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

 arstechnica.com  08/29/2019 16:06:24   Kiona N. Smith
Color photo of a paleoanthropologist in a hat holding a fossil Australopithecine skull
Enlarge / Yohannes Haile-Salassie poses in the field with the newly discovered skull.

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.

Meet A. anamensis

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 assumptionsalbeit very likely onesbased 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 doesnt sound like much to go on, but the sizes and shapes of teeth changed noticeably between hominin species, so theyre 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, its 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 hominins 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 hadnt yet started developing our infamous big brains in A. anamensis day.

  • Is this the face of one of our earliest ancestors, or just a distant cousin?
  • The cranium was found in two large pieces and a handful of smaller fragments.
  • This reconstruction shows what A. anamensis' face may have looked like. Note how the face narrows moving upward from the cheekbones.
  • The diagrams show three possible relationships between A. anamensis, A. afarensis, and their last common ancestor.
  • Archaeologists dated the skull based on the ages of the volcanic deposits just above and below it.
  • The lower face of A. anamensis protrudes forward much more than A. afarensis or later hominins, who have flatter faces.

A tangled family tree

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 cant say much about the hominin species on the scene just before the emergence of A. afarensiswho, its thought, led directly to us.

Although there are some clear directions in evolutionary changes, its 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 arent sure how all that hominin diversity fits together, or how all those species relate to each otherand 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. Finally looking A. anamensis in the eye sockets may make things even more complicated.

Pliocene sister act

For years, conventional wisdom has said that A. anamensis and A. afarensis were basically earlier and later versions of the same species (something paleontologists call a chronospecies). A. anamensis gradually evolved into A. afarensis, and the older species just sort of faded away in the process. In part, that assumption came from the fact that A. anamensis' teeth didn't look like they had many derived featuresdistinctive features a species develops on its own after it has branched off from a common ancestor. Between that and the relative dates of the fossils, it looked as if A. anamensis and A. afarensis had to be mother and daughter species.

But A. anamensis' face has a surprising number of derived features, some of which are distinct from those of A. afarensis, even though they follow the general Australopithecine trend toward more robust bones to support chewing. That makes it look more plausible that A. anamensis and A. afarensis could actually be branches from a common ancestor; in other words, they could be sisters, not mother and daughter.

Haile-Salassie and his colleagues put the measurements of several early hominin fossils into a program designed to calculate their relationships based on how similar their features were. When they included MRD in their data set, A. anamensis plotted as a sister species, not an ancestor, to A. afarensis.

Having an A. anamensis skull to compare other fossils also prompted Haile-Salassie and his colleagues to take a second look at some other specimens, including a 3.9 million-year-old frontal bone from the Middle Awash region of Ethiopia. The specimen included the area behind the eyes where the skull narrowed, which is another feature that seems to be fairly distinctive for each species. The older fossil didn't narrow nearly as much as A. anamensis; in fact, it looked a lot more like A. afarensis.

If that's correct, the two species overlapped in time by at least 100,000 years, making it more likely that either the two species share a common ancestor or that A. afarensis branched off from an A. anamensis population that continued on.

So, who should we invite to Grandparents Day?

A study earlier this year used statistical methods to test how likely it is for a species to exist at the same time as its own ancestor. That study ruled out another early hominin, Australopithecus sediba, as an ancestor of the genus Homo, because the 800,000-year overlap between A. sediba and the earliest known Homo fossil was too unlikely. But in this case, an overlap in time of just 100,000 years isnt enough to rule out A. anamensis as an ancestor of A. afarensisit just means branching, rather than linear, evolution.

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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