Sexual dimorphism in Lucy’s species as extreme as in gorillas

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It may not always feel like it, but males and females of our species are incredibly similar. Homo sapiens has the lowest level of sexual dimorphism (aka the anatomical differences between the sexes) in the great apes, and one of the lowest in primates overall. Sure there are still differences, but they’re nowhere near as extreme as you find in, say, a gorilla¹.

Meanwhile, chimps are also on the lower end of the sexual dimorphism spectrum (although still a bit more variable than us)¹. Which raises the question: what were our ancestors like? Surely it can’t have been that extreme, given it’s low in both us and our chimpanzee cousins.

However, a growing body of evidence suggests this is not the case. It turns out early human relatives, like Australopithecus afarensis (Lucy’s species), may have had as much sexual dimorphism as gorillas! And that our reduced differences might be a more recent development².

Taking steps towards sexual dimorphism

It turns out that investigating this problem is harder than it seems. Whilst we have plenty of fossils to work with, you have to remember Australopithecus afarensis lived for hundreds of thousands of years. As such, we can’t be sure any differences between them aren’t just due to change over time^1,3.

For instance, Kadanuumuu is an Au. afarensis maybe 2 feet taller than Lucy. Whilst it could be sexual dimorphism, they are separated by 300,000 years⁴.

So researchers Villmoare, Hatala, and Jungers began studying footprints as an alternate way to investigate sexual dimorphism. These form over a few days at most, so provide a crucial snapshot into prehistoric groups.

And fortunately, Australopithecus afarensis left us with the famous Laetoli tracks. These footprints track three individuals marching through volcanic tuff, 3.7 million years ago. Villmoare et al. were able to compare this to a range of other foot data, including the US army and gorillas².

<insert joke about how they’re totes the same here>.

Given the Laetoli sample is relatively small, particularly given what they were comparing it to, these researchers used bootstrapping for their tests. This powerful machine learning technique breaks up the samples into comparable sub-groups for repeated analysis².

All of which revealed the Laetoli footprints contained massive variation, on par with gorillas more than modern humans. This confirms there were big differences in body size between male and females in Lucy’s species².

Fossils and footprints

The discovery of big sex differences in body size may seem shocking, given the low levels of dimorphism in both us and our closest living relatives. However, there has been some evidence pointing in this direction for a while.

Obviously, the most dramatic example is the aforementioned Kadanuumuu, whose name literally means “big man”. However, they weren’t the only fossil that shows Lucy’s species contained a lot of variation. For instance, the catchily named “NFR-VP-1/29” is an Au. afarensis jaw chunkier and more robust than any other found⁶. Could this belong to another “big man”?

Crucially, statistical analyses reveal these are legitimate trends within the species^1,7. Not just me looking at a random assortment of bones and going “I guess they look kind of different”.

However, it turns out this dramatic sexual dimorphism isn’t body-wide. Most notably, their canines show much less variation than chimps and gorillas. The males of other apes use their big knashers (and body size) to fight other males for dominance. Perhaps the fact that Lucy’s species lacked this weaponry means they had a different social system that didn’t revolve around “alpha males”¹.

Contrary to what misogynists will tell you, we still lack that alpha-male social structure. Maybe these changes in Australopithecus marks the first steps towards our modern mating systems.

References

1. Plavcan, J.M., 2012. Sexual size dimorphism, canine dimorphism, and male-male competition in primates. Human Nature, 23(1), pp.45-67.
2. Villmoare, B., Hatala, K.G. and Jungers, W., 2019. Sexual dimorphism in Homo erectus inferred from 1.5 Ma footprints near Ileret, Kenya. Scientific reports, 9(1), p.7687.
3. Reno, P.L. and Lovejoy, C.O., 2015. From Lucy to Kadanuumuu: balanced analyses of Australopithecus afarensis assemblages confirm only moderate skeletal dimorphism. PeerJ, 3, p.e925.
4. Haile-Selassie, Y., Latimer, B.M., Alene, M., Deino, A.L., Gibert, L., Melillo, S.M., Saylor, B.Z., Scott, G.R. and Lovejoy, C.O., 2010. An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia. Proceedings of the National Academy of Sciences, 107(27), pp.12121-12126.
5. Behrensmeyer, A. K. (2010). Paleoenvironmental context of the Pliocene A.L. 333 “First Family” hominin locality, Hadar Formation, Ethiopia. Geological Society of America Special Papers, 446, 203–214.
6. Haile-Selassie, Y., Melillo, S.M., Ryan, T.M., Levin, N.E., Saylor, B.Z., Deino, A., Mundil, R., Scott, G., Alene, M. and Gibert, L., 2016. Dentognathic remains of Australopithecus afarensis from Nefuraytu (Woranso-Mille, Ethiopia): Comparative description, geology, and paleoecological context. Journal of Human Evolution, 100, pp.35-53.
7. Gordon, A.D., Green, D.J. and Richmond, B.G., 2008. Strong postcranial size dimorphism in Australopithecus afarensis: results from two new resampling methods for multivariate data sets with missing data. American Journal of Physical Anthropology: The Official Publication of the American Association of Physical Anthropologists, 135(3), pp.311-328.