Two fossils suggest human family is older than thought

The mottled wedge of rock was lying on a hill in the Ethiopian desert as if waiting to be noticed.

Now elated scientists say it is a 2.8 million-year-old fossilized jawbone -- and the oldest known remnant of the family that includes modern humans.

The fossil, described in this week's Science magazine, has been hailed as a "missing link" between two classes of species: the group that includes us and a group of earlier, more apelike beings such as the iconic Lucy. A second, unrelated study in this week's Nature provides yet more evidence of humanity's deep roots, suggesting that our family's origins date back much farther than suggested by previous fossil evidence.

Some scientists reacted with caution to the new findings, illustrating the deep divisions among scholars about the origins of our species, Homo sapiens.

"This is a debate that has been going on forever – more than 50 years," says evolutionary biologist Christoph Zollikofer of the University of Zurich, who was not part of either team and is skeptical of both studies' conclusions. "Only more fossils will tell us what's going on."

Debate is sure to be fueled by the extraordinary new fossil, a jawbone studded with a half-dozen teeth. Discovered in 2013 in an Ethiopian research area known as Ledi-Geraru, it is by far the oldest fossil belonging to Homo, the species group that includes modern humans and our direct ancestors. The runner-up, a jawbone known as A.L. 666-1, dates back only 2.3 million years.

The new jaw is a strange patchwork of the primitive and the more modern. The receding chin is characteristic of some species of Australopithecus, the early, ape-like group of human relatives that include Lucy and related species. But the teeth and landmarks of the jawbone identify it as a member of Homo, says study co-author William Kimbel of Arizona State University.

The Ledi fossil is a "missing link," says Philipp Gunz of Germany's Max Planck Institute of Evolutionary Anthropology, an author of the Nature paper. The combination of his team's results and the new fossil "make a very convincing argument that early human evolution probably happened … 2.8, 2.5 million years ago," much deeper in the past than some scientists have argued.

Gunz and his colleagues sought new clues not in the sands of Africa but in another Homo fossil unearthed there in the early 1960s. By reconstructing a 1.8-million-year-old jawbone badly crushed before its discovery, the researchers determined that it, too, had very primitive characteristics, including an ape-like shape to the row of teeth.

Strangely, the reconstruction shows this Homo jawbone is more primitive in some ways than its counterpart A.L. 666-1, which is older. That means they must share a common ancestor that lived before A.L. 666-1, which has been dated to 2.3 million years ago, Gunz says.

Other scientists, though, are reserving judgment on the new findings. The new fossil and the reconstruction are "new and interesting additions," says evolutionary biologist Jeffrey Schwartz of the University of Pittsburgh, but he questions each study's assumptions and analysis and argues that researchers need to revisit the classification of many fossils.

The Ledi fossil may belong not to our own Homo grouping but to the older, more ape-like Australopithecus, says Adam Van Arsdale of Wellesley College. Kimbel argues, though, that the Ledi fossil's teeth and bone anatomy mark it as Homo.

All the same, "I would hesitate to move the starting point for … Homo back half a million years on the basis of half a jaw," Van Arsdale says. "I wouldn't necessarily rewrite the textbooks because of this one find."