Evolution: Branches on the Tree

The phrase "family bush" doesn't trip off the tongue the way "family tree" does, but anyone talking about human origins had better get used to it. For years scientists have known that the simple linear model in which one ancestor evolved into another is a myth. Starting 4 million years ago, half a dozen species of Australopithecus lived in Africa at the same time. Experts thought that once the Homo lineage debuted 2.5 million years ago in East Africa with Homo habilis, things settled down, with habilis evolving into Homo erectus, who evolved into Homo sapiens—us. But two fossils discovered in Kenya in 2000 (it takes scientists years to figure out what fossils mean) suggest evolution was a lot messier than that.

One fossil, found just east of Kenya's Lake Turkana, is the upper jaw of a habilis from 1.44 million years ago. This species was thought to have gone extinct about 1.6 million years ago. The other find, from the same site, is an erectus skull from 1.55 million years ago. The more recent date for habilis shows that he and erectus were contemporaries for half a million years, until 1.4 million years ago, when habilis died out. That long period of coexistence "makes it unlikely that Homo erectus evolved from Homo habilis," says Meave Leakey, coauthor of the paper announcing the discovery in the journal Nature. Instead, evolution took a "let 1,000 flowers bloom" approach—see which species survives and thrives. "It underlines the diversity in the human-evolution story, with experimentation in what the genus Homo should be like," says paleoanthropologist Ian Tattersall, of the American Museum of Natural History in New York, who was not involved in the find. The result is the human family bush, with numerous branches at every point in time.

There is no evidence about how, or even whether, erectus and habilis interacted. Each probably had his own diet and way of living. The teeth and jaws in habilis are larger, suggesting habilis ate tougher food, such as vegetation, while erectus ate more meat. Habilis was the first of our ancestors to have a brain bigger than a chimp's and the first toolmaker. But erectus was the first to have an asymmetric brain, which enables cognitive specialization and therefore complexity, perhaps giving him an evolutionary edge.

As often happens in the science of human evolution, other researchers are not sure the discoverers got it right. Tattersall, for instance, questions whether the specimens are indeed a habilis and an erectus. Neither "looks anything like" the specimen that defines the species, he says: "It doesn't look like erectus as we know him. It might be a new, as-yet-unnamed species." The discoverers are sticking by their ID.

The presence of numerous Homo species, whoever they were, would still mean many human ancestors living simultaneously. That leads to the real puzzle. If throughout human evolution several species of ancestors lived at the same time, why is Homo sapiens the one and only species of human on the planet today?