Were early humanlike species neighbors?

CHICAGO — Two small fossils unearthed in Kenya — the top of a skull and half of a jawbone — fill an important gap in the evolutionary story of how humans came to be, yet have created as many questions as they have answered.

The similar age and location of the fossils suggest that two early humanlike species, Homo habilis and Homo erectus, closely coexisted rather than coming one after the other on the evolutionary road to modern humans, according to a paper published today in the journal Nature.

"Showing these two lines living side by side means the picture of a simple line from habilis to erectus can't be right," said co-author Susan Anton, a paleoanthropologist at New York University.

The fossils were discovered in the Lake Turkana region of Kenya by the Koobi Fora Research Project, a group led by the notable anthropologists Meave Leakey and her daughter, Louise. The area is rich in the remains of early hominid species.

Homo habilis, which means "handy man," was previously thought to have existed in East Africa from approximately 2.3 million to 1.6 million years ago. A short, long-armed species, it looked very different from modern humans yet appears to have developed some of the earliest tools excavated by paleontologists.

Homo erectus, meaning "upright man," is thought to have existed between 1.8 million and 250,000 years ago, and fossils have been found in parts of Europe and Asia as well as Africa. With a larger head and evidence of more advanced behavior, erectus is considered by some to be an immediate predecessor of modern humans, Homo sapiens.

Despite some overlap in the fossil record, the two species were not believed to have coexisted in a significant way. But finding a 1.4 million-year-old H. habilis jaw fossil in the same region as a 1.5 million-year-old H. erectus skull fragment throws that interpretation into doubt.

If the two species coexisted for roughly 500,000 years in the same East African domain, as the authors argue, H. erectus may not have evolved from H. habilis, as was previously thought.

"The differences are strong enough and persist for a long enough period of time to suggest two separate gene pools that weren't mixing," Anton said. She compared their relationship to that of modern chimpanzees and gorillas, who share the same habitats and some food sources without interbreeding.