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When “Lucy” the now-famous Australopithecine was discovered in Ethiopia in 1974 by paleoanthropologists, researcher’s minds were full of questions, but the biggest question of all was: what did Lucy’s ancestors look like? That question has now been answered, at least in part, thanks to the discovery of a nearly complete skull of Australopithecus anamensis.
The skull, with its protruding jaw and large canine teeth, dates back some 3.8 million years, meaning its very likely that A. anamensis overlapped with Lucy’s species, Australopithecus afarensis for at least 100,000 years, according to LiveScience. A few fragmentary fossils had been found previously, but they didn’t provide researchers with enough evidence to go on. Until this discovery.
A. anamensis looks quite similar to Lucy but there are a few noticeable differences.
“What we’ve known about Australopithecus anamensis so far was limited to isolated jaw fragments and teeth,” study co-author Yohannes Haile-Selassie, a paleoanthropologist with the Cleveland Museum of Natural History told journalists at a press conference announcing the discovery. “We didn’t have any remains of the face or the cranium except for one small fragment near the ear region.”
Before A. anamensis’ discovery, researchers had little to work with. But that changed on February 10, 2016, when Haile-Selassie and his colleagues unearthed the cranium in two good-sized pieces in the Afar Region of Ethiopia’s Godaya Valley. The fossil was entombed by the sands of what was once an ancient river delta emptied near the shore of a lake, according to Beverly Saylor, a professor of stratigraphy and sedimentology at Case Western Reserve University. Saylor was also speaking at the press conference. She was also the leader of the team of paleobotanists, geologists and paleontologists who worked out the fossil’s age and its geological context.
Saylor added it’s likely the river moved the skull away from where the hominin (human ancestor) died. But she surmised it only traveled a short distance because the skull wasn’t very abraided.
She said she also thinks this individual “probably was living along the river and the shores of this lake.” The shores of the lake would have been forested but the surrounding landscape was arid, with scrubby vegetation. Saylor and her team were able to accurately date minerals and volcanic tuffs in the area and determined the fossil was 3.8 million years old. Based on the size of the bones, the researchers also believe the specimen was a male.
“This specimen fills an important gap in our knowledge of the cranial anatomy of Australopithecus during this period,” said Amélie Beaudet, a paleoanthropologist at the University of the Witwatersrand in South Africa. While Beaudet wasn’t involved in the research, she said the fossil helps illustrate the changes in Australopithecus through time, and this may also illuminate geographical connections among species. The skull, she told LiveScience, shares features with Australopithecus africanus, a species that lived in South Africa.
The face of A. anamensis wasn’t quite as rugged as Lucy’s, but it was still pretty robust, the researchers noted August 28 in the journal Nature. Indeed, this individual, which researchers have dubbed “MRD” (an abbreviation of its specification classification) does have smaller canine teeth than earlier hominids, but the teeth were still larger than those of Lucy and her kind. And MRD’s protruding lower jaw is very ape-like, unlike the jaw of modern humans and other species of the genus Homo, which appeared around 2.8 million years ago.
MRD’s brain was about a quarter of the size of a modern human’s, The Guardian notes, but despite those smaller canines, that protruding jaw and prominent cheekbones (that are quite similar to Lucy’s) provide evidence that MRD and his kind were able to chew tough vegetation during dry periods when food was scarce.
MRD’s discovery challenges the long-held notion of linear evolution, which holds that one species vanishes before it is replaced by a new one. But in the case of A. anamensis, the timeline spans from 4.2 million to 3.8 million years ago, meaning that while MRD is Lucy’s ancestor, it was still around when Lucy’s kind branched off from the parent lineage. Along with that, there’s geological evidence that this was a landscape of steep hills, rifts, volcanoes, and lava flows that could have created isolated populations, thus allowing them to diverge.
And all of this worked to isolate subgroups of hominins, which means interbreeding sparked changes that lead to entirely new species. Meanwhile, their parent species continued to thrive and survive elsewhere. This switches things up a bit for Haile-Selassie.
“This is a game-changer in our understanding of human evolution during the Pliocene,” he says.
Afarensis continues to show up in the fossil record until three million years ago, and up until now, it’s been considered a likely candidate to have given rise to the Homo lineage that we humans come from. But the discovery that several lineages existed simultaneously makes the hypothesis of linear evolution considerably less certain, the researchers say.
“Having multiple candidate ancestral species in the right time and place makes it more challenging to determine which gave rise to Homo,” said Stephanie Melillo of the Max Planck Institute for Evolutionary Anthropology in Germany.
So it seems that on the one hand A. anamensis has solved one mystery while creating another one by further complicating our human story. A story, it turns out, that has many secrets yet to be revealed.
In the remarkable video below, artist John Gurche, who did the reconstructions of A. anamensis seen here takes us on a trip through human evolution.