The new fossil skull from Dmanisi published this week on Science adds again more questions than answers. The specimens from this site, dated to 1.8 My, display a considerable degree of variation, at the same time showing some resemblance with Homo ergaster, H. erectus, H. habilis, and even with the genus Australopithecus. This last individual has massive facial structures, with a very small cranial capacity which even clashes with the common ranges associated with the human genus. Assuming that all the skulls from Dmanisi are members of the same species, and assuming that morphology is a good indicator for the specie level, the variation at Dmanisi is such that we could lump most of those cited taxa into a single and heterogeneous group. But, of course, there is no reason to think they represent a single species. Most of all, we already know that morphology can be a very bad source of information on species identity. So, why do we paleontologists insist to define species if we know that it is but a meagre speculation? We can put it in nice colours, appealing statistics, and pretty doll-like nicknames. But it is still a personal opinion, not an analytical solution. Of course, we have to call the attention of the media, that’s a good point. But this way paleoanthropology will never be a robust scientific field. If every descriptive study will be associated with firm, striking, and unverifiable statements, we will keep on swinging from one breaking announcement to another, without a solid development of the field. Maybe we have to change a bit the perspective, or even the aims. And maybe we have to focus more on biology and anatomy, their functions, meaning, and evolution, more than keeping on naming rocks.
Tags: Dmanisi, human genus, species concept
Tags: brain atlas, Katrin Amunts
The BigBrain Project has received lot of attention: a whole human brain sectioned in 7400 slices with 20 microns thickness, stained for evidencing the cellular bodies and then digitized. An exciting success, indeed, not only for anatomical advances but also for technical and methodological challenges. Data storing and exploration is a major issue in our digital era, and I am fascinated by the numbers discussed in the paper published on Science. A recent digitized mouse brain at 1 micron resolution can be stored in 8 Terabytes. A similar attempt for a single human brain would therefore require 21.000 Terabytes! Just one specimen. Difficult to store, impossible to handle with the current technology. Computers can store a huge and amazing quantity of information, but they can roughly store or handle the information of one human brain. And we are here dealing only with the raw spatial organization. In terms of function, we are still entangled in a famous cybernetic paradox: a computer can perform incredible and unimaginable calculations, but we have not been able to make it properly … walk!
Tags: chimpanzee, geometric morphometrics, hemispheric asymmetries, human brain
Cerebral asymmetries are a thorny issue in both paleoneurology and human neuroanatomy: despite their relevance, their origin and actual variations are elusive. Conceptual and technical problems tend to hamper conclusive statements, as evidenced by the many disagreements and uncertainties in the field. A recent paper on asymmetries in human and chimpanzee brains has added a geometric perspective to the topic. This shape analysis suggests that humans and chimpanzees have the same kind of asymmetries, with humans showing a larger degree of variation and expression of those patterns. Hence, again it seems it is more a matter of grade than of novel characters, at least in terms of geometry. Interestingly, allometry has a very minor role (if any) in intraspecific differences. In both species, shape variation and asymmetries are especially marked in the parietal areas.
A recent analysis of lateral brain proportions in modern humans confirms once more that “microcephaly” is probably a box in which we place phenotypes that have something in common but that are likely the result of very different processes. Robust conclusions are hampered by small sample size usually available to investigate such processes. Robust conclusions are hampered by the limited knowledge we have on cranial variation and cranial dynamics of our species. In the last years, the interest in microcephaly has been raised because of mass-media attention to the Flores human remains. According to the (large and expert) literature, there is irrefutable evidence that this specimen represents an extinct human species, probably related to Homo erectus or to some archaic taxon undergoing insular dwarfism. There is also irrefutable evidence that this specimen is but a microcephalic modern human. Hence, the only irrefutable evidence is that there is no irrefutable evidence. The Flores hominids tell us something: that we need more craniological studies.
Tags: encephalization, frontal lobes, Robert Barton
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.
Tags: Neandertals, occipital lobes, orbits, visual cortex
Paleoneurology gets new tools. By using living species, Eiluned Pearce and colleagues look for correlations between neural and non-neural anatomical elements, and then apply their results to fossils. This approach opens up an incredibly large set of new research lines. They find a correlation between orbit size and visual cortex which, after a delicate series of normalization processes, suggests that Neandertals had larger visual areas of the brain. At the expense of the parietal ones. This new information seems to provide further support for the hypothesis of relative dilation of the parietal lobes in our species. The long chain of numerical transformations the authors applied to control any possible scaling factor is really efficient and successful. Evidently it also introduces many assumptions and estimations which call for caution when dealing with strict interpretations of the final outputs. Also, we know that only in modern humans and Neandertals the prefrontal brain areas lie on the orbital roof, generating complex constraints between brain and eyes that may produce departures from the schemes of other species. Nonetheless, with these limits in mind, we must recognize that this study supplies new and fresh ideas to our field, and relevant new information on Neandertals. It is also useful in reminding us once more that different lineages may undergo different cognitive evolution, which in the case of the brain means improving or demoting different behavioural skills. This recalculation of the different cortical areas in fossil hominids also supplies new estimations of parameters associated with primates’ biology and social structure, such as the average group size.
Many people are warning against “paleo-phrenology”, but this should not be the case. This is a correlation study. A theory can explain a correlation or fail to do it, but the correlation is there, and it can reveal underlying information. Caution is recommended, but this is always recommended in science. It is strange how we are critic against a lots of details for complex models in evolution, yet we accept overly simplistic alternatives like those associating cognition with a single molecule or gene …
A comment on National Geographic.