Friday, December 28, 2007

On the origin of humans

Interesting observations regarding brain evolution

Homo sapiens distinguish themselves among the general primate lineage in three distinct ways- bipedalism, smaller modified jaws and teeth, and a very large brain. It is likely that at some point these principal developments coalesced functionally allowing for new opportunities in the game of evolutionary dynamics. It is not hard to imagine this unique combination beginning to form ever strengthening feedbacks with each other and eventually becoming the critical ingredients enabling, in time for the creatures we are today.

In fact, it may very well be this interplay that set one distant hominid ancestor off on the road to becoming Homo sapiens. The common belief that brain size or brain to body size relates to intelligence (at least the kind of intelligence we would consider human) is too simplistic. The elephant brain is four times larger than ours yet they posses no where near the interactive complexity of humans, therefore brain size does not equal complexity or correlate to human like intelligence. A straight brain to body size ratio do not work either as for example the mouse lemur has a brain to body ratio of 3% whereas humans come out at 2%.

Brain size grows as bodies become larger, albeit more slowly as body size increases, but ultimately there is no direct link to increased intellectual complexity and just larger brain size. The real relation between brain and body size is best described by a non-linear power ratio which is 0.75. With this ratio, humans indeed have the largest brains relative to body size.

Among all mammals, Homo sapiens have the largest brains, and primates in general tend to have larger brains than other mammalian species. It is interesting to note that there are exceptions and overlaps with other mammals though. For example, simian primates generally have relatively larger brains than prosimians but there is overlap at this level with some large brained prosimians and other mammals such as seals.

As primates go, contrary to what many think, the gorilla has a small brain size relative to body ratio compared to monkeys and the New World capuchin monkey has the highest non-human relative brain to body size ratio. This interesting capuchin fact offers researchers an opportunity for study that could potentially reveal some evolutionary insight into the brain expansion of our own ancestors.

One fascinating observation when considering brain size in mammals is, even though Homo sapiens have a much higher brain/body ratio (three times larger) than expected, it is not the fact that we have larger brains that stands out first from an evolutionary perspective.As previously implied, mammals in general tend to have larger brains, which seems to be a favorable quality for this branch of animals.

In fact, endocast studies of mammalian brain fossils has led to the incredible realization that, through time, brains have tended to increase in size across all lineages. This is indeed a humbling thought for the hominid line. The differences in modern species brain sizes have come about through differing rates of brain expansion. It seems what really set us apart in the mammalian world is the rate of expansion- just a few million years to gain our present brain size- that is truly unique.

The brain is a very expensive organ to develop and maintain. It seems the mammalian way of life facilitated the selection of a type of brain complexity that favored their survival. This then begs the question of how a hominid line was pressured towards attaining such a large brain.

Here, there is much speculative thought that revolves around the uniquely human evolutionary triage previously mentioned. Probably by serendipity and the inexorable flow of evolutionary pressure, this tiny hominid lineage stumbled upon a formula that allowed it the ability to feed its brain beyond the capacity of other mammals.

It seems then that our humanness is, at least in large part due to the inseparable interplay between bipedal walking, a remodeled jaw for eating high energy food, and a ready predisposition for large brains.


Jones S, Martin R, et al. Human evolution. Cambridge university press. Cambridge, UK. Part three 107-108.

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