Human brain variation

One year ago Croxson and colleagues published a survey on human and macaques brain variation, a paper which has been issued this month in Cerebral Cortex. They considered variation in white and grey matter, comparing inter and intra-specific patterns, and discussing similarities between the evolutionary and individual degree of variability. This study evidences the importance of variation as a source of evolutionary possibilities and constraints. The survey was based on only 10-20 individuals and, despite any statistical reassurance, we have to recognize that this is an unusual sample size for a study targeted to describe and quantify intra-specific diversity. Furthermore, in these kinds of analyses one has constantly the sensation that phylogenetic differences (macaque-human) are still interpreted as evolutionary differences (ancestral-descendant), which is definitely an inappropriate perspective when dealing with extant species. Also, the fact that we keep on using the term “monkey” when referring to one single species of hundreds of living, independent and diverse ones, denotes a still-alive linear approach to the evolutionary schemes (the old fashion progression monkey -> ape -> human). This paper was then commented by Aida Gómez-Robles, who discussed the pros and cons of this study. Some months later, Reardon and colleagues published a similar analysis, but on a huge sample. In her review, Aida Gómez-Robles pointed to endocasts as a potential source of additional information on intra-specific brain variation. Definitely a good point, and a valiant position to be presented in a mainstream journal on cognition. Endocasts and macroanatomy are issues which are often neglected in neuroscience. Nonetheless, two aspects must be taken into account. First, macroanatomy and morphology still hide many issues which suffer a dramatic lack of information, and that can reveal unexpected suprises. This is also true taking into account traditional neuroanatomy and, for example, in our last survey on human brain variation (on 265 individuals) the precuneus still stands as a major source of gross morphological human diversity. Second, although endocasts can’t provide a comprehensive information on brain biology, they can remarkably help to increase the sample size when dealing with primates and especially hominoids, because of the many collections available as dry or digital skulls. A recent study on the degree of endocranial metric variation in apes, humans, and hominids can be found here.


2 Responses to “Human brain variation”

  1. 1 Michel Thiebaut November 29, 2018 at 13:35

    Thanks for your interest in our work.
    I would like to mention that the sample size was determined using measures of replicability reported in the supplementary material. Results were duplicated several times in humans with different sample size. Therefore the critic on the statistics is not correct. We apologise if this point was unclear in the manuscript. An important point is that evolutionary interpretation were drawn from comparative anatomy between two species. I totally agree with your criticism and we should investigate many more species. This work in currently in progress.

    • 2 emilianobruner November 29, 2018 at 15:58

      Thanks Michel for participating in this blog! It is really a pleasure to have feedback from the authors. I understand the replicability issue, that’s why I was mentioning “statistical reassurance”. I normally work with intra-specific variation, and I think that statistical representativity is a pillar of any quantitative analysis, but then you have the biological representativity, which is something different. This latter parameter has less to do with numbers, and it can deal with many factors ranging from biogeography to population genetics. This issue is the one we can’t control for with a statistical test.

      Nonetheless, I must confess that my comment was not necessarily a criticism to this sample size. I work with brains, skulls, fossils, and I perfectly know the problems associated with sampling. Sample limitation is not a problem, if this is properly taken into account. Sample limitations are an intrinsic factor in science. What I am concerned about, instead, is a different weight people give to this factor in distinct fields. As author, collaborator, editor or reviewer, in paleoanthropology or neuroanatomy I have seen so many times harsh criticisms on “small samples” when dealing with tens (or hundreds!) of specimens. Even when “statistical reassurance” was supplied. In turns, in neurobiology many studies are based on few individuals. The same sample limitations which are strongly criticized when you deal with anatomy (macroanatomy) are generally accepted if cells or complex algorithms are involved. The same for the number of species: many surveys are criticized when are based on “only some tens of species”, but in neurobiology two species are generally sufficient to talk about “primates”. In neurobiology, it seems that biological reductionisms or mathematical complexity make small samples acceptable, at least when compared with traditional morphological fields or with evolutionary and zoological topics. I think this may be a major problem in some disciplines and, furthermore … an interesting epistemological issue!

      In turns, my criticisms on the evolutionary perspective in neurobiology are sincere. Macaques have been evolving as long as us, having their own derived traits (which we ignore). And, Macaca mulatta is one of 300 different primate species, so we can’t give for granted that it properly represents “monkeys” or “the primate brain”. I am publishing right now a review exactly on this topic, I will really be happy to know your opinion on that when published …

      Once more, thanks so much to contribute this post with your comments! And … do not hesitate to tell more about your results and conclusions. Your article discusses so many aspects which merit much attention. For example, in humans (and in comparisons with chimps) we found an amazing variation in the precuneus. In the Cerebral Cortex cover, dedicated to your paper, this region seems pretty “colored”. But in the paper there is no particular discussion about that. I would really like to know how to integrate the morphological results with those obtained from your studies on variation … Are these results compatible? Do they deal with different aspects or converge on some shared conclusion? Keep in contact!

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