Do Neurologists Understand Brain Evolution?
Jane Bradbury wrote a feature piece for PLOS Biology recently,1 entitled, “Molecular Insights into Human Brain Evolution.” Help us find the insights. First, she marvels on how “humans sit on top of the pile when it comes to relative brain size.” Then she marvels at how quickly the human brain apparently evolved compared to apes. Next, she complains that we don’t have good maps of the differences between ape and human brains, so “it is hard to make meaningful comparisons between our brain and that of chimpanzees.” She calls on Karl Zilles (Germany) to explain:
Already, Zilles has discovered that there is much more interindividual variation in human brain organisation than anyone suspected. This means, says Zilles, “that a general statement like ‘the neocortex is bigger in human brains than in ape brains’ actually tells us very little. It gives us the general direction that evolution has taken but not whether an ape brain is different because of its sensory, motor, or association areas.” (Emphasis added in all quotes.)
Still hunting for insights, we find Bradbury finding an apparent case of convergent evolution between whale brain evolution to human brain evolution, but that assumes the brains evolved rather than providing insights into how they could have. Next, we find Bradbury wondering how a large brain could have evolved, because it costs a lot to run it. Worse, it would have had to simultaneously get reorganized as it grew bigger:
For one thing, a big brain is a metabolic drain on our bodies. Indeed, some people argue that, because the brain is one of the most metabolically expensive tissues in our body, our brains could only have expanded in response to an improved diet. Another cost that goes along with a big brain is the need to reorganise its wiring. “As brain size increases, several problems are created”, explains systems neurobiologist Jon Kaas (Vanderbilt University, Nashville, Tennessee, United States). “The most serious is the increased time it takes to get information from one place to another.” One solution is to make the axons of the neurons bigger but this increases brain size again and the problem escalates. Another solution is to do things locally: only connect those parts of the brain that have to be connected, and avoid the need for communication between hemispheres by making different sides of the brain do different things. A big brain can also be made more efficient by organising it into more subdivisions, “rather like splitting a company into departments”, says Kaas. Overall, he concludes, because a bigger brain per se would not work, brain reorganisation and size increase probably occurred in parallel during human brain evolution. The end result is that the human brain is not just a scaled-up version of a mammal brain or even of an ape brain.
Still hunting for those elusive insights into brain evolution, we find Bradbury wondering how natural selection could have done the job. “For natural selection to work,” she explains, “the costs of brain evolution must be outweighed by the advantages gained in terms of fitness.” So were they? She quotes those who speculate about possible selection pressures, such as the need for better diet and more social group coherence, but provide no data to explain why apes are sociable and physically fit but retain small brains. Whatever selection pressures there might have been, though, she is sure were not guided: they had to “work on the raw material of random gene mutations,” she reminds us.
Here Bradbury provides a smidgeon of data. Several teams have suggested that a gene named ASPM, which is implicated in the shrunken-brain disease microcephaly, might have been a factor. But then again, one researcher said, “we really have no idea yet how or even if ASPM is involved in brain evolution.” Some other “candidate genes” are being studied, but “functional studies” on genes are “difficult to do,” she cautions, so there are only suggestions at this point.
Considering how much must have changed since humans made their first evolutionary strides from apes, some genes show no clear evolutionary pattern, according to a surprise announcement by the Max Planck Institute. They recovered DNA from a Neanderthal skull said to be 75,000 years old, and discovered that the gene for osteocalcin was identical to that in modern humans. Furthermore, they found a marked difference in the sequence for this gene in gorillas compared to other primates in mammals. All they can promise is that the possibility of additional gene comparisons from fossils might help “better understand the phylogenetic relationships” between primates (implying that they are not well understood now).
But then again, maybe it is not just the genes, but the way they are expressed, that became important sometime in the human evolution saga. One team has found 100 genes so far that are differentially expressed in human and chimpanzee brains. That led Todd Preuss (Emory U) to remark, “All told, it seems that the human brain may be more dynamic than ape or monkey brains. The human brain seems to be running hot in all sorts of ways.” This still begs the question of how or why that should be so.
We finally reach her last paragraph, subtitled, “Scratching at the Surface.” Just when we were hoping for a surprise treasure chest full of insights, we find ourselves empty-handed:
As far as understanding how our brains evolved, more questions remain than have been answered. One problem is that we don’t really know enough about how our brains differ from those of other mammals and primates, although work by Zilles and others is helping here. We also know very little about how the areas of our brain are physically linked up, and we need to understand that before we can see how we differ from our nearest relatives. And as far as identifying the gene changes that were selected during evolution, although we have several candidates, we don’t know how or if these gene variants affect our cognitive abilities. It is one thing, concludes Dunbar, to identify genetic or anatomic differences between human and ape brains, but quite another to know what they mean in terms of actual cognitive processes.
1Jane Bradbury, Feature: “Molecular Insights Into Human Brain Evolution,” Public Library of Science: Biology, Vol. 3, Issue 3, March 2005.
We want our money back. We were promised some insights, and all we got were excuses. Pay up, Darwin Party, or we are going to the ID show across the street, where all the crowds are gathered.