What makes us human?
What makes us human? Our ability to form complex social structures? Our use of language? Our concept of self? It is undeniable that, of the plethora of species that inhabit the Earth, humans are unique.
While similarities with our closest relative, the chimpanzee, are easy to spot, our furry friends are still very much animals and not people. So the recent publication of the draft sequence of the chimpanzee genome stands to teach us much not only about our closest evolutionary relative, but also about ourselves and what it is that makes us who we are.
Yet DNA sequencing is just the start of the story. You can think of the chimp genome as being a lot like a new textbook; it needs to be read before anything can be learnt from it. The way in which scientists plan to ‘read’ the 2.8 billion pairs of DNA letters that make up the chimp blueprint is to compare it with the human genome sequence published a few years ago. The aim is to gather information that will tell us about the important differences between these two species. So, while a quick flick through the genome has allowed scientists to glean a brief overview of how the two genomes differ, there is still a lot of work to be done.
Just how similar our DNA is to that of a chimp may come as a surprise to many when you look at just how much we vary on the outside. Consider our large cranial capacity, the fact that we walk on two legs, and our advanced brain development allowing the use of complex language. Plus it is rare to find a human with quite as much hair as a chimp! So when you consider the somewhat clichéd fact that only 1.2% of our DNA sequence actually differs from that of our great ape cousin, it becomes obvious that this small percentage of changes must have had a big impact on our evolution.
This importance of a small subset of genes in our human-ness has in fact been known for some time following the observation that humans share 88% of our genes with rodents and 60% with chickens, despite being little like either species. Because of the fairly recent divergence of humans and chimpanzees from their last common ancestor, we would expect the genomes of the two species to be much more similar than that seen with previously sequenced genomes such as that of the rat as, in an evolutionary scheme of things, there hasn’t been enough time for a huge number of changes to take place between humans and chimps. So when scientists compare the sequences, they are not focusing on those regions that are the same, but rather on the differences.
The real challenge is in determining which differences in our DNA are actually responsible for making humans distinct from apes. Although only 1.2% of the genome differs between chimps and humans, when you consider that there are around 3 billion nucleotides in a genome, it works out that there are tens of millions of differences between the two species. Some of the changes will simply be silent mutations that have occurred during the 5 million or so years since our evolution took a different route to that of the chimp and won’t actually have any effect on us. It is the mutations that change the DNA code in such a way that the encoded protein is altered that are of interest to scientists. Natural selection has chosen mutations based on the advantage they offer over the original. The combined effects of all of these mutations over the millennia have made us as we are today. A significant number of protein changing mutations appear to be clustered in genes involved in reproduction and immunity; giving a good starting point for further investigation of what makes us human.
Discovering exactly how the evolution of humans has diverged from that of our closest cousins is a big task that will be greatly helped by the upcoming publication of the genome sequence from a number of other primates, such as the orang-utan. Comparing the genome sequences of several species will allow evolutionary biologists to determine which changes are unique to Man and may therefore play a role in making us human. Hopefully, as well as gathering a better understanding of Human evolution, the chimp sequence might help us understand diseases, such as Aids, Malaria and Alzheimer’s, which affect people but not great apes.
