| The Difference May Lie Not in the Sequence but in the Expression |
Most genes are shared across all animals. More than ninety-nine percent of human genes have a related copy in the mouse. As one examines animals that are more distantly related, the proportion of the genes they share decreases; however, despite about 500 million years of evolutionary separation, half the genes in the lowly sea squirt correspond to those found in humans. This remarkable conservation of gene structure is striking considering how much these animals differ in morphology, physiology, and behavior.
If they share so many of the same genes, why are different animals so different? Differences among species result largely from differences in the time and location of the genes' expression. Let us consider our closest relative, the chimpanzee. Not only do chimpanzees and humans share nearly all of the same genes, but the DNA sequences of those genes also are very similar between the two species. Svante Pääbo sequenced three million bases of the chimp genome and found that chimps and humans differ overall by less than two percent at the sequence level. (See the Human Evolution unit.) Based on the low sequence divergence, Pããbo hypothesized that the difference between humans and chimpanzees was due mainly to how the genes were expressed in the different species.
To test this hypothesis, Pããbo compared the expression pattern of 20,000 human genes in humans and chimps. He found that while expression levels were similar in liver cells and blood, there were larger differences in brain cells. This suggests that the human brain has increased the use of certain genes compared to those same genes in a chimp. So, it not so much the sequence of the genes that is important, but how they are expressed to make the cell's proteins that determines the unique characteristics of each organism.