A chance encounter in an Ethiopian goat pen has yielded a one-of-a-kind fossil: the nearly complete skull of a human ancestor that died some 3.8 million years ago.
The new specimen—described today in the journal Nature—is the oldest skull ever found of an australopithecine, a pivotal group of early human ancestors that lived between 1.5 and four million years ago. It’s also the first skull ever found of Australopithecus anamensis, one of the earliest members of that genus.
“It takes you back about 3.8 million years to think about what our ancestors looked like at that time,” says the study’s lead author, Yohannes Haile-Selassie, a paleoanthropologist at the Cleveland Museum of Natural History. “It’s really an exciting moment.”
The find could fill in several important gaps in the study of human evolution. Fossils of hominins, or human ancestors, that old are exceedingly rare and often mere fragments of bone. In contrast, the newfound skull is mostly complete, which should reveal many details on how our earliest ancestors lived and evolved.
“It’s the skull we've been waiting for,” says Carol Ward, a paleoanthropologist at the University of Missouri who wasn’t involved with the study. “Hominin skulls are these exceptionally rare treasures, and to find one this old and this complete is almost unprecedented.”
'His hands were shaking'
The roots of the tangled human family tree trace back to Africa more than four million years ago, to a menagerie of ancient primates such as Ardipithecus and Sahelanthropus. It took until three million years ago for our genus Homo to arrive, an evolutionary saga in which ancestors such as Australopithecus afarensis played a starring role.
Best known by fossils belonging to an individual scientists call “Lucy,” this early hominin and its later kin had bigger brains than earlier primates, the ability to walk on two feet, and strong jaws that let them eat a wide variety of foods. That flexibility would have come in handy: During the heyday of A. afarensis 3.5 million years ago, natural shifts in climate were making East Africa cooler and drier, shrinking the woodlands that our earliest ancestors called home. Over time, evolution sculpted A. afarensis and its successors to take advantage of more open, varied environments.
But A. afarensis wasn't the first creature on the scene with these traits. In 1995, scientists described A. anamensis, an even earlier australopith and the probable ancestor of A. afarensis. The species tantalized scientists, since it shared key features with Lucy and later australopiths. But A. anamensis remained stubbornly out of view. Its only known remains consisted of just teeth and jaw fragments. “Despite the many skulls of of A. afarensis, we did not know what the face of very early members of the genus was,” says Zeray Alemseged, a paleoanthropologist at the University of Chicago who wasn’t involved with the study.
Clarity would start to come on February 10, 2016, thanks to the astounding luck of a herder named Ali Bereino.
At the time, an expedition co-led by Haile-Selassie was digging at Woranso-Mille, a field site in Ethiopia’s Afar region less than three miles from Miro Dora, where Bereino was herding. According to Haile-Selassie, Bereino had tried for years to get himself hired on Haile-Selassie’s team. He sometimes claimed that fossils emerged from eroded rock; when Haile-Selassie had visited in the past, he hadn’t see any.
On this particular day, Bereino was digging an addition to a temporary goat pen when he noticed a bone exposed in the sandstone surface. Bereino got in touch with a local government official, who agreed that it might be something Haile-Selassie would find interesting.
When the official called Haile-Selassie, he remained skeptical, replying that Bereino should mark where he found the fossil and walk it over to his camp. When Bereino and the official arrived, Haile-Selassie soon realized the magnitude of the find. Bereino had found a maxilla, or upper jawbone, belonging to an ancient hominin.
Haile-Selassie immediately stopped what he was doing and walked the 2.5 miles to Bereino’s goat pen. Just feet away from where Bereino had found the maxilla, Haile-Selassie soon spotted what turned out to be most of the remaining skull. “I didn’t even pick it up, and I started jumping up and down,” Haile-Selassie says. “The [official] looked at me and told his local friends, ‘What is going on with the doctor? Is he going crazy?’”
Once Haile-Selassie saw that the maxilla and skull fit together, he returned to camp with the fossils, cradling them in his bandana and a borrowed scarf. “I’ve never seen him so happy in my life,” says study coauthor Stephanie Melillo, a paleontologist at the Max Planck Institute for Evolutionary Anthropology and an expedition member. “He couldn’t even get words out of his mouth; his hands were shaking.”
A clock made of ash
The next day, Haile-Selassie, Melillo, and their team hiked to Miro Dora. Determined to sieve for even the tiniest fragment of bone, they fanned out across an area five meters to a side. But being thorough meant getting dirty. The area included a stinking heap: years of goat dung in layers up to a foot thick. Enduring the stench was worth it. Over the next few days, researchers found more of the skull, including a crucial cheekbone, beneath the dung pile.
Back in the lab, Haile-Selassie’s team found that the skull’s jawbones and teeth most closely resembled those of A. anamensis. But identifying the skull was only part of the mystery. When and where did this A. anamensis live and die?
To find out, a team of geologists led by Beverly Saylor examined the Woranso-Mille terrain in detail. In particular, they were on the lookout for tuffs, sediment layers created by ancient volcanic ashfalls. Some minerals in tuff contain traces of radioactive potassium-40, which decay like clockwork from the time the minerals were created until the present. By counting up the decay products, Saylor’s team could tell when the crystals—and the tuff as a whole—were created. So to date the skull, the team needed to find two tuffs that sandwiched the fossil’s sediments.
In a second study published in Nature, Saylor’s team says that one tuff above the skull formed between 3.76 and 3.77 million years ago, and a second below the skull formed slightly more than 3.8 million years ago. In addition, researchers pieced together the skull’s burial environment: They found that the skull was buried in a river delta on a lakeshore, surrounded by shrubland and patches of trees. “It probably was either along the river or along the shores of this lake. It died there, and then it was transported down and buried in the delta,” says Saylor, a stratigrapher at Case Western Reserve University.
Evolution's many modes
In many ways, the face fits in nicely with researchers’ expectations. Like other australopiths, the face of A. anamensis was long and sloping, unlike the flat faces of modern humans. The dimensions of its teeth and jaws also make sense: Later australopiths had large, wide faces to accommodate the bones and muscles needed to power through tough diets. While A. anamensis had a more robust face than earlier primates, it wasn’t as large as those of its later cousins.
But if Haile-Selassie and Melillo are right, the skull could raise more questions about how A. afarensis evolved.
One key feature in early hominin skulls is how much the skull narrows behind the eye sockets. Older, more primitive hominins tend to have more constricted skulls than younger ones. The new skull of A. anamensis narrows considerably behind the eye sockets. That feature could clarify the identity the “Belohdeli frontal,” a 3.9-million-year-old fragment of australopith skull found in 1981.
When the Belohdeli frontal was first discovered, some researchers thought it belonged to A. afarensis, but they couldn’t be sure. The situation got murkier once A. anamensis was discovered. Researchers couldn’t confirm whether the bone belonged to A. anamensis, since there were no clear-cut frontals from that species.
“This fossil was hanging out in a taxonomic limbo for decades,” says Melillo.
Now that they have the new skull for a reference, Melillo and Haile-Selassie say that the Belohdeli frontal isn’t A. anamensis and instead belongs to A. afarensis.
Because the Belohdeli frontal is older than the new skull, the find suggests that A. anamensis and A. afarensis overlapped in time from 3.8 to 3.9 million years ago. That’s an evolutionary shakeup: Scientists had thought that successive generations of A. anamensis evolved into A. afarensis, a straight-line process that would have precluded any overlap. Instead, the researchers argue that by 3.9 million years ago, one group of A. anamensis had branched off from the rest and evolved into A. afarensis while other groups of A. anamensis stuck around.
Some scientists say that confirming this evolutionary scenario will require more fossils. “In order to have confidence ... one needs really good sample sizes, both within time planes and across times,” says William Kimbel, a paleoanthropologist at Arizona State University’s Institute of Human Origins who wasn't involved with the study. “You cannot make a strong claim on the mode of evolution based on only two specimens.”
The research team says they have more studies in mind, including a more detailed look at how A. anamensis and A. afarensis may have differed in their diets and lifestyles. But even now, scientists poring over the fossil have to contend with a new delight: a fossil that can stare back.
“It really fills you with wonder,” says Melillo. “To be able to see the face of this entity I was already so familiar with and have so many ideas about was super-cool.”