, Biology, and team have found that repetitive DNA, not whole chromosome duplication, may be why the red vizcacha rat has the largest genome of all mammals. A native species of Argentina, the rats’ genome is roughly two-and-a-half times as large as the human genome, 102 chromosomes versus 46 for humans. That is twice the size of one of its closest relatives, the mountain vizcacha rat. The two species had a common ancestor as recently as five million years ago, which is a short period of time in evolutionary terms, according to Evans.