Previous studies have found that many of these same genes have also evolved rapidly in humans, accumulating changes in their coding sequence as well as in expression rates. "Together," they add, "these findings raise the possibility that the function and regulation of transcription factors have been substantially modified in the human lineage."
This is a very efficient way to make big changes with very little effort, according to Gilad. By altering transcription factors, the entire regulatory network can change with very few mutations, increasing the impact and minimizing the risk.
"The big question," he said, "is why are humans so different? What sort of changes in the environment or lifestyle would drive such a rapid shift in the expression of genes -- in this case in the liver -- in humans and in no other primate?"
Part of the answer, he suspects, is rapid alterations in diet, probably related to the acquisition of fire and the emerging preference for cooked food. "No other animal relies on cooked food," he said. "Perhaps something in the cooking process altered the biochemical requirements for maximal access to nutrients as well as the need to process the natural toxins found in plant and animal foods."
This is just the first of a series of similar studies, said Gilad, that will look at changes in gene expression over evolutionary time. The next steps are to look at larger arrays of genes and to focus on other tissue types.