Posts Tagged 'frontal lobes'

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!

Advertisements

Brains and eyes

After our first survey on the morphological relationships between eyes and brains, here a comprehensive second study on this same topic. We have analyzed data from computed tomography (orbits and endocranial space) and magnetic resonance (eyes and brain), investigating modern humans, apes, and fossils. Soft tissue variation mainly deals with the distance between eyes and temporal lobes. Cranial variation mainly concerns the orientation of the orbits, probably influenced by parietal morphology and variation of the head functional axis. Phylogenetic differences are generally associated with the distance between orbits and braincase, with fossil humans showing an intermediate position between modern humans and apes. Here a Skull Box post with more details.

Frontal surfaces

beaudet-and-bruner-2017

More surfaces. This week we have published a surface comparison of the frontal endocranial morphology in OH9, Buia, and Bodo. The methods are the same applied previously by Amélie Beaudet and colleagues. Despite the importance generally assigned to the frontal cortex in our species, paleoneurology has not managed to reveal clear and patent changes in its gross form. Endocasts can only supply information on the general external appearance of the cortical anatomy, so we should expect they cannot be used to trace many aspects  associated with evolutionary variations. Also, the bad habits to defend firm statements based on single (and often reconstructed and fragmented) individuals unpleasantly crashes against the basic scientific principle of hypothesis testing, something that needs quantification, large samples and statistics. In this paper we compare these three specimens with the general scope of discussing some issues about frontal lobe evolution and paleoneurology. When compared with a modern human endocast, the younger fossils (Buia and Bodo) display flatter dorsal-lateral areas, while the older one (OH9) show a more extensive flattening of the whole dorsal surface. They all fit within a general trend observed in humans and hominoids: the more the eyes go below the frontal cortex, the more the frontal lobe bulges. So it seems reasonable to think that the curvature of the frontal lobes is but a structural consequence of the spatial relationships between face and braincase. In paleoneurology, we should exclude structural changes (cranial constraints and secondary consequences) if we want to localize functional ones, or if we want to reveal specific adaptations and primary evolutionary variations. Surface analysis is one more tool to go in that direction.

Eyes and brains

Brain and Eye - PaleoneurologyThis week, with a team coordinated by Michael Masters (Montana Tech), we have published a correlation analysis to evaluate the relationships between eye, orbit, and brain, in adult modern humans. As already evidenced in other studies in anthropology and primatology, the correlation between eye size and orbit size is very modest. Therefore, the orbit is really a poor predictor of the eye morphology, at both evolutionary and species-specific level. There is also a minor size correlation between the eye and the occipital cortical areas, probably because of their shared visual functions. However, there is also a similar (and even higher) correlation between eye and frontal lobe. In this case there is a structural issue: the frontal lobes lie just above the orbits, generating a spatial interaction between facial and neurocranial elements. Within hominids, this spatial proximity between prefrontal cortex and eyes is generally observed only in modern humans and Neandertals. These two taxa, possibly because of such vertical constraint, enlarged their frontal lobes mainly laterally. These correlations between soft and hard tissues, when dealing with inter-specific trends, can be useful to make inferences on brain proportions based on osteological evidence, providing an heuristic tool for indirect paleoneurology.

So, back to modern human evolution, the situation of the eye was pretty difficult: large eye (due to brain size increase), small orbit (due to facial reduction), upper constraints (the frontal lobes right on the orbital roof), posterior constraints (larger and closer temporal lobes). And, in industrial Countries we can also add more fat between eye and bone. Hard times for the eyeballs, forced to minor deformations blurring images on the retinal screen: myopia. Luckily for us crossing the 40s, the brain stops growing, but the face does not: it grows bigger, giving more space to the eye, which can enjoy a more comfortable environment year by year.

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.

Brain and eye

Paleoanthropologists are now fairly convinced about the importance of integration in biology and evolution. It is a rare pleasure to see such perspectives successfully applied to every-day life problems. Michael Masters, with a very well documented study on human evolution and functional craniology, suggests that myopia (the primary source of reduced vision throughout the world) may be the consequence of our recent anatomical evolution. Large brains placing the frontal lobes on the orbital roof and constraining the orbital space, while at the same time facial reduction provides further structural limitations. That is, in our species the orbit cannot acknowledge properly the morphological requirements of the eye. Brain and eye compete for space, and the advantages associated with the former are paid with the problems associated with the latter. The consequent packing deforms the eye, leading to vision problems. Allometric and brachycephalic proportions make the situation even a bit more difficult in women and East Asian populations. This hypothesis is an excellent example of interchange between evolutionary biology and medicine. Until now, myopia has been mostly studied considering the eye like an isolated unit. Masters has now provided a very effective example of how induction and deduction can be improved mixing fields, in this case integrating medicine with functional craniology and paleoanthropology.  Interestingly, similar deformations associated with the frontal lobe spatial packing have been also described for some psychiatric disorders.


Enter your email address to follow this blog and receive notifications of new posts by email.

RSS Brain News

RSS Cognitive archaeology

  • Summer 2018 Courses at The Center For Cognitive Archaeology
    Courses offered for the Summer 2018 Semester (June-August) The Center for Cognitive Archaeology (CCA) provides both undergraduate and graduate students the opportunity to study the evolutionary development of cognition in humans and other primates. The CCA offers 12 different online courses, which are taught by professors from the University of Colorado, Col […]

RSS The Skull Box

  • A History of Surgery
    The Chirurgeon’s Apprentice is a popular blog on the website of medical historian Dr Lindsey Fitzharris who received her doctorate from University of Oxford in medical, technology and science history. Dr Fitzharris discusses the apt naming of the blog with the word ‘chirurgeon‘ the first historical reference to a practitioner of surgery. The website illumina […]

RSS Anthropology

RSS Human Evolution

  • An error has occurred; the feed is probably down. Try again later.

RSS Neurophilosophy

  • Researchers develop non-invasive deep brain stimulation method
    Researchers at MIT have developed a new method of electrically stimulating deep brain tissues without opening the skullSince 1997, more than 100,000 Parkinson’s Disease patients have been treated with deep brain stimulation (DBS), a surgical technique that involves the implantation of ultra-thin wire electrodes. The implanted device, sometimes referred to as […]

Disclaimer

This blog publishes texts and comments of the author, which can not be referred to institutions or contexts outside of the blog itself. The published material may be partly derived or reported from the Web, and therefore evaluated in the public domain. If some content violates copyright or if it is considered inappropriate, please contact me, to promptly remove it. On the other hand, please cite this source whenever using images or texts from this website.
Advertisements