Stunning ancient skull shakes up human family tree

For months, herder Ali Bereino had been trying to get a job working for a team of fossil hunters in northeastern Ethiopia. The Afar man hung around, watching and learning. One day in February 2016, Bereino dug a burrow to keep his baby goats safe from hyenas. He noticed teeth protruding from the hard-packed sand and pulled out a jawbone, which he brought to the team's leader, Ethiopian paleoanthropologist Yohannes Haile-Selassie of the Cleveland Museum of Natural History in Ohio. Shoveling aside nearly half a meter of old goat droppings and sieving through sediment, the team unearthed the nearly complete skull of an enigmatic human ancestor, the oldest member of the genus that eventually led to our own.

After 3 years of analysis, researchers have dated the fossil to 3.8 million years old and identified it as Australopithecus anamensis, a hominin long thought to be the direct predecessor of the famed "Lucy" species, A. afarensis. The new fossil could reshuffle that ancient relationship, the authors argue this week in two papers in Nature.

Researchers hail the skull as one of the most significant hominin discoveries in decades. "It's a spectacular find," says Carol Ward, an evolutionary anatomist at the University of Missouri School of Medicine in Columbia. "A number of teams—mine included—have been looking for an australopith skull like this. … This is the specimen we've been waiting for."

Still, not everyone is convinced it clarifies the relations of the australopithecines, a genus of upright apes that lived between 4.2 million and 2 million years ago throughout eastern and southern Africa.

A. anamensis was first identified in 1995, mostly on the basis of 4-million-year-old teeth and jaws from Kenya. Given the dates, plus several telltale anatomical similarities, most researchers concluded that A. anamensis gradually transitioned into and was replaced by A. afarensis, which lived from about 3.7 million to 3 million years ago.

The new Ethiopian specimen, named MRD after Miro Dora, the site where it was found, was probably a male with a brain size of about 370 cubic centimeters, about that of a chimpanzee. He had jutting cheekbones, elongated canine teeth and oval-shaped earholes—all features that strongly suggest membership in A. anamensis rather than the bigger-brained, flatter-faced A. afarensis, Haile-Selassie says. The team dated the skull using the radioactive decay of isotopes of argon in the surrounding sediments.

Fred Spoor, a paleoanthropologist at the Natural History Museum in London, says features such as MRD's projecting cheekbones and primitive earholes resemble those of later hominins, including South Africa's A. africanus and Kenya's Kenyanthropus platyops. The similarities, he says, may make some researchers wonder whether A. anamensis—and not A. afarensis, as thought—was the ancestor of those later hominins.

MRD's anatomy also helps pin down the identity of a puzzling 3.9-million-year-old forehead bone found in Ethiopia in 1981; Haile-Selassie says the comparison suggests the skull fragment belonged to A. afarensis. If he's correct, Lucy's species would predate the new anamensis skull. Haile-Selassie concludes that the two species overlapped for about 100,000 years. The team still thinks A. afarensis descends from A. anamensis, but suggests Lucy's species branched off anamensis, rather than simply replacing it.

Ward and William Kimbel, a paleoanthropologist at Arizona State University in Tempe, agree that the new skull belongs to A. anamensis, but both say it will take more fossils to convince them that two distinct species of australopithecines roamed the Afar region at the same time. "That issue rests on the comparison of the new specimen with the single frontal" bone, which is the only A. afarensis specimen suspected of such antiquity, Kimbel says. "It's difficult to make a strong argument because we have only the two specimens."

In a statement, Tim White, a paleoanthropologist at the University of California, Berkeley, who served as Haile-Selassie's doctoral adviser years ago, praised the discovery but says the studies' evolutionary implications are "a bridge too far." He thinks individual variation alone can account for the differences between the two specimens, and that the idea that afarensis replaced anamensis still makes sense.

Regardless of how things shake out for hominin taxonomy, the finding proved a boon for Bereino. "Obviously, it guaranteed him a hire," Haile-Selassie says.