Stressed Lemurs and Grass-Eating Humans

In his 1960 presidential address to the South African Archaeological Society, the anthropologist Louis Leakey cast the fossil humans that had been found in that country as little more than a collection of evolutionary dead-ends. Leakey didn’t put it quite like that – that would have been rude – but he did utilize the platform to push the southern fossil humans away from our ancestry and underscore the importance of a discovery made the year before by his wife Mary at Olduvai Gorge. The wide-cheeked, deep-jawed skull she had found there went by a number of names – Titanohomo mirabilis, Zinjanthropus boisei, “Dear Boy”, and “Nutcracker Man” – but, regardless of what you called it, Louis was nearly certain that the fragmented skull represented the early glimmerings of our own genus in prehistoric eastern Africa. The older fossil australopithecines from South Africa, in Leakey’s view, represented an early experiment in human evolution that did not pan out, and were only related to Louis’ beloved Zinj by a deeper common ancestry.

But Leakey was wrong. The features he claimed as being proof of a connection between Zinjanthropus and our species – such as “the form of the brow ridge, the shape of the external orbital angles, the development and position of the nasal spines”, and so on – were not as distinctive as he proposed. The Olduvai skull was actually little different from similar fossils given the name Paranthropus by the Scottish paleontologist Robert Broom in 1938, and what had truly swayed Leakey was the discovery of stone tools at Olduvai. Since the Zinjanthropus skull had been found near stone tools, and tool creation was a hallmark of our genus, then clearly the Dear Boy had to be one of our ancestors – at least until further searches of Olduvai by the Leakey family turned up the remains of what Louis would cast as the true toolmaker, Homo habilis. Within five years, the creature we now know as Paranthropus robustus went from human ancestor to part of an evolutionary sideshow.

The three known species of Paranthropus were our evolutionary cousins. In addition to Zinj – now called Paranthropus boisei and thought to have lived between 2.6 – 1.2 million years ago in eastern Africa – there was the 2.5 million year old Paranthropus aethiopicus from Kenya and the 2 – 1.2 million year old Paranthropus robustus from South Africa. (The spacing and timing of these species hint that there are other species yet to be found.) Together, these three were part of a separate lineage of humans which split from our own side of the family over 2.5 million years ago. Their thick cheeks, large teeth, and deep jaws are their claims to fame, and have led scientists to refer to them as the “robust australopithecines.”

Why these humans had such formidable-looking teeth and jaws has been a matter of persistent discussion. The early nickname “Nutcracker Man” undoubtedly framed the debate. The Paranthropus species certainly looked as if their skulls were suited to pulverizing and cracking open seeds and nuts, but looks can be deceiving. What a creature was capable of and what it actually did are two different things. Two new papers bear on the ongoing discussion about why Paranthropus became adapted in such a different way from our closer human relatives.

The more recent paper, published by an international team of scientists led by Thure Cerling of the University of Utah, indicates that a more apt nickname for the robust Olduvai skull might have been “Lawnmower Man.” The secret was the chemical makeup of Paranthropus boisei teeth. As the old saying goes, “You are what you eat”, and the proof is in chemical isotopes locked within teeth and bone.

Like other mammals, prehistoric humans only got two sets of teeth to last them a lifetime. They had an early set of milk teeth, later followed by the full set of adult teeth. Now here’s where the isotopes come in. As the adult teeth were forming in the jaw, the diet and environment of the individual influenced the characteristics of the oxygen and carbon isotopes that became incorporated in the teeth – an aquatic animal would have very different oxygen isotopes from a land-dwelling one, for example, and teeth from a grazing animal would bear different carbon isotope signatures from one that fed on fruits and leaves. This technique has been used to investigate when legged whales began to paddle about in the ocean, what prehistoric horses ate, and, in this case, whether or not Paranthropus was cracking seeds and nuts.

Keeping track of species is important here. While Paranthropus robustus from South Africa have been reconstructed as dining on hard objects thanks to studies of tooth wear, similar investigations failed to find evidence of hard-object feeding on the teeth of Paranthropus boisei from eastern Africa.  What goes for one Paranthropus species might not hold for all. In the case of the new study by Cerling and colleagues, P. boisei was the focus, and the results of the research are consistent with the idea that the eastern species was doing something different from its southern relative.

What the research boils down to is a detectable difference between plant food sources. The carbon isotope values of trees, shrubs, and herbs – categorized as C3 plants – predictably vary from those in C4 plants such as grasses and sedges. Within the sample of twenty two P. boisei individuals recovered from strata in Kenya spanning 1.9 to 1.4 million years ago, the teeth contain carbon isotope values consistent with a diet mostly made up of grasses. (Specifically, C4 plants were said to make up 77% of the diet on average, ranging from 61-91% across the sample.) In terms of diet, the east African Paranthropus were closer in diet to the horses that grazed in the same habitat than to the other Paranthropus species in South Africa! The only other species of primate with a similar diet was extinct baboon named Theropithecus oswaldi (a prehistoric cousin of today’s grass-eating gelada of Ethiopia).

Whereas the southern species of Paranthropus retained a more general diet – incorporating fruits and various C3 plant foods – the species from eastern Africa apparently specialized on grasses for a span of half a million years. What this discovery means for the evolution of these creatures is unclear. On the basis of minute wear patterns on teeth, paleoanthropologists have proposed that earlier australopithecines like Australopithecus anamensis and Australopithecus afarensis (“Lucy” and her kin) had diets similar to that of P. boisei. It had been assumed that the connection was a diet of hard foods, but this new study would seem to indicate that what they shared in common was a reliance on grasses or similar C4 plants. The assumption that early humans had diets heavy on fruits and leaves, like modern apes, might be in error. “[T]his study suggests that the prevailing ideas [about early human diets] based on morphological and biomechanical considerations are at least partly in error,” Cerling and colleagues concluded, “and that our understanding of the dietary basis of masticatory differentiation within the hominin lineage may require revision.”

Virtual skulls of Archaeolemur (a,b) and Hadropithecus (c,d) showing stress on the skull created during a reconstructed bite at the second molar position. The virtual Hadropithecus skull suffers higher levels of stress - indicated by the different colors - than Archaeolemur. From Dumont et al., 2011.

Support for a revised view of the Paranthropus diet has also come from an indirect source. Though the robust australopithecines were a unique part of our close family, their skull shape is not entirely unique among primates. Hadropithecus – a huge lemur that lived on the island of Madagascar until the arrival of humans around 2,000 years ago – also possessed a short skull and a deep jaw set with large teeth. Like the traditional image of Paranthropus, therefore, it was thought that Hadropithecus had a skull adapted to a diet of seeds and other hard foods, but a study published last month by scientists Elizabeth Dumont, Timothy Ryan, and Laurie Godfrey suggests otherwise.

The paradox of Hadropithecus, Dumont and co-authors point out, is that teeth of the lemur show pitting and other signs consistent with the idea that it was chewing on hard foods while the enamel of the primate’s teeth is thin and would have been susceptible to fracturing. The wear patterns did not match the anatomy. In the closely-related and also-extinct species Archaeolemur edwardsi, however, similar wear-patterns were seen on thickly-enameled teeth set in a longer jaw – Archaeolemur better fit the profile of a hard-object feeder. Chemical isotopes supported the split between the two lemurs. Archaeolemur was primarily feeding on parts of C3 plants, whereas Hadropithecus went after C4 plants that would have lacked large, hard seeds and nuts. The short and robust skull shape of Hadropithecus had led paleontologists astray.

In order to test the idea that the two lemurs were different kinds of herbivores, the scientists created virtual models of Hadropithecus and Archaeolemur skulls to test their biting abilities. The results were in accord with what paleontologists had begun to suspect on the basis on skull anatomy. Archaeolemur was able to open its jaws wider to accommodate large, difficult-to-crack foods, and its skull was better able to cope with the stresses required to bust into seeds and nuts. By comparison, the skull of Hadropithecus was that of a primate which efficiently processed large quantities of plants like grass or leaves, and the pitting seen on the lemur’s teeth might have been caused by dirt and grit that clung to its preferred food. Hadropithecus may have dined like a savanna baboon – foraging for bulbs and corms of grasses during times when more nutritious foods were scarce – or, alternatively, may have specialized on the succulent, desert-adapted plants similar to the Madagascar ocotillo. In either case, however, the plant foods were low-quality and low-energy. Hadropithecus would have had to consume large amounts of corms, bulbs, or leaves to survive, and the peculiar construction of its skull indicates that this lemur likely chewed through copious amounts of these foods rather than specializing on hard objects.

The study of Hadropithecus preceded the paper on Paranthropus boisei by a month, but it is remarkable that both derived similar conclusions about two distantly-related, short-faced primates. Contrary to their superficial resemblance to toy nutcrackers, these creatures chewed on bushels of grass and other nutrient-poor food. A short skull and big teeth cannot automatically be taken as an indication that a fossil primate preferred tough-skinned fruits, tubers, and seeds. Even the pits and scratches left on teeth may be ambiguous, and, if the eastern and southern Paranthropus species are any indication, the same anatomical tools can be put to different uses. Though always enigmatic, the variable diets of the robust australopithecines and Madagascar’s recent lemurs make these fossil primates stranger still, and paleontologists have yet to crack the mystery of how such peculiar features evolved in the first place.

Top Image: The skull of the “Dear Boy”, Paranthropus boisei. Image from  Ungar PS, Grine FE, Teaford MF, 2008 Dental Microwear and Diet of the Plio-Pleistocene Hominin Paranthropus boisei. PLoS ONE 3(4): e2044. doi:10.1371/journal.pone.0002044

References:

Cerling, T., Mbua, E., Kirera, F., Manthi, F., Grine, F., Leakey, M., Sponheimer, M., & Uno, K. (2011). Diet of Paranthropus boisei in the early Pleistocene of East Africa Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1104627108

Dumont, E., Ryan, T., & Godfrey, L. (2011). The Hadropithecus conundrum reconsidered, with implications for interpreting diet in fossil hominins Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.0528

Leakey, L. (1961). Africa’s Contribution to the Evolution of Man The South African Archaeological Bulletin, 16 (61) DOI: 10.2307/3887411

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