Evolution’s Surprise: Fossil find uproots our early ancestors

In a discovery that upends the study of human origins, scientists have unearthed remains of what they say is the earliest known member of the human evolutionary family. Investigators led by anthropologist Michel Brunet of the University of Poitiers in France estimate that the creature, officially dubbed Sahelanthropus tchadensis, lived between 7 million and 6 million years ago.

The researchers call their find Touma, which means “hope of life” in the language of an African group that resides near the fossil site.

The nearly complete skull, two lower-jaw fragments, and three isolated teeth attributed to this previously unknown hominid hold a pair of major surprises.

First, a small braincase like that of living chimpanzees connects to a face and teeth resembling those of bigger-brained hominids dating to 1.75 million years ago, perhaps even early Homo specimens. No one had predicted that elements of later skulls–in particular, a short, relatively flat face, pronounced brow ridge, and small canine teeth–coexisted with a chimp-size brain in early hominids.

Second, Brunet and his colleagues made their discovery in Chad, a central African nation located far from established fossil-hominid sites in eastern and southern Africa. It appears that, between 7 million and 5 million years ago, hominids evolved into a wider variety of lineages across a broader area than scientists had assumed, says anthropologist Bernard Wood of George Washington University in Washington, D.C.

“This is an astonishing find,” remarks anthropologist Daniel Lieberman of Harvard University. “Hominid species in eastern and southern Africa appear to have been a small part of a more complicated evolutionary process.”

It’s hard to know whether Touma represents a direct ancestor of modern humans, Brunet’s team notes. Touma’s unusual anatomy distinguishes it from chimps, gorillas, and other fossil hominids (SN: 3/24/01, p. 180: Fossil Skull Diversifies Family Tree; 7/14/01, p. 20: Earliest Ancestor Emerges in Africa).

The researchers excavated soil layers that had been eroded by sandstorms in a surrounding desert. The fossil remains of 42 animal species were found in the hominid-bearing sediment, the researchers report in the July 11 Nature.

These creatures include fish, crocodiles, turtles, hippopotamuses, monkeys, rodents, and antelopes. This mix indicates that the region once contained a large lake with nearby forests and grasslands studded with stands of trees, the scientists say.

No absolute age was calculated for the site because it contains no volcanic ash, the material usually analyzed in such dating. The estimated age rests on a comparison of the Chad finds with animal species at other African sites with established ages.

Although “a huge diversity of humanlike forms” evolved in Africa, it’s unclear whether Touma was an early ape or hominid, as argued in the new report, comments anthropologist Christopher B. Stringer of the British Museum in London. If the skull is that of a female, then it may be a small version of male skulls in an ancient ape lineage. If it’s a male, then its small face and teeth give credence to its hominid status, Stringer says.

Brunet’s team suspects that Touma was a male because of its visorlike brow. In Stringer’s view, the group will need to find lower-body fossils to confirm the specimen’s sex and see whether it had an upright gait, a cardinal hominid trait.