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Our family split from the other apes around 7 million years ago. The best evidence we have for this crucial point in human evolution comes from Ardipithecus. This 4.5 million-year-old hominin is best known from “Ardi” a fossil female more complete than Lucy, shedding valuable light on our early evolution1.

Ardi, the most complete Ardipithecus skeleton (left) and Lucy (right)
Ardi, the most complete Ardipithecus skeleton (left) and Lucy (right)

That is, assuming Ardi actually belongs to the human family. More than a decade ago researchers claimed she doesn’t have enough “human” features to place us in our lineage2. Of course, science has progressed since then, and several more Ardipithecus fossils have been found.

These have allowed researchers to re-evaluate the claims against Ardi. And it turns out the critics don’t have a branch to stand on, and Ardipithecus can happily return to their rightful place in human evolution3.

Ardipithecus unravelled

Evolution comes with a bit of a problem: it means things change, and humans are no exception. Many of the traits that make us recognisable, like big brains and chins, evolved quite recently. Obviously, this makes spotting an early human relative very hard, given they’ll lack those key traits.

As such, any time someone thinks they’ve found an early member of our family it sparks a whole lot of debate4. Ardi was no exception.

A possible early hominin (left) and chimp (right). How to tell them apart?

Some skeptics pointed to specific anatomical traits that linked Ardi with other apes. For instance, both her and chimps have relatively thin tooth enamel, whilst ours is nice and thick. This might sound minor, except for the fact that earlier purported members of our family also had thicker enamel, making Ardi the odd one out2.

Others were more cautious on principle, noting that there were countless ape species knocking about Africa at this time. Such a bushy family tree would make it incredibly hard to tell different branches apart, particularly given the scanty fossil evidence we have5.

Early ape evolution features a lot of question marks, so telling lineages apart is difficult5.

Nevertheless, the researchers behind these discoveries made a compelling case Ardi was one of us. As such, these skeptics never gained much traction3 (outside of creationist quote-mining seeking to undermine evolution).

Yet their concerns couldn’t be ruled out either.

Recalculating Ardipithecus

So, back in 2004 Strait and Grine tried to come up with a method for reliably spotting early hominins6.

Building on earlier work, they developed a big list of traits that help tell living apes apart (including us). Thus, by using this to compare the congruence between fossils and living species, maybe we can finally figure out where bones belong6.

When they ran this for Ardipithecus their results did indicate they were part of our family. However, the lack of fossils meant Ardi only preserved 26% of their trait list. Plus, some of their tests grouped Ardi with other apes6.

Ardi barred from the reunion falling outside the human family (green box)6.

As such, there was some ambiguity left about where Ardipithecus belonged. Strait and Grine provided good evidence Ardipithecus belonged to our family, but not quite good enough to silence the critics completely. Doubters persisted for years after this study was published5.

But since then more Ardipithecus fossils have been found6. So Mongle treamed up with Strait and Grine to re-run these tests using this new and exciting data. Thanks to it, they were able to examine 78% of their ape-indicating traits in this species3.

Their results clearly confirm Ardi belongs in the family after all.

Ardi’s denouement. Again, green box is the human family3.

The significance of Ardipithecus

Ardi’s back in the family. So what? Well, not only is this species back in with the cool kids, it’s back in close to the base of the family tree. As such, they provide vital insight into the early phases of human evolution.

And how it’s nothing like we expected.

Whilst the classic “march of progress” has us rising up from a knuckle-walking ancestor, Ardi paints the opposite picture. She didn’t walk on four legs. Instead she was a biped, but was using it to get around in the trees. Thus, our lineage wasn’t “rising up” from a four-legged animal but dropping down from a two-legged climber.

A more accurate view of human evolution, based partly on Ardi

Also, compared to apes their sexual dimorphism is reduced. This hints at early changes to social systems in our lineage, and perhaps an early emergence of monogamy.

And of course, Ardi really annoys creationists.

Altogether this is a vital piece of the puzzle that we can now be a bit more certain about where it slots in.

References

  1. White, T.D., Asfaw, B., Beyene, Y., Haile-Selassie, Y., Lovejoy, C.O., Suwa, G. and WoldeGabriel, G., 2009. Ardipithecus ramidus and the paleobiology of early hominids. Science326(5949), pp.64-86.
  2. Senut, B., Pickford, M., Gommery, D., Mein, P., Cheboi, K. and Coppens, Y., 2001. First hominid from the Miocene (Lukeino formation, Kenya). Comptes Rendus de l’Académie des Sciences-Series IIA-Earth and Planetary Science332(2), pp.137-144.
  3. Mongle, C.S., Strait, D.S. and Grine, F.E., 2019. Expanded character sampling underscores phylogenetic stability of Ardipithecus ramidus as a basal hominin. Journal of Human Evolution131, pp.28-39.
  4. Wolpoff, M.H., Senut, B., Pickford, M. and Hawks, J., 2002. Palaeoanthropology (communication arising): Sahelanthropus or’Sahelpithecus’?. Nature419(6907), p.581.
  5. Harrison, T., 2010. Apes among the tangled branches of human origins. Science327(5965), pp.532-534.
  6. Strait, D.S. and Grine, F.E., 2004. Inferring hominoid and early hominid phylogeny using craniodental characters: the role of fossil taxa. Journal of human evolution47(6), pp.399-452.
  7. Simpson, S.W., Levin, N.E., Quade, J., Rogers, M.J. and Semaw, S., 2019. Ardipithecus ramidus postcrania from the Gona Project area, Afar Regional State, Ethiopia. Journal of Human Evolution129, pp.1-45.

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