For a long time it was assumed that humans engaged in particularly human cognitive activities such as creative thinking, future planning, language, and high-functioning memory because humans, compared to other apes, have a higher frontal lobe to brain ratio. In an important experiment, Semendeferi et al. (1997) found evidence that severely countered this assumption.
The experimenters measured the volumes of frontal lobes and whole brains of living human brains and of post-mortem brains of the chimpanzee, gorilla, orangutan, gibbon, and the macaque. To measure the volumes they took magnetic resonance (MR) scans of the brains and then three-dimensionally reconstructed the scans. Major landmarks common to all species were used to distinguish the boundaries of the frontal lobe. In addition, they measured the volumes of the 3 frontal lobe sectors (dorsal, mesial, orbital) across the species. In order to account for any possible differences between living and dead brains, like shrinkage, they compared the living human frontal lobe to a post-mortem human frontal lobe.
The results of the experiment indicated that humans do have the largest absolute volume of both the brain and the frontal lobe; however, all of the hominoids have a similar frontal lobe to brain ratio. In other words, humans do not have a larger frontal lobe than a human-sized primate brain would be expected to have. In addition, the relative size of the three sectors were similar across the primate species.
Semendeferi et al. would, however, call attention to some qualifications. First of all, the relative human and chimpanzee frontal lobes values are most similar, while there is a slight, but steady decrease in relative frontal lobe size as the species are located further up the human phylogenetic tree. For example, the relative values for the orangutan brain are at the low end of the human range, as are the gorilla values. The researchers suggest that the data may indicate that frontal lobes becoming relatively larger is a trend of hominoid evolution, but not specifically of hominid evolution. After the split of chimpanzees from the common African hominoids, there was apparently no further increase in relative frontal lobe size all the way to modern humans.
There is, of course, plenty of scientific critique to this study, and the researchers know it. Their sample size is small, and they admit their samples may include outliers (especially the gorilla!). In this 1997 study, however, they intended merely to begin a database of brain volume measurements, using repeatable techniques, to which future studies may contribute. A humble and meager start, which can only produce tentative conclusions.
The study, nevertheless, debunks a classically held assumption, and leaves scientists really wondering what it is about the human brain that is unique. If the human and the chimpanzee have equal relative frontal lobe sizes, and yet only one can speak and write and create grand oeuvres d’art, what accounts for this great difference? Certainly, the study did not address other aspects of the frontal lobe besides overall relative size: perhaps organization of the frontal lobe changed during hominid evolution. Gazzaniga (2008) lists four hypotheses regarding how the human frontal lobes can account for high functioning: reorganization and enlargement of some cortical areas, richer connections between frontal sectors, modification of circuitry in frontal subsectors, and other micro- or macroscopic subsectors added or removed. In addition, the relative prefrontal cortex was not measured in this study, and perhaps the prefrontal cortex to frontal lobe ratio may be important in distinguishing between human functioning and ape functioning. The role of the prefrontal cortex in high functioning is especially implicated in a study by Baare et al (1999), which found that in schizophrenic patients, problems with verbal and visual memory and semantic fluency may be related to a decrease in volume of frontal lobe (especially prefrontal) structure.
This study, especially, delivers a slight blow to a philosophical understanding of human personality as being very distinct from that of an animal’s. Here, we do not see relative frontal lobe enlargement as being a unique derived character during hominid evolution, but it is a shared character among all hominoids. The clearest line this study draws is not between humans and the rest of apes, but divides humans and chimps from the rest of the apes. This is further evidence that humans are not qualitatively distinct from other apes, but that brain changes amongst species can be pinpointed on a rather blurred continuum. A win for reductionism of the human mind? Not necessarily. Much more work needs to be done to show how high-functioning human brains materially evolved over time from lower-functioning ape brains.
References:
Semendeferi, K., H. Damasio, R. Frank, G.W. Van Hoesen. 1997. The evolution of the frontal lobes: A volumetric analysis based on three-dmensional reconstructions of magnetic resonance scans of human and ape brains. Journal of Human Evolution 32:375-88.
Gazzaniga, M.S. 2008. Human: The Science Behind What Makes Us Unique. Harper Collins:
Baare, W.F.C, H.E. Hulshoff, R. Hijman, W.P.T. Mali, M.A. Viergever, R.S. Kahn. 1999. Volumetric analysis of frontal lobe regions in schizophrenia: relation to cognitive function and symptomatology. Biological Psychiatry. 45:1597-1605
At the end you pointed to an interesting conclusion...that increased intelligence might be "reduced" to brain size. In other words, functionality is reducible to size.
ReplyDeleteEagles are known to have excellent vision, better than humans. But their eyes are smaller than human eyes, showing that the correlation (if there is one) between function and size isn't causal.
If there is a correlation, is THAT reduced to some deeper correlation between complexity and processing parts? That is probably the story closer to the truth.