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Early hominin skull fills in "a major gap" in the fossil record

A 3.8 million-year-old fossil skull is giving anthropologists their first look at an early Australopithecine, the hominin genus that eventually led to modern humans. The skull belongs to a member of a species called Australopithecus anamensis, which many anthropologists have considered the ancestor of the fossil hominin Lucy and the rest of her species, Australopithecus afarensis. But the find suggests that, as with most of these things, the story may be more complicated.

A. anamensis lived in Eastern Africa between 3.8 million and 4.2 million years ago. Like Lucy, they would have walked upright, but with a gait that we would probably pick out as a little odd. They probably would have still had upper arms adapted to the physical strains of climbing, especially as young children. At the moment, however, those are just assumptions—albeit very likely ones—based on what we know about other Australopiths. That's because, until now, anthropologists knew A. anamensis only from its teeth and jaws. In fact, skulls are hard to find at all in the fossil record before 3.5 million years ago.

That doesn't sound like much to go on, but the sizes and shapes of teeth changed noticeably between hominin species, so they're very handy for identification. In fact, paleoanthropologist Yohannes Haile-Salassie and his colleagues identified their newly found skull as A. anamensis based on the size and shape of its canines, which had certain anatomical features that stood out from A. afarensis and other close relatives.

But now anthropologists have a complete skull to work with. Formally known as MRD, it's mostly intact after 3.8 million years buried in sandstone, sandwiched between two layers of volcanic debris. The find, from the Waranjo-Mille site in the Afar region of Ethiopia, reveals what A. anamensis looked like, the kind of diet it was adapted to eat, and how its brain had grown compared to apes and to other hominins.

The lower half of the hominin's long face juts forward beneath its wide, heavy cheekbones, then narrows above them. Those broad cheeks and narrow upper face give A. anamensis a clear family resemblance to Lucy and other, later Australopiths. Overall, it's a strong, heavy-looking face, built on a frame of bones robust enough to support powerful muscles for chewing tough plant foods. In the dry shrubland around the shores of the ancient lake where MRD lived and died, nearly everything edible would also have been tough enough to make chewing serious work.

But if A. anamensis had the face of a later Australopith, its cranium looks more like those of apes and older hominin species. Its skull narrows just behind the eye sockets, like earlier hominins and apes, and its brain case, at 365cc to 370cc, is smaller than that of A. afarensis. Clearly, hominins hadn't yet started developing our infamous big brains in A. anamensis' day.

The find "fills a major gap in the fossil record," as Haile-Salassie and his colleagues wrote. Because skulls are so scarce in the East African fossil record before 3.5 million years ago, anthropologists can't say much about the hominin species on the scene just before the emergence of A. afarensis—who, it's thought, led directly to us.

Although there are some clear directions in evolutionary changes, it's increasingly clear that throughout the Pliocene (5.3 million to 2.6 million years ago), hominin species split into a profusion of new branches, trying out variations on the themes of bipedalism, strong chewing, and eventually larger brains. Some of those evolutionary experiments failed, some succeeded for awhile, and at least one succeeded long enough to ultimately lead to us.

Fossils unearthed in the last few decades have shown us that early hominins were a diverse group, and it was normal for multiple species to exist at the same time. In fact, we may be the first hominin species to ever not be sharing the planet with another one.

Anthropologists still aren't sure how all that hominin diversity fits together, or how all those species relate to each other—and to us. Trying to trace the path of our own lineage among all those sister and cousin species is much harder than it seemed a few decades ago, when we knew about fewer species and the whole story looked deceptively simple.

[...]

Perhaps more importantly for our understanding of our own origins, it also means that more than one hominin species was living in Africa 3.8 million years ago, just before the first members of Homo emerged. If A. anamensis was around at the same time as A. afarensis, then one species could be our ancestor just as easily as the other could. That implies that we can no longer take A. afarensis for granted as our ancestor. Stay tuned; that claim is likely to spark some debate.

Nature, 2019. DOI: 10.1038/s41586-019-1513-8 (About DOIs).

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