Recent genetic studies, however, support Sarich and Wilson's early work. The complete human genome was published in 2004, and the chimp genome followed in 2005. That year, scientists at Arizona State University and Pennsylvania State University compared modern human mitochondrial, or maternal, DNA with chimpanzee, macaque and mouse DNA to determine the point at which each lineage diverged from our common ancestor. "Though we will never know the exact date of the split, we can estimate that date using differences in their DNA," explains Blair Hedges, an evolutionary biologist at Penn State. These differences, or mutations, are assumed to occur at a constant rate, which can be used to estimate how much time has passed since lineages diverged. This method, called the molecular clock, indicated that the human and chimp lineages split five million to seven million years ago, although more fossil-based research is needed to confirm that idea.