Archaic Hominin Gender Dimorphism Or Multiple Archaic Species?

John Hawks points out a potential alternative interpretation of data from a recent fossil footprint study

eLife has published a paper by Fidelis Masao and colleagues describing new footprint trails from the famous site of Laetoli, Tanzania: “New footprints from Laetoli (Tanzania) provide evidence for marked body size variation in early hominins”.

The scientific theme of the paper is about body size and dimorphism. The species presumed to have made all the trackways is Australopithecus afarensis, the only species that has so far been reported from fossil remains at Laetoli, although the tracks at 3.66 million years old are a bit more ancient than any of the fossils. This is the same species as the Lucy skeleton, which was found at Hadar, Ethiopia, and the “First Family” series of fossils from Hadar in the locality known as A.L. 333. In 2010, Yohannes Haile-Selassie and colleagues reported a partial skeleton from Woranso-Mille, Ethiopia, some 3.6 million years old, which also seems to represent a large male individual, that stood just under 160 cm tall. Based on a regression of foot size to stature, the new footprint trail in test pit L8 represents an individual that probably stood around 165 cm, with 10 cm or so error either way. 

Here’s a neat graphic showing stature estimates for early hominins up through early H. erectus:

Figure 12 from Masao et al. 2016. Original caption: "Estimates of predicted stature of fossil hominin individuals by species over time for the interval 4–1 Ma. Solid symbols (or crosses in bold) refer to stature estimates based on actual femur length; open symbols refer to stature estimates based on estimated femur length, in turn based on femur head diameter. For Laetoli and Ileret, stature estimates are based on footprint length (see Materials and methods). For Laetoli, Ileret and Woranso-Mille, the average value and range of predicted stature are shown. Colours are associated to the geographical location of each fossil/ footprint site on the map." 

That’s a bit complicated but the point is pretty clear. A. afarensis overlaps with H. erectus substantially in stature. If we consider only the tiny Lucy skeleton (the lowest “x” in the figure at less than 110 cm), we get a misleading view of body size in this early hominin species. But at the same time, Lucy and some other specimens of A. afarensis really are quite a lot smaller than any H. erectus specimens. The conclusion made by Masao and colleagues, applying some statistics, is that A. afarensis was more variable and sexually dimorphic than humans and H. erectus. 

This idea of higher dimorphism in early hominins has been the subject of pointed debate over the past fifteen years, a debate that has been driven by insufficient fossil data. . . .  

I think we should also be skeptical about whether these footprints were really produced by A. afarensis. That species already has problems at Hadar and at Woranso-Mille, where some researchers now recognize multiple species are present. At Laetoli, we should probably apply a level of skepticism to the idea that only one fossil species could be present.

Hawks then goes on to explain how the standards for documenting fossil finds have grown much more rigorous over the last forty years as it has become practical to do better documentation.