The New Old Man
As a little girl, Meave Leakey loved doing jigsaw puzzles, except for one thing: she found them too easy. To give herself more of a challenge, she flipped all of the pieces face down, and only then, with just their shapes to guide her, put the puzzle together. Good career move. Having married into the famous fossil-finding Leakey family--Meave is the wife of paleontologist Richard, himself the son of the late Louis and Mary--and become a leading paleontologist at the National Museums of Kenya, she has had to put all her puzzle-solving talents to work, and not only to assemble ancient fossil skulls from pieces the size of macadamia nuts. Recent finds in East Africa have scrambled the picture of human evolution, leaving a far more complicated--and fascinating--scenario than paleontologists suspected only a few years ago.
Now Leakey's latest find, a battered skull unearthed on the western shore of Kenya's Lake Turkana, threatens to topple the one thing we thought we knew about our earliest ancestors: that we are direct descendants of the diminutive species whose most famous member, Lucy, roamed East Africa 2.9 million to 3.9 million years ago. "For more than 20 years we thought that Australopithecus afarensis [Lucy's species] was our common ancestor," said Leakey as she dashed for a flight in California last week. "With this find, we now have at least two possibilities for who our ancestor was." It's like discovering a long-lost great-aunt--and then realizing that she, and not the woman you always called Grandma, might be your family's real matriarch.
The discovery of the ancient skull, announced last week in the journal Nature, came in August 1999. Justus Erus, a research assistant who had been probing for fossils by hand in the charred ground around the lake, came upon a piece of bone. "I thought maybe it's a monkey," he says. "I put the mark, and then I waited for Meave Leakey to come and look at it." She pronounced judgment in less than half an hour. "She called all of us and said, 'This is a human skull.' She was very happy," says Erus. That evening Meave's daughter Louise broke out the beer. "Whenever we find hominids [members of the family of man], we have bottles of beer and a goat," she says. The celebration lasted far into the night.
Only a year later did the full significance of the discovery dawn on anyone. Pieced together, the skull looked like nothing ever seen before. "It's the flat face that gives it away" as a new genus in our family tree, says Louise, 29. The team named him Kenyanthropus platyops--flat-faced man of Kenya. "The find tells us that our genealogical tree is much more complicated," says Louise, who as a child accompanied her parents on digs. "At 3.5 million years ago, the same time Lucy lived, we have got something much different." The old notion of a single, almost Biblical lineage in which Lucy begot Homo habilis, who begot Homo erectus, who begot Homo sapiens (us), is out the window. Humans have "a complicated, diverse past with lots of different species, many of which became extinct," says Meave Leakey.
That shouldn't surprise anyone. The family trees of every other mammal are full of branches that led nowhere--failed evolutionary experiments. There were multiple hominids before Lucy and after Lucy, "so it didn't make sense to have only a single species 3.5 million years ago," says Leakey. After the ape and human lineages split 5 million to 6 million years ago, the chimplike Ardipithecus ramidus inhabited the dense woods of what is now Ethiopia 4.4 million years ago, and Australopithecus anamensis lived in Kenya 4.2 million to 3.9 million years ago. Anamensis was probably the direct ancestor of Lucy's species. Small-brained, big-toothed, 3.2 million-year-old Lucy, found by Donald Johanson and his team in 1974, had long arms, short legs and a protruding face. Afarensis probably hadn't completely abandoned life in the trees. After Lucy came an evolutionary parade of australopithecines, followed by various Paranthropus and Homo species. But until platyops we had but one ancestor in the middle Pleiocene, 3.5 million years ago: Lucy and her afarensis kin. Now it looks as if we have two. Platyops has a flatter face and smaller teeth than Lucy. Since different-sized teeth indicate a different diet, the two hominids probably did not compete for the same food in the woodlands and grasslands of ancient East Africa.
Once a species makes a breakthrough adaptation, "which in our case was bipedalism, the ability to walk upright," Johanson says, "there is usually a great radiation of new species." By that reasoning, if the arid landscape beside Turkana's alligator-infested waters yields up more of humankind's long-lost relatives, paleontologists may eventually find that platyops and afarensis had company. The number of known species of human ancestors has almost doubled in the past 15 years. What determined which would live and which would die, who would be an evolutionary dead end and who our ancestor? "Species that became too specialized probably met their demise once the climate changed and there was greater competition for food and other resources," says Johanson, who is taking the possible dethroning of Lucy graciously. "They lost out to others that were better at exploiting their environment. But apart from that, we may never know the exact reasons." What we do know is that, in the search for humankind's roots, the tree of life is looking downright bushy.