Early Hominin Skull Fills in “a Major Gap” in the Fossil Record

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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.

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