Monday, April 5, 2010

What Makes Chimps and Humans Different

Yeah..yeah...chimps are hairier and while in heat the bare skin of a female chimp's bottom gets all red and swollen..I haven't noticed that in humans...but really deep down the question  most people want to ask is why is there so much cognitive difference between chimps and humans.

 Almost As Brainy

What is the genetic basis of the brain differences between chimps and humans and how did they come about through evolution?

On NPR's Cosmos and Culture blog, Ursula Goodenough has a terrific post on gene regulation and brain development and research that is beginning to tease out the reasons for the differences between chimps and us.

...brains build themselves. Bottom up. When A happens, that allows B and C to happen; B allows D and E to happen; and so on.

Because brain development is so contingent on what has gone on before, it's pretty easy to alter what happens. For example, if the pioneer neurons in our example carried a switch mutation that prevented them from secreting the nerve growth hormone at the appropriate time, the next phalanx of neurons wouldn't move towards them and might, instead, pick up on a more distant hormonal signal from another brain region and move in that direction, forming synapses with a new set of neurons altogether. A brain is still constructed, but it will have different kinds of neural pathways and connections and hence, perhaps, different ways of doing things.

So now we can return to the chimp-human question. If the chimp and human protein-encoding genes are virtually all the same, then are there any interesting differences in their switch regions? Given the bottom-up nature of development, mutant switches could have large-scale consequences.

The answers that is emerging is ..yes..there seems to be differences in switch regions between chimps and humans.

The post is a quick lesson in the genetic innovations that occured through the transition from unicellular to multicellular life and fast-forwards to brain development and the role of genetic switches in evolving human-specific patterns of regional differentiation of the brain.

The question of when these changes in the brain occurred is not dealt with but a reasonable answer would be.. beginning with the Homo lineage about 2 million years or so and continuing ..even today? It was about 2 mya when we begin to see in the hominid fossil record a noticeable departure (increase) in brain size compared to the earlier fossil hominids and modern chimps ..accompanied by a more varied tool.

I suspect these kind of discoveries will provide fodder for the many controversies about the emergence of "modern humans". Did the sudden emergence of a new genetic switch enable symbolic behavior or ...language?

This post by Ursula Goodenough deals with the developmental aspects of brain differences. An evolutionary narrative awaits.


  1. this is interesting. on a slight tangent, I saw somewhere that the genetic variation within homo sapiens is quite a bit less than you find among most other primates, which suggests that at one or more times the population of our species was reduced to very small numbers--probably in the thousands. and as recently as 50,000 years ago, we shared the earth with a lot of other kinds of "people." really fascinating stuff, this.

  2. yeah the bit about variation being less than primates has been reported but what is the cause is still not understood. a population bottleneck is a reasonable answer.

    Neanderthals were certainly around 50K ago and possibly other hominids in east Asia as well.