One of the first things a biology student hears is that chimp and human DNA are 99% identical. But what exactly does that mean? And what is it about the difference that makes us human? What set us on the path to humanity?
It was Charles Darwin who recognised that chimps and humans are related (and he was ridiculed and tormented for it) and it was genetic research that revealed just how closely the two species are linked; a comparison of chimp and human DNA reveals that it's 99% identical. However ...
... on closer inspection we differ by 1.2% in the functional genes that code for proteins. And we also differ by about 3% in the non-coding DNA regions, so called "junk DNA" - although this phrase seems to be losing meaning as some of these regions regulate genes and possess as yet unknown functions. So overall we can say that chimp and human DNA is about 96% identical - which is still very close. If you were to lay both genomes out side by side you would see that base for base they are 96% similar.
A Common Ancestor
For millions of years chimps and humans were as one, and then about 6 million years ago (give or take) we split, and our evolutionary paths went in different directions. DNA comparison of both species should give us the answers as to why chimps remained chimps, with their dense body hair and protruding jaws, and why our brains grew, we managed to walk upright, and started to speak. Presumably, the answers to what turned us into humans are contained in the 4% of chimp and human DNA that differs?
The 4% difference between chimp DNA and human DNA represents 35 million single letter (nucleotide) differences, and about five million insertion/deletion events. That's a lot of DNA to be studied and as yet there hasn't been a single discovery where a scientist can say "ah yes, because of a mutation in that gene, our ape-like ancestors branched off from chimps." However, there are several regions which are promising targets.
For example there is one region of DNA that contains a gene important for speech FOXP2. It is highly conserved in humans, showing very little variation, which is suggestive of its importance. But in chimps there is much more variation.
That most of the DNA differences lie in the non-coding regions of DNA could mean that what accounts for chimp human differences are not the protein coding genes themselves, but how they are regulated and controlled.
Rapid DNA Changes
A 2006 study by scientists at UC Santa Cruz looked at highly conserved DNA sequences between chimpanzees, rats, and mice. These were then compared to the human genome to find areas that had changed rapidly since chimps and humans went their separate ways all those years ago. The thinking being that these rapidly changing areas could account for the differences.
The scientists discovered 202 "highly accelerated regions" and only three of these regions contained genes that code for proteins. The significant changes occurred in non-coding regions, and the most dramatic change was observed in a region, HAR1 that is believed to make a stretch of RNA that could be involved in brain development.
The answers are present in the chimp and human genomes, it's just that we haven't learned to read all the clues and signposts yet. But surely the time will come? One day we will know how we evolved to build cities, create concertos, write films, and why a chimp on Wall Street (make up your own joke) would look ridiculous.
The answers will not just satisfy our curiosity; they could help with the prevention and treatment of genetic diseases in chimps and humans, as well as securing more rights and protection for our 'cousins under the skin.'