Two million years ago was a crucial time for our family. Several different lineages began to emerge, including the one that ultimately became us. But were these different branches in conflict, or did they co-exist peacefully? Their diet could hold the answer.
Except our understanding of our ancestors' diet just got a whole lot more complicated.
Australopithecus sediba - one of the species living at this crucial time - was thought to have a relatively unique diet featuring a broad range of foodstuffs, including bark. As such, they would not have been competing with the other lineages; raising the possibility that all these branches of humanity co-existed
However, a new study suggests that they may have had a more narrow diet that didn't feature bark. But is there any meat to this claim (hahaha food joke).
Food of the ancients
Paranthropus is the granddaddy of odd diets in the human family; and they also lived during this turbulent period of human evolution. The first members of this group discovered were nicknamed "nutcracker man" because their jaw was so odd. Nowadays we know they weren't actually routine nutcrackers. Instead, they ate a lot of hard, brittle, and abrasive foods.
Homo erectus was the first member of our family to be "modern". They lived alongside all these other species, so where do they fit in? Well, it seems like they weren't competing with Paranthropus. Their teeth appear to indicate a focus on softer foods than that other group, possibly including. As such, they could have happily lived side by side with these other families as they weren't in direct competition with one another.
Homo naledi also lived alongside these other species (assuming early estimates of its age are right). And much like we don't know too much about its age, we don't know that much about its diet. Yet. Plenty of teeth and jawbones have been found. Information about these - and other bits of their anatomy - is trickling out as its analysed, but there's not yet any real word on their diet.
And finally we get to Australopithecus sediba, the controversial one. Previous research indicated that they had a varied diet, like Homo erectusi, but that included hard food, like Paranthropus. However, this diet focused on a different group of plants (known as C4 plants) to Paranthropus. Thus, the two species could have lived side by side without fighting each other for resources. Which is good, because they literally lived alongside each other. Contemporary fossils of Au. sediba and Paranthropus have been found only 15 km apart.
Australopithecus sediba: on a different diet?
So, Austrlaopithecus sediba occupied their own special niche; allowing them to live alongside other species happily. Or did they?
This is what the new research claims. The authors used engineering techniques to create a model of just how well Australopithecus sediba 's skull could deal with the pressure of biting hard foods. They found that whilst the bones themselves were up to the challenge, the muscles were in a rather shoddy position for it.
The joints between the skull and jaw form a triangle with the location of the bite. If the resulting forces fall within this triangle, everything is dandy. However, if they don't then this produces sideways forces that could dislocate the jaw. To avoid this, Australopithecus sediba would had to have chewed rather gingerly. Not quite the adaptation you want for eating hard food.
Based on this, the authors conclude that Au. sediba wasn't eating hard food regularly. If it was then evolutionary forces would have begun to rectify this failing. But they didn't so they weren't.
All the hominins were kung fu fighting
So, we're faced with two competing claims. How do we know which one to believe? Why not both?
It turns out that these two claims aren't actually in conflict ( sorry daily mail). This new analysis doesn't rule out the possibility that Australopithecus sediba consumed hard food. They may well still have. In fact, we know they definitely did because we've found bits of hard food in their calculus (the gunk on your teeth formed from food, not the difficult branch of maths).
Instead, the claim is simply that these hard foods weren't a vital part of their diet. Hence the apparent lack of adaptations for eating them. These foods may have been rarely consumed, perhaps only when Au. sediba was desperate. This isn't inconsistent with previous evidence, which can't really identify how frequently these sorts of food were eaten.
And yet we know they did eat this hard food. Could this mean they were actually desperate? Perhaps they were in competition with species like Homo erectus and being driven to extremes by the fact they were loosing this fight. They had to eat these tough foods that they weren't very well adapted for because they'd been driven away from their preferred foodstuffs by the big bad human ancestors. Their kung fu was just not strong enough
Alternatively, this new paper overestimates how much jaw power is needed to eat these "tough" foods. Capuchin monkeys also have a tough diet that features bark and nuts, yet their bite is even weaker than Australopithecus sediba's. It's a rather boring answer by comparison, but can't be ruled out yet. If we could find more Au. sediba fossils perhaps we could identify if the use of bark was a rare thing, indicating potential conflict, or the authors are just exaggerating how much bite is needed for bark. Maybe these fossils have been found already.
tl;dr
Australopithecus sediba wasn't as adapted to eating hard foods as we thought. Thus the fact they did could indicate they were being pushed to extremes by competition with other species of human.
References
Berger, L.R., Hawks, J., de Ruiter, D.J., Churchill, S.E., Schmid, P., Delezene, L.K., Kivell, T.L., Garvin, H.M., Williams, S.A., DeSilva, J.M. and Skinner, M.M., 2015. Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa. Elife, 4, p.e09560.
Chalk, J., 2011. The effects of feeding strategies and food mechanics on the ontogeny of masticatory function in the Cebus libidinosus cranium (Doctoral dissertation, The George Washington University).
Henry, A.G., Ungar, P.S., Passey, B.H., Sponheimer, M., Rossouw, L., Bamford, M., Sandberg, P., de Ruiter, D.J. and Berger, L., 2012. The diet of Australopithecus sediba. Nature, 487(7405), pp.90-93.
Ledogar, J.A., Smith, A.L., Benazzi, S., Weber, G.W., Spencer, M.A., Carlson, K.B., McNulty, K.P., Dechow, P.C., Grosse, I.R., Ross, C.F. and Richmond, B.G., 2016. Mechanical evidence that Australopithecus sediba was limited in its ability to eat hard foods. Nature Communications, 7.
Scott, R.S., Ungar, P.S., Bergstrom, T.S., Brown, C.A., Grine, F.E., Teaford, M.F. and Walker, A., 2005. Dental microwear texture analysis shows within-species diet variability in fossil hominins. Nature, 436(7051), pp.693-695.
