Humans ,Chimps and the 2% difference

The difference between the human and chimp genome is a mere 2%. That is all. And yet why are we so different? Neuro biologist Robert Sapolsky looks at the various points at which the genome differs and his findings are quite a revelation.

The two percent distinction involves the following
- chimps have more genes related to olfaction while we don't, which is very obvious
- chimps and humans have different parasite vulnerability. chimps are immune to malaria, while humans handle TB better than chimps
- the anatomical difference in the reproductive organs is pretty significant

Now all these differences make sense. But what contributes to the vastly different brains? Which are the brain specific genes that have evolved in very different directions in the two species? Apparently there aren't any that fit the bill.

Robert Sapolsky says that 'If you examine the neuron from a human brain under a microscope and do the same with the neuron from a sea slug, they look the same. They have the same complement of neurotransmitters, they are all the same building block. The main difference being the sheer number of neurons. The human brain has 100 million times the number of neurons in a sea slug's brain. Where do those differences in quantity come from?

At some point in their development, all embryos - human, chimp, rat, frog - have a single cell committed toward generating neurons. That cell divides to 2, then 4, then 8, then 16. After a dozen cell divisions there are enough neurons to run a slug. Go another 25 rounds or so and you've got a human brain. Stop a couple of rounds short you've got one for a chimp. Vastly different outcomes, but relatively few genes regulater the number of rounds of cell division in the nervous system before calling a halt. And it's precisely some of those genes, the ones involved in neural development, that appear on the list of differences between chimps and human genomes. That's it: that's the 2 % difference. What is shocking is the simplicity of it.

Humans to be humans need only a few more rounds of cell division that produce neurons. Our braininess arises from having humongous number of just a few types of off-the-rack neurons and from the exponentially great number of interactions between them. Genes have some thing to do with the quantity and thus the complexity of the quality that emerges.