Figure 2.

Parallel evolution and loss of an innate self-signal. Humans cannot synthesize Neu5Gc, because human CMAH was inactivated over two million years ago (red). The inactivating mutation apparently fixed rapidly after originating, which suggests that the loss could have been adaptive—driven by pathogen avoidance, reproductive conflict, or a combination of the two. Independent losses of Cmah function have recently been found in New World Primates, Mustelids and several other groups. Figure modified from Springer and Gagneux (9). In some lineages, such as bats and toothed whales, only certain species lost the capacity to make Neu5Gc (indicated by lines that are both blue and red).