Human sialic acid biology is unusual and thought to be unique among mammals. 
Humans lack a functional cytidine monophosphate-N-acetylneuraminic acid 
hydroxylase (CMAH) protein and cannot synthesize the sugar Neu5Gc, an innate 
mammalian signal of self. Losing this sugar changed how humans interact with 
some of our deadliest pathogens: malaria, influenza, and streptococcus among 
others. We show that the New World monkeys, comprising the third of all primate 
species, have human-like sialic acid biology. They have lost Neu5Gc because of 
an independent CMAH inactivation ~30 million years ago (mya) (compared to ~3 mya 
in hominids). This parallel loss of Neu5Gc opens sialic acid biology to 
comparative phylogenetic analysis and reveals an unexpected conservation 
priority. New World monkeys risk infection by human pathogens that can recognize 
cells in the absence of Neu5Gc. This striking molecular convergence provides a 
mechanism that could explain the long-standing observation that New World 
monkeys are susceptible to some human diseases that cannot be transmitted to 
other primates.