30 million years. In the north, where Maeve Leakey and her colleagues focused on the site at Lomekwi, the Great Rift Valley is desolate. The mean temperature is 86 degrees F, and vegetation is sparse, with scrub brush and an occasional acacia tree rising above the dry river beds. Only Lake Turkana, a vast inland sea tinged jade with algae, breaks up the barren hills.
Yet millions of years ago, research suggests, the whole region was gradually transitioning from a dense forest to open grasslands. It was about that time, scientists say, that hominids first began to walk with two legs, beginning the evolutionary journey toward Homo sapiens. Indeed, the Great Rift Valley has become known as the "cradle of humankind."
Recently, this cradle has been revealing numerous secrets about mankind's infancy, causing something of an upheaval in the anthropological community. In addition to findings that have shown great diversity in the relatively recent past, a French team working in Kenya last month declared that it had located fragments of a hominid that are 6 million years old - the oldest ever found. Moreover, the bones of the so-called "Millennium Man" seem to be more human than Lucy, throwing the human family tree into further doubt.
The overall effect of this and other discoveries has been a catalyst for reexamining every scenario for human evolution.
"What this is doing is opening a whole new set of possibilities that we weren't able to consider before," says Dr. Tattersall. "People are more prepared to accept diversity."
And while the current byproduct of these revelations is uncertainty, anthropologists say the end result will be greater understanding.
"Everything is more complex than what we thought 50 or 100 years ago," says Christopher Stringer of the Natural History Museum in London. "Now we have a new model, and this will lead to clearer thinking."
(c) Copyright 2001. The Christian Science Monitor