MadSci Network: Evolution

Re: How did we get our big brains?

Area: Evolution
Posted By: Brad Keele, Grad student Neuroscience Graduate Program
Date: Tue Jun 25 08:50:29 1996

Hello Christopher,

This is a very interesting question. First, I'd like to offer my opinion on why we have large brains, then address Harris's hypothesis and finally, information theory.

To understand why humans have large brains, I think we have to look back at the environmental and evolutionary factors that were working on early hominid species. I'll bet you've heard about one of the earliest hominids, named Lucy ( Australopithecus), who lived about 5 million years ago. Lucy had a small brain, but walked upright. It is believed that due to tectonic activity in East Africa (the Great Rift Zone, today) land was pushed upwards into mountains that kept the rain from falling on the eastern-most part of Africa (today, Ethiopia). This event may have influ enced the speciation of early hominids. Because the primates to the east of the mountains found food more scarce in the dry savannah, they had to go farther to find food and had to be more efficient in finding food. Some believe this is why early hominids began walking upright. An interesting note here, Chimpanzees (the closest cousin to modern humans since we share about 98% gene identity) and humans both have a muscle, the glutius medius, that attaches the pelvis to the leg (swing your hips side-to-side , that's the glutius medius). Chimps don't use this muscle when walking on all fours, but they do use it when climbing vertically. Perhaps this is evidence suggesting both humans and other primates had the "physiological" means to maintain an upright posture, but do to extraordinary environemtal factors, only the early hominids began to walk on two legs (as bipeds).

One of the next class of early hominids are Paranthropus robustus, c. 2-3 million years ago. Although they had a skull that more closely resembled modern humans than other primates (high forehead, not sloping), they too had small brains. Robust us is important in the early evolution of man because they are the first hominids we're a ware of that began to use tools. They developed the ability to efficiently use tools because of an anatomical change. They developed a "pincer grip." That is, an opposable thumb and forefinger. Chimps and apes can grasp small objects between thumb and fo refinger, but they use the side of the forefinger, whereas Robustus could use the tip of his forefinger. Take-home message here, they still had small brains.

The first early hominid to really look "human" was Homo Erectus, who walked fully upright and began using vocalizations (primitive speech). But still, Homo Erectus had reletively small brains. Beleive it or not, the first early hominid to have brain size comparable to modern humans are the Neanderthals. However, there is some uncertainty regarding which hominid was a direct ancestor of modern humans. It seems clear to anthropologists that Erectus were ancestors of both Neanderthals and modern humans, but there is no clear consensus whether Neanderthals are direct ancestors of modern humans. That is, there is some evidence to suggest that Erectus broke into two branches: one evolving into Neanderthals, the other into an ancestor of modern humans, the Neanderthals becoming extinct with the expansion of early humans.

Neanderthals were sort of the "kings" of the Ice-Age (~10-20 thousand yea rs ago). They learned to use fire, to make weapons for hunting, and to make clothes to keep warm. In essence, they began to control their environment. Also, they seemed to have an appreciation for the after-life: they buried their dead, and it seems they appreciated art, music and other "cultural" things that today are uniquely human. Perhaps all of this is because they were the first hominids to have such massive brains.

OK, you may have guessed by now, I don't believe the "escape the heat by walking upright" hypothesis. I think that the preponderance of anthropological evidence would suggest that early man thirved for several (at least a couple) million years in the heat of East African savannah with a relatively small brain. Alternatively, our brains did not grow as a result of an evolutionary attempt to protect neurons from heat stress. However, Harris (and Fialkowski) are not the first to suppose this idea. The ancient Greeks (maybe Aristotle or Plato) believed the brain was sort of a big radiator. They noticed all the convolutions in the cortex and reasoned that this served to increase surface area, and thus was more effiecient in allowing heat to escape.

As for interpreting the evolution of large human brains in the context of information theory, sure, the bigger the better (within limits). I think it is widely believed today that the brain functions as a multitude of modules working in parallel (i.e. parallel, distributed processing). True, this would allow for a bit of "play" in the system: if one module goes down, there are others that can pick up the function given time and training. This is exactly what occurs following certain neurological traumas. However, it is completely different to suggest that the benefits of redundancancy in brain circuits drove the expansion of brain size. It's a kind of "chicken or the egg" thing. Evolution works by natural selection, selecting the behaviors which promote reproductive fitness (and propogation of the species). It is hard for me to imagine that large brains are the result selecting for reduncancy of function (in fact, it is counter-intuitive). I would postulate that as early hominids gained the ability to perform more extensive behaviors (tools, hunting, sewing, other ceremonial acts) that promoted survival of the species, the outcome was larger brains which only by chance tended to store functional circuits in parallel.

So, in summary, I'd say that there is a lot of credence in trying to understand brain function in terms of information theory. However, the teleological principles at work on "why" we have large brains is surely contested. I don't happen to believe the "heat theory," but there are plenty of other, more knowledgeable, people out there who do. I've given you my 2=A2 (well, maybe $2 worth), and I hope it's been of some use to you. My home page My home page is devoted to brain stuff, and I have some links to other brain stuff at Brain stuff

Keep the good questions coming!


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