Posts Tagged 'brain size'

Humans, chimps, and brain size

Amélie Beaudet and colleagues have now published a new review on the evolution of the modern human brain. They introduce many traditional issues in paleoneurology, including frontal lobe evolution, asymmetries, lunate sulcus, brain growth, and brain shape. They also provide a detailed discussion of the information we have on the evolution of brain size. Studies on this topic are frequently biased by statistical or taxonomic problems, because of the intrinsic limitations of the fossil record. Actually, any model (gradual, random, punctuated, etc.) can be supported by the few and scattered data, generating disagreements and debates. Isaac Asimov said “Where any answer is possible, all answers are meaningless”. In this paper, they describe their own approach, trying to deal with such limitations. They support a gradualist perspective, although with some discontinuities within some clades. The review strictly deals with brain evolution, but I really appreciate the taxonomic considerations at the beginning of the article, defending their reasons to include humans and apes in one single family. I belong to the opposite faction, namely to the resisting supporters of two distinct families for this group, with the term hominids restricted to humans and (probably) australopiths. Firstly, because I think that taxonomy should not try to trace phylogeny too strictly, constrained and forced by cladistic schemes. The real phylogeny is unknown (we use genes as a proxy, which is but an estimation, with pros and cons), and the phylogenetic hypotheses are frequently changing. Instead, a taxonomy based on the whole biological model (that includes anatomy, physiology and so on) is more stable and, importantly, can add more information on the actual evolutionary, zoological and ecological organization and role of a group of species. Secondly, because I think that differences are the great value of evolution, and taxonomy should acknowledge such differences. In this case, we must admit that our lineage is particularly dissimilar from all the other apes. This does not mean that we are better, but surely much different, and taxonomy should take into account the importance of such outstanding changes. Many anthropologists give all these taxonomical issues for granted, using one label or another just by repeating or copy-pasting what they hear around, generally following a mainstream without a personal or competent opinion. But passively repeating statements is proper of dogmas and mantras, something that should be left out of science.  That’s why I really appreciate that, in this article, Amélie and her coauthors take a clear position, explaining their reasons.

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More on humans and chimps can be found in this other recent review on human paleoneurology. For Spanish readers, here a provocative dissemination article on humans and apes, and another one on the immense value of diversity.

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Naledi

Ralph Holloway and colleagues have just published a paleoneurological study of Homo naledi. They used seven cranial portions from at least five individuals to provide a general view of an endocast of this species. The study is comprehensive and very detailed, indeed. It turns out that, despite the very small endocranial volume (about 500 cc), the brain general organization is very similar to all the other human species. Beyond some particular features in Neanderthals and modern humans, all human (Homo) species display the same general sulcal pattern. If there were differences in their sulcal organization, these should have been pretty minor or hardly recognizable on an endocast, at least according to what we can test with the small samples generally available in paleoanthropology. So, it is not surprising that Homo naledi has a Homo brain form. But the interesting thing is the association between a human brain morphology and a small brain size, as suggested by this current study. If true, we have two main conclusions. First, our brain cortical complexity and our large brain size are two independent features. They have evolved together in many cases, but not in others. Second, our human cortical folding scheme is not simply an allometric (scaled) version of the apes’ one. Cortical folding is largely influenced by mechanical factors, most of all size-related effects, so one could think that our brain morphology, although distinct from apes, is a secondary consequence of having a big brain. The results presented in this study suggest that this is not the case. We humans have a specific cortical organization and, furthermore and additionally, a big brain too. Reasonably, both features have an influence on our cognitive capacities.

Of course, these results must be confirmed on a larger perspective. Remember that here we don’t have a “brain”, but some scattered endocranial surfaces of a few specimens. That’s not sufficient to reach detailed and reliable conclusions on the brain itself, not to say on cognition. Also, the species Homo naledi (and its chronology) is at present strictly associated with one specific site and needs further corroboration from a wider geographical scenario before supporting firm or generalized statements. Its striking feature is the very small brain size. In this sense, it is worth noting that we often use to mention “average” values, sometimes forgetting about their associated variation and variability. We modern humans have a normal cranial capacity spanning a range of more than 1000 cc. In this paper, Holloway mentions the case of Homo erectus, spanning from 550 cc to 1200 cc. Therefore, caution is still necessary when interpreting the small brain size of these individuals. Of course, the fact that this species (as the Flores hominid) could have undergone brain size reduction or small brain retention does not point against the importance of brain size and encephalization. According to the available fossil record, most human species bet on big brains. Exceptions are expected, but do not break the rule.

I want to focus on one more aspect of this article. Although the topic was definitely “sexy”, the authors avoided any speculation on cognition or phylogeny. Such attitude is so professional and definitely welcome, thank you!

Plasticity

Gomez-Robles et al 2015Brain evolution involves changes in size and morphology, but also changes in the capacity to be changed. Plasticity refers to the range of phenotypic variation allowed within a given genetic structure. Environment is a major factor influencing the phenotypic expression, and we humans have a special additional environmental component called “culture”. The ecological, cultural, and social niches, shape each others, with dynamics which are far from being understood. Aida Gómez-Robles and colleagues have now published a morphological analysis of human and chimp brain, taking into consideration heritability. In both species cranial capacity is highly heritable, more for humans than for chimps. Also the general dimensions of the main brain areas show in both species an apparent genetic component. The situation is different when dealing with sulcal morphology, which are still heritable for chimps but not that much for humans. This means that brain morphology in chimps has a stricter genetic program, while humans are more sensitive to non-genetic factors and individual responses. Environmental influences are supposed to be the key, mostly when considering the altricial condition and heterochronic changes associated with the human brain growth and development. In a recent review on the evolution of visuospatial integration with Atsushi Iriki we focused on the necessity to understand to what extent brain changes associated with human evolution are due to genetic, epigenetic or environmental factors. We pointed to the sensitivity of the brain to be “trained” through feedbacks between biology and culture as a crucial variable targeted by selection. This new study stresses further the possibility that selection can act on the capacity to change, more than on the change itself.

Pandora

Endocast (C Zollikofer and M Ponce de León)Christoph Zollikofer and Marcia Ponce de León have published a review on human evolution and endocasts: Pandora’s Growing Box. The article provides comments on topics mostly related to brain size, brain growth,  encephalization, cranial evolution, and comparative neuroscience. There are useful notes and images on the correspondence between brain and endocast.

Neurocracy

After giving (too) much importance to brain size for more than one century, we are now focusing our attention on functions, parts, and relationships. Nonetheless, the many recent papers by Suzana Herculano-Houzel stress that size matters, indeed. More than size per se, what is definitely relevant is the number of neurons. The number of “computational units” in mammals, primates, or hominids, is incredibly correlated to many biological factors. Patterns are amazingly constant, and linear. After estimations on the number of neurons in fossil hominids, now a recent paper considers the relationships between brain size, metabolism, and foraging. The models developed by using the available information on primates strongly support the theory that the human genus could have had the possibility to increase brain size only thank to food preparation. Otherwise, we should spend all the day searching for something to eat, to maintain such a brain budget!


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