Stunning Ancient Skull Shakes Up Human Family Tree

There is an interesting story that led to a stunning discovery. For months, local herder Ali Bereino tried to get a job with a team of fossil hunters in northeastern Ethiopia. The Afar man watched and learned while tending to his goats. One fine day in February 2016, Bereino dug a burrow to protect his baby goats from hyenas. He noticed some teeth protruding out from the hard-packed sand and managed to expose a jawbone. When he showed this to the team’s leader, Ethiopian paleoanthropologist Yohannes Haile-Selassie of the Cleveland Museum of Natural History in Ohio, he got excited. The team shoveled aside nearly half a meter of old goat droppings and sieved through sediment to unearth the nearly complete skull of an enigmatic human ancestor, considered as the oldest member of the genus that in due course led to our own ancestor.

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

Researchers claim the skull as one of the most significant hominin discoveries in decades.

Carol Ward, an evolutionary anatomist at the University of Missouri School of Medicine in Columbia says, “It’s a spectacular find. A number of teams—mine included—have been looking for an australopith skull like this. … This is the specimen we’ve been waiting for.”

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

A. anamensis was initially identified in 1995, primarily on the basis of 4-million-year-old teeth and jaws from Kenya. Considering the dates and several telltale anatomical similarities, most researchers determined that A. anamensis gradually transitioned into and was replaced by A. afarensis, which lived from around 3.7 million to 3 million years ago.

The new Ethiopian specimen, termed MRD after Miro Dora, the site of its discovery, was probably a male with a brain size of about 370 cubic centimeters, similar to that of a chimpanzee. He possibly had jutting cheekbones, elongated canine teeth, and oval-shaped earholes. All these features strongly suggest membership in A. anamensis rather than the bigger-brained, flatter-faced A. afarensis, according to Haile-Selassie. The team dated the newly discovered 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 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 isolate the identity of a puzzling 3.9-million-year-old forehead bone found in Ethiopia in 1981. Haile-Selassie explained that the comparison suggests the skull fragment belonged to A. afarensis. If this is correct then Lucy’s species would predate the new anamensis skull. Haile-Selassie concludes that the two species overlapped for approximately 100,000 years. The team still believes A. afarensis descends from A. anamensis, but proposes Lucy’s species branched off anamensis, rather than just replacing it.

William Kimbel, a paleoanthropologist at Arizona State University in Tempe, agrees with Ward that the new skull belongs to A. anamensis, but both concur that it will take more fossils to convince them that two distinct species of australopithecines roamed the Afar region at the same time.

Kimbel says, “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. 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 also served as Haile-Selassie’s doctoral adviser years ago, praised the discovery but admits that the studies’ evolutionary implications are “a bridge too far.” He contemplates individual variation alone can account for the differences between the two specimens, and that the idea that afarensis replaced anamensis still seems possible.

Irrespective of how things turn out for hominin taxonomy, this finding has proved a boon for Bereino. “Obviously, it guaranteed him a hire,” Haile-Selassie remarks.

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