For the first time in 3.8 million years, we can look into the face of the oldest-known member of our family tree. The dramatic discovery in Ethiopia of an almost-complete skull of a primate species called Australopithecine anamensis – the earliest australopithecine – was reported by a team of scientists Wednesday in the journal Nature. Up to now, all that had been found of this ancient creature were teeth and jaw fragments.
The cranium was discovered in 2016 on the shore of an ancient lake in Woranso-Mille, an area in the Central Afar that had been a stomping ground for multiple types of australopiths and probably other hominid species as well.
Mild signs of abrasion on the ancient primate’s skull suggest that he didn’t die there: his remains were washed down a river until they landed in sediment, geologists on the team say.
The skull’s dating to 3.8 million years ago goes against the traditional theory that human evolution was linear. Australopithecine anamensis had been thought to be an ancestor of the more advanced Australopithecine afarensis species. But a specimen of the more advanced species that had previously been found in the same location was dated to 3.9 million years ago. That would mean that the two species coexisted at the site for at least 100,000 years.
Anamensis – the species of the newly discovered skull – was in some ways closer to a chimpanzee than a human, including in brain size. And body size.
It was tiny, though we can’t tell from the skull alone how tall it was, let alone estimate averages. And it wasn’t a child.
“We can’t tell exactly how old it was other than saying it was an adult, and based on the wear on the teeth, we can infer that it was old,” lead author Prof. Yohannes Haile-Selassie of the Cleveland Museum of Natural History tells Haaretz.
Anamensis specifically might be ancestral to Homo habilis, who is considered the first member of our own genus – homo; or he might have turned into a dead end on the evolutionary tree. We can’t know.
But the discovery of his almost-complete cranium sheds new light on the earliest stages of human evolution – mainly that it wasn’t linear after all, but rather a messier affair.
Crested head, small brain
Not least because of the sheer rarity of hominin fossils, we don’t know specifically who begat whom. We can say which species predated which, to a degree. Multiple species of hominins – the family that we Homo sapiens belong to – coexisted at different times, and many interbred. It is apparently only in the age of modern humans, the most recent tens of thousands of years, that we have existed as the sole hominin on the planet.
For instance, at Woranso-Mille, there was coexistence among Australopithecine afarensis, Australopithecine deyiremeda (whose features are still being studied) and probably another species represented by the Burtele Foot fossil (which dates to 3.5 million to 3.3 million years ago and had a primitive opposable big toe), Haile-Selassie says. As noted, there was also the 100,000-year overlap between anamensis and the younger afarensis.
Now we know
There were several types of australopithecine, whether defined as different species, subspecies or just different types. As a group, the australopithecines spanned more than three million years. Fossil evidence for them dating from over four million years to 1 million years ago has been found in Chad, in eastern Africa and in South Africa, Haile-Selassie says. But until the momentous discovery of the skull, we didn’t know what the australopithecines older than 3.5 million years looked like.
Now we know. The oldest australopithecine is anamensis and he would have looked quite strange to us.
Anamensis had a prominent sagittal crest – a bone ridge on the top of his skull, a primitive feature shared with the even older species of hominins. That includes “neither chimpanzee nor human” Ardipithecus also found in the Afar, and Sahelanthropus tchadensis, which lived around 6 or 7 million years ago, shortly after the homo line split from the chimp.
Ardipithecus is believed to have begat the australopiths (and as for tchadensis, some researchers suspect it isn’t hominin but ape, despite its relatively flat face).
Australopithecine anamensis’ brain volume was 365-370 cubic centimeters, the researchers estimate, about the size of a chimpanzee’s or a medium-sized orange. Its jaws were powerful and thrust forward, with strong chewing muscles anchored in the skull crest, and its face had broad cheeks but was elongated. Anamensis was petite, going by the size of his head, but how tall he stood is anybody’s guess.
He or she?
The specimen found at Woranso-Mille had reached adulthood: his post-canine teeth were heavily worn, having lost almost all their enamel. He also had large canines – in fact, in the hominin fossil record, only Ardipithecus ramidus had canines as big as anamensis, Haile-Selassie says.
He? Apparently. “A female would have had small canine teeth and no sagittal crest,” Haile-Selassie explains to Haaretz.
So, do the canines mean it was a predator? Do the teeth tell us what it ate? They do not.
“The large canines don’t have any dietary signal. The earliest hominins have generally large canines,” Haile-Selassie explains. “It is the primitive condition. We’ll have to wait to sample its teeth for carbon isotope analysis. Right now, we don’t know.”
Stephanie Melillo of the Max Planck Institute, a coauthor on the work, notes that australopiths in general “were animals with a big, massive face,” while the homo line have much more slender faces. On the newly discovered anamensis skull, that massive face is right there, just as is typical of australopiths. In any case, whatever anamensis ate, his teeth indicate that it was tough.
Is it Daddy?
In fact, there is little clarity on the line of human evolution, and as for this specimen, it can’t be stated whether he begat later australopithecines, including afarensis, best known from the skeleton dubbed Lucy, let alone whether he is an ancestor of our homo genus. (Lucy, by the way, was also found in the Afar.)
Anamensis was discovered in 1995 through reanalysis of teeth and bone fragments, including the initial find, an arm bone discovered in northern Kenya in 1965. More bones were found in 1994. Altogether, fragments from about 20 anamensis individuals have been discovered so far. The species features some primitive traits: though capable of bipedality, it also had the elongated arms associated with climbing trees. Separate research on a baby australopith’s feet theorized that the young ones may have even lived in the trees for protection. Having dwelled on some of their primitive characteristics, it bears adding that australopithecines in Dikika, Ethiopia were the earliest known users of stone tools, 3.4 million years ago. The male adult anamensis found in Woranso-Mille lived in predominantly dry shrubland, with some riverside forest, grassland and wetland, a second paper on the discovery suggests. It describes how the cranium was dated and elaborates on its environmental context.
Anamensis, dating to 4.2 million years, predates afarensis, but is it a direct lineage? The hypothesis of anamensis being the ancestor of afarensis is not refuted by the new discovery, Haile-Selassie makes clear.
Nor is it supported. “The primitive cranial morphology links this fossil to even more ancient hominins, such as Sahelanthropus and Ardipithecus, and casts doubt on previous assumptions about a direct link to the younger Australopithecus afarensis,” Haile-Selassie and the team write. Apropos primitive features, the Burtele Foot species also found at Woranso-Mille had an opposable big toe like that of Ardipithecus.
What the findings do suggest, the team explains, is that the anamensis and afarensis lineages may have overlapped and developed together for at least 100,000 years (“cladogenesis”) and split, rather than the former preceding the latter in a single evolving lineage (“anagenesis”) as previously hypothesized, Haile-Selassie says.
Volcanic eruptions, massive lava flows and earthquakes could plausibly have resulted in geographical isolation of small groups of australopithecines – which could have led to their evolution into different species. That diverse, active environmental setting of these primates’ lives is key to understanding how it is that multiple australopithecine species lived in the same place at the same time. Africa was rifting, explains Beverly Saylor of the Case Western Reserve University in Cleveland – meaning that the region was prone to geological instability. Saylor worked on the context of the anamensis skull discovery.
And, could this earliest-known australopith have been our ancestor?
“As far as the currently available fossil record goes, some australopiths are on the line that eventually led to our genus,” Haile-Selassie replies. “However, the question is which ones of them are on the line of ancestry and which ones are side branches.”