David Haussler and colleagues have identified a 118 base pair sequence that has evolved really fast along the human lineage relative to the chimpanzee lineage (Carl Zimmer has a good review). In fact, this sequence differs by two base pairs out of 118 between chimpanzees and chickens, and 18 out of 118 between chimps and humans. Differences in relative rates usually indicate changes in selection regimes along at least one lineage. These changes could be due to increased selective constraint along one lineage, relaxed constraint, or adaptive evolution. More on that later.
Also interesting is that this sequence is not translated. It is located within a non-protein coding RNA (see Alex’s review of RNAs for more info), which performs some yet to be determined function. The authors did determine that the gene is expressed in the developing brain (specifically the neocortex) which means this is the gene that makes us so much damn smarter than the other apes. Or not. It’s also expressed in the ovaries and testes — quite normal for a rapidly evolving gene.
As I mentioned before, unequal rates of evolution can be due to a number of factors. Thankfully, these guys have heeded Andy Clark’s advice and did not just settle for comparative genomic data in reconstructing the evolutionary history of this gene:
Checking against available single nucleotide polymorphisms (SNPs; http://www.ncbi.nlm.nih.gov/SNP/) and resequencing this region in a 24-person diversity panel indicates that all 18 substitutions are fixed in the human population. Evidence from preliminary resequencing of a 6-kilobase (kb) region containing HAR1 shows levels of polymorphism and a positive skew in the frequency spectrum that are typical of European samples, suggesting that a recent selective sweep in this region of the genome is unlikely (Supplementary Notes S4). Thus, the changes in HAR1 clearly occurred on the human lineage, but probably took place more than 1 Myr ago.
This is exactly the type of study I was talking about when I discussed overcoming the rift between computational and wet lab biology. Good timing on that publication, guys. The researchers used computational approaches to identify sequences of interest (in this case sequences with high rates of evolution along the human lineage). They then performed some laboratory analyses on the function of one of these genes of interest and collected more data to determine when and how selection acted on this gene. Mad props on a study well done.