The 2% Difference

This article is from Discover, April and it is about the 2% difference between chimps and us humans; what those differences are. I'll not post the whole article but the most interesting part about the difference in our brains and theirs: "...That all makes sense. Still, chimps and humans have very different brains. So which are the brain-specific genes that have evolved in very different directions? It turns out that there are hardly any that fit that bill. This, too, makes a great deal of sense. Examine a neuron from a human brain under a microscope, then do the same with a neuron from the brain of a chimp, a rat, a frog, or a sea slug. The neurons all look the same: fibrous dendrites at one end, an axonal cable at the other. They all run on the same basic mechanism: channels and pumps that move sodium, potassium, calcium around, triggering a wave of excitation called an action potential. They all have a similar complement of neurotransmitters: serotonin, dopamine, glutamate, and so on. They're all the same basic building blocks. The main difference is the sheer number of neurons. The human brain has 100 million times the number of neurons a sea slug's brain has. Where do these differences in quantity come from? At some point in their development, all embryos - whether human, chimp, rat, frog, or slug - must have a single first cell committed toward generating neurons. That cell divides and gives rise to 2 cells; those divide into 4, then 8, then 16. After a dozen rounds of cell division, you've got roughly enough neurons to run a slug. Go another 25 rounds or so and you've got a human brain. Stop a couple rounds short of that and, at about one-third the size of a human brain, you've got one for a chimp. Vastly different outcomes, but relatively few genes regulate 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 the chimp and human genomes.

That's it; that's the 2 percent solution. What's shocking is the simplicity of it. Humans, to be human, don't need to have evolved unique genes that code for entirely novel types of neurons or neurotransmitters, or a more complex hippocampus (with resulting improvements in memory), or a more complex frontal cortex (from which we gain the ability to postpone gratification). Instead, our braininess as a species arises from having humongous numbers of just a few types of off-the-rack neurons and from the exponentially greater number of interactions between them. The difference is sheer quantity: Qualitative distinctions emerge from large numbers. Genes may have something to do with that quantity, and thus with the complexity of the quality that emerges. Yet no gene or genome can ever tell us what sorts of qualities those will be. Remember that when you and the chimp are eyeball to eyeball, trying to make sense of why the other seems vaguely familiar."

[Acknowledgement: Discover Magazine]

That was only like 1/3 of the whole article yet it is really fascinating to know that the number of neurons actually makes us so different from the chimps. There was a picture of the foot of a chimpanzee and it resembles so much like our limbs with the lines and everything. There were other differences mentioned in the article for example that "chimps have a great many more genes related to olfaction than we do; they've got a better sense of smell because we've lost many of those genes. The 2% distinction also involves an unusually large fraction of genes related to the immune system, parasite vulnerability, and infectious diseases; chimps are resistant to malaria, and we aren't; we handle tuberculosis better than they do. Another important fraction of that 2% involves genes related to reproduction - the sorts of anatomical difference that split a species in two and keep them from breeding." They make a whole lot of difference don't they.

Examine a neuron from a human brain under a microscope, then do the same with a neuron from the brain of a chimp, a rat, a frog, or a sea slug. The neurons all look the same: fibrous dendrites at one end, an axonal cable at the other.