Posts Tagged 'encephalization'

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!

Brain ontogeny

Hublin et al 2015After that recent article on endocasts, the team from the Max Planck Institute has now published one more review, this time on brain ontogeny and life history. This paper introduces issues concerning encephalization, energy budget, birth, maturation, ecology, and culture. It provides also many general perspectives on hominid paleoneurology, resuming much paleontological evidence published in the last decade. It is a good and effective recompilation of literature and concepts, integrating morphometrics, development, and evolution. It is part of a special issue dedicated to brain, birthweight and the immune system.

Functional craniology

Bruner et al (Front Neuroanat 2014)Chet Sherwood and Suzana Herculano-Houzel are editing a Frontiers volume entitled “The Human Brain’s Place in Nature: Evolution of Large Brains”, cross listed between Frontiers in Neuroanatomy and Frontiers in Human Neuroscience. There you can find review and research papers dealing with pros and cons of evolving large brains. Our article presents issues in functional craniology, with topics joining evolution and medicine. The article begins with an introduction to functional craniology and brain-braincase structural relationships. Features associated with sutures and brain spatial organization are interesting in evolutionary neuroanatomy and in surgery as well. Brain thermoregulation is a major factor in both fields, and modelling can help to test the influence of brain form changes in heat dissipation patterns. Changes in the frontal lobes proportions and position during human evolution are discussed as a probable background for visual impairment, in particular myopia, because of spatial conflicts between brain and orbits. The dilation of the parietal areas in modern humans and the complexity of the deep parietal elements are then integrated with information on neurodegenerative processes, in particular Alzheimer’s disease, in an evo-neuro perspective. Evolutionary neuroanatomy and medicine share information, tools, methods, and samples, being interested in the same characters and processes for different reasons and different targets. Functional craniology is the bridge we use to integrate these fields.

Frontal matters

Saccopastore 1 (EBruner)A re-evaluation of brain volumetric data, adjusting for scaling and phylogeny, adds further evidence against the hypothesis of morphological changes in the frontal lobes for our species. Despite a century of firm claims on the patent role of the frontal lobes in our evolution, there are now many different indications suggesting that those statements were probably excessive and not well demonstrated. It seems that there is no clear specific change in the general morphology of the frontal lobes in Homo sapiens, and even the correspondence between anatomy and functions has lost strength. We must take into consideration the possibility that differences may be subtle but important. A minor shift from the general tendency may be irrelevant for the statistical thresholds but important in biological terms (for example, this can be the case for white matter proportions). There may be also changes which have not been detected yet, as well as changes that are not evident from gross morphometrics. Furthermore, even if volumetric changes in our frontal lobes are those expected for our large brain size, the increase in terms of absolute size is patent, and this may be a relevant difference anyway. Throughout this debate, it is interesting to note how the paleoneurological information is generally ignored. Despite the many inferences on the evolutionary changes in the brain human form, there is no mention of the notable advances published on the evolution of brain geometry in our species. This is even more imprudent when considering that anthropology is currently employing very complete and powerful morphometric tools, while in neuroscience most data still refers only to general size measures. However, even using just basic morphometric variables, we know that modern humans and Neandertals experienced at least a change in the proportions of the frontal areas. Excluding the fossil evidence from the debate does not seem to be a good idea, at least when dealing with evolutionary studies.

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.

Brain scaling in mammals

A recent paper by Boddy and colleagues provides an excellent and updated review on encephalization in mammals. Theories and methodologies are introduced and compared, and a sample of 630 living species is used to evaluate encephalization according to phylogeny. The allometric scaling of brain and body size is discussed in extant and extinct taxa. On the same issue, have also a look to this paper by David Alba on encephalization and intelligence, with relevant methodological cues.


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