Author: zcofran

Guest Post: Jerry and Julie on their latest paper

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“Good news, everyone!” to quote Prof. Farnsworth. Our good friends Jerry DeSilva and Julie Lesnik just published a paper in the Journal of Human Evolution, about neonatal brain size in primates [1]. Rather than talk and talk about it, probably missing the important stuff, I made some calls. The authors were kind enough to make a cameo appearance at Lawn Chair to talk about their paper about their paper. Thanks, Jerry and Julie! Here’s what they had to say:

Summary:

This paper presents a regression equation that can be used to calculate the size of the brain at birth in different hominin species.

Significance:

Knowing the size of the brain at birth is critical for understanding obstetric constraints and brain development throughout human evolution. Unfortunately, it is very unlikely to find fossil evidence of how big the brain was at birth in human ancestors (though see below). This paper presents a way to get around the absence of fossil evidence and calculate the size of the neonatal brain in early homs using what we know about brain development in modern primates.

Things Jerry liked about the paper:

Humans are so unusual, and in biological anthropology we often study ways in which humans are different from other primates. However, what this study finds is that humans are no different from other primates in terms of the adult/neonatal brain scaling relationship. This means that we have exactly the brain size at birth expected given the size of our brains as adults. Because of this, we can infer that our extinct ancestors and relatives also followed this ‘rule’ of adult/neonatal brain size, and can calculate the size of the brain at birth from reliable estimates of brain size in 89 adult fossil crania that have been unearthed.

I am also thrilled that Julie and I may have solved the “% brain size at birth” issue that has been all over the literature lately. Did Homo erectus have a more human-like or a more chimpanzee-like pattern of brain growth? What about australopiths? Well, we’ve found that the whole issue of % brain size at birth is simply a function of the scaling relationship between adult and neonatal brain size. Because they do not scale 1:1, but instead scale 1:0.73 (roughly), as the adult brain gets bigger, the neonatal brain gets proportionately smaller. Therefore, less of brain growth occurs in the womb as overall adult brain size increases. If you know the size of a hominin brain as an adult (which we do from the many preserved fossil crania), you can calculate the size of the brain as a baby, and then easily take a % of how much of that brain growth is achieved by birth.

Again, because of the negative allometry (m=0.73), we argue that % of brain size at birth in hominins was never “chimpanzee-like” or “human-like”, but instead followed a gradual progression from a chimpanzee-like ancestral condition to what we have today.

Things Julie liked about the paper:

So much is going on when we think about hominid evolution, especially in the early Pleistocene. With the emergence of Homo brain size is increasing, bipedality is becoming more efficient, and tool use is becoming more advanced. What I like about this paper is that understanding neonatal brain size is one way of tying all of those elements together. Humans are considered to be secondarily altricial meaning that they are born in a more underdeveloped state than their ancestors. Selection for this smaller neonatal size is often considered to be linked to the constraints placed on the pelvis by selection for more efficient bipedal locomotion. A small brain size at birth and a large adult brain always seemed exceptional for Homo. What our paper shows is that the relationship is entirely normal across anthropoids. So, where is the selective pressure? On the larger brain as an adult or on the smaller brain as a newborn? I am now more apt to lean towards larger adult brain. Efficient bipedality is important for exactly that reason; it’s efficient and therefore requires less energy to walk upright and allows the body to allot that energy to other tasks, such as maintenance of a large brain. Add tool-use advancement to the equation and it seems bigger brains and more advanced cognitive abilities were of primary importance at this stage of human evolution.

What we’d do different:

I would have included Neandertals. Julie and I made a statement in the introduction that the discovery of neonatal crania was bordering on impossible. Just days before our paper appeared on-line, however, Marcia Ponce de Leon published a fantastic paper in PNAS on a neonatal Neandertal cranium from Mezmaiskaya Cave in Russia [2]. What is very exciting to me is that this newly described fossil allows us to test our regression equation. How accurate is it in predicting the size of the brain at birth in Neandertals (which we now know because of this new specimen)? Our regression would predict a brain size of about 425 cc, which is very close to the size of the brain at birth in the Mezmaiskaya infant and well within the 95% CI. When two independent methods arrive at the same result, it is reasonable to argue that the method is valid.

Referenecs

1. DeSilva J, and Lesnik J. 2008. Brain size at birth throughout human evolution: a new method for estimating neonatal brain size in hominins. Journal of Human Evolution, corrected proof in press.

2. Ponce de Leon M, Golovanova L, Doronichev V, Ramanova G, Akazawa T, Kondo O, Ishima H, and Zollikofer C. 2008. Neanderthal brain size at birth provides insights into the evolution of human life history. Proceedings of the National Academy of Sciences 105: 13764-13768

New twist from teeth

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Peter Ungar, Fred Grine and Mark Teaford recently reported in PLoS ONE on their results of studying the microwear on Australopithecus boisei molars. Their study showed that the microwear differs from that of A. robustus, arguably boisei‘s South African counterpart, and from A. africanus. Here’s the abstract:

The Plio-Pleistocene hominin Paranthropus boisei had enormous, flat, thickly enameled cheek teeth, a robust cranium and mandible, and inferred massive, powerful chewing muscles. This specialized morphology, which earned P. boisei the nickname “Nutcracker Man”, suggests that this hominin could have consumed very mechanically challenging foods. It has been recently argued, however, that specialized hominin morphology may indicate adaptations for the consumption of occasional fallback foods rather than preferred resources. Dental microwear offers a potential means by which to test this hypothesis in that it reflects actual use rather than genetic adaptation. High microwear surface texture complexity and anisotropy in extant primates can be associated with the consumption of exceptionally hard and tough foods respectively. Here we present the first quantitative analysis of dental microwear for P. boisei. Seven specimens examined preserved unobscured antemortem molar microwear. These all show relatively low complexity and anisotropy values. This suggests that none of the individuals consumed especially hard or tough foods in the days before they died. The apparent discrepancy between microwear and functional anatomy is consistent with the idea that P. boisei presents a hominin example of Liem’s Paradox, wherein a highly derived morphology need not reflect a specialized diet.

Note that they refer to boisei and robustus as “Paranthropus,” whereas I (and others) refer to them as Australopithecus. A. boisei and robustus are two “robust” australopithecines, described as such because their skulls and teeth suggest these guys were adapted for prolonged, powerful bouts of mastication (it means chewing, get your mind out of the gutter). Some people argue that these two taxa form a monophyletic group; that is, they share a last common ancestor that is not shared by any other taxon. If this is the case, the generic distinction (Paranthropus) can be made, separating them from the other australopithecines. Though I tend to lump groups, I really think that these taxa do not form a monophyletic group, that they have different ancestors (that their superficially similar masticatory apparati were independently evolved), and that they should stay in the genus Australopithecus. Right now, this issue (wherein I am very interested) has yet to be resolved.
Anywho, what’s important here is that the two robust australopithecines differ in their microwear patterns, which suggests that the two subsisted on different diets. Similarly, Wood and Constantino (2007) report that the stable carbon isotope signal from boisei (yet unpublished, but communicated to them personally by Matt Sponheimer) is different from the A. robustus and africanus. Together, these two data indicate that the robust australopitheciens (not to speak about A. aethiopicus…) were quite different in their diets (and possibly lifestyles?). Interestingly, A. robustus‘s molar microwear and stable isotope signals are very similar to that of A. africanus, who was present in the same regions as robustus but a bit earlier in time. This bolsters the scenario in which A. robustus is evolved from A. africanus, or something like it. Could this suggest also that A. boisei is not descendant from A. africanus? Or, is it simply that there were different foods available in the Plio-Pleistocene of South and East Africa?
Another important note that the authors bring up is the fact that of the seven specimens examined, none appeared to have eaten tough or hard foods that might necessitate the use of their (we assume) powerful masticatory muscles. Now why the hell would they have such a derived face, jaws and teeth if they were not eating things that would have required such an apparatus? One proposed scenario about the “hyper-robust” masticatory apparatus of A. boisei is that it is an adaptation for only the toughest of times, when survival might have hinged upon the ability to process and ingest the lowest quality (and hardest to eat) foods. Ungar et al.’s data suggest that this may well be the case, that the powerful masticatory apparatus came in handy only very rarely, and so the dietary signal from microwear reflects what these critters usually ate (and preferred to eat).
If this is really the case, then it might suggest that the robust face of boisei was almost completely genetically acquired, that epigenetic factors did not contribute greatly to produce boisei‘s face. This could be important for teasing out criteria (i.e. skeletal, craniofacial traits) useful in phylogenetic reconstruction. For example, it could be that certain robust features of boisei‘s face indicate a shared genetic ancestry, whereas those of robustus were more epigenetic in nature, acquired over a lifetime of experiencing high chewing forces. Contrariwise, these traits might be the result of these two taxa’s shared ancestry.
Either way, this paper presents interesting new information about the most bizarre hominin evolutionary dead-end, the robust australopithecines.

References

Ungar PS, Grine FE, and Teaford MF. 2008. Dental Microwear and Diet of the Plio-Pleistocene Hominin Paranthropus boisei. PLoS ONE 3(4):e2044.

Wood B, and Constantino P. 2007. Paranthropus boisei: Fifty years of evidence and analysis. American Journal of Physical Anthropology 134(S45):106-132.