Our species is just one branch of a withering part of the evolutionary tree, the great apes. Along with the handful of species of chimpanzees, gorillas, and orangutans, we are all that is left of the hominids, and considering the threats our close relatives face we could very soon be the only great apes left. It has not always been this way. During the swath of prehistory ~23-5 million years ago known as the Miocene a variety of ape species inhabited forests through much of Africa, Europe, and Asia, and a new study published in the Proceedings of the Royal Society B looks at what the teeth of three fossil apes from Europe might tell us about their diets.
Around 11.9-11.8 million years ago, right in the middle of the Miocene, there were at least three different apes living in the forests of what is now northeastern Spain. Dryopithecus, Pierolapithecus, and the recently-discovered Anoiapithecus have all be recovered from the same deposits, and they are especially interesting as they are relatively closely related to living great apes. Although the evolutionary relationships of Dryopithecus have been difficult to tease out, as a whole these apes represent a radiation of types which would have been close to the last common ancestor of living great apes, and the their discovery has allowed scientists to investigate details of our own evolution before the origin of the first human between 5 and 7 million years ago.
The trait the authors of the present study decided to look at was tooth enamel thickness. This characteristic has often been discussed in reference to shifts in diet, and the scientists wanted to see how the three fossil apes compared to living great apes. To do so, however, they needed to look inside teeth from each genus, something some researchers have been reluctant to do since it has typically involved breaking open the precious few fossil ape teeth known. Rather than break out the hammers, though, the scientists behind the new study used industrial computed tomography to create images of each tooth's internal anatomy without damaging it, and what they found was that at least two of the ancient apes had heavier dental equipment than their living relatives.
According to the results of the study, the enamel of both Anoiapithecus and Pierolapithecus was considerably thicker than that of any living ape. Instead the thickness of their enamel was similar to that seen in most other fossil apes, including most humans, but Dryopithecus was different. The enamel on its teeth was much thinner than in either of its contemporaries, and in this respect its teeth were most similar to that of living chimpanzees and gorillas (orangutans have slightly thicker enamel).
What all of this means is that the fossil apes with thick enamel were probably eating different foods than Dryopithecus. From studies of living apes, it seems that thin enamel is related to a diet of relatively soft foods like fruit and leaves, especially since teeth with thin enamel make it easy for their owners to maintain shearing crests on their teeth useful for processing soft foods. Species with thick enamel, by contrast, have teeth better suited to crushing less desirable foods such as unripe fruits and nuts, and the authors state that thick enamel may prolong the life of a tooth if its own is regularly eating tough foods (which is especially important since mammals only get one set of adult teeth). Hence, on the basis of the enamel thicknesses, Anoiapithecus and Pierolapithecus were probably eating a variety of tougher foods while Dryopithecus may have maintained itself on softer fare. When ripe fruit was lacking, Anoiapithecus and Pierolapithecus probably ate nuts or other hard foods while Dryopithecus, like living chimpanzees, may have switched over to leaves, and so the differences on enamel thickness might signal different preferences in "fallback foods" which the apes relied on when their preferred food was not available.
If this hypothesis is correct it may have important implications for figuring out the evolution of apes. If Anoiapithecus and Pierolapithecus were closely related to the last common ancestor of all great apes then they may indicate that thick-enameled teeth was the ancestral condition. This might possibly explain how apes could have spread out from Africa; their thick teeth would have allowed them to survive on harder foods found outside of a forest setting, and once they had spread into distant forests some forms (such as Dryopithecus) became adapted to have thin-enameled teeth. As the authors note, further research will be required to better test this idea, but it may be that thickly enameled teeth allowed apes to travel far and wide in the lush Miocene world.
Alba, D., Fortuny, J., & Moya-Sola, S. (2010). Enamel thickness in the Middle Miocene great apes Anoiapithecus, Pierolapithecus and Dryopithecus Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.0218
(When I first saw the picture of the skulls, I thought one of them was Sahelanthropus. HA!)
Nicely done, Brian. I've often thought that linking fossil hominins to modern humans on the basis of dental enamel thickness alone wasn't enough, because teeth are so responsive to diet. Now that more detailed studies are being done on teeth and their plasticity, I think we'll have a much better understanding of what the evolutionary and dietary forces were that were driving hominin (and hominoid) morphology.
I'd be careful about imputing dietary preferences based solely on tooth morphology. Take Didelphis for instance. If all we knew about it was its teeth, could we infer its fondness for ripe persimmons? My guess is that all three of these extinct ape species were opportunistic omnivores whose diets overlapped quite a bit. Perhaps SEM studies of microscopic scratches on enamel surfaces could provide more information on diet.
I'm puzzled about the thin enamel= fruit and leaves, thicker enamel= nuts etc. What about thick enamel for grinding tough meat or fibrous grasses and leaves (like cows have big thick grinders that can be scraped across by jaw motion to break the cellulose before it gets to the first stomach etc)? I thought thin enamel would be useless for fruits, as the fruit acid burns it away very quickly (I'm a thin enameled human). I'm not sure what eating nuts a lot would require- they really only need crushing and are not acidic- would they wear down enamel swiftly or not...it's a mystery... bring on the experimentalists! What sort of teeth did humans need for chomping on un-milled grains before discovering it was more versatile ground up and cooked? Hmm.