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Ajit Varki from University of California, San Diego believes to have found out what it means to be human. He’s been investigating a molecule called Neu5Gc, a variant of sialic acid, and his progress has been summarized in this news article. Neu5Gc functions as a marker or tag which identifies cells and helps them stick together. It also helps regulate immune response.

Chimpanzees, bonobos, gorillas, orangutans are able to produce Neu5Gc, but curiously humans do not produce Neu5Gc. Humans do produce Neu5Ac, a precursor to Neu5Gc. But we do not have the enzyme that helps tack on an extra oxygen atom to make Neu5Ac to Neu5Gc. Neandertals, also, do not have the enzyme to convert Neu5Ac. Clearly there was potentially a strong human lineage specific loss of function mutation somewhere during human evolution. The Neu5Gc that is found in us is found from red meat and milk products.

Varki believes that this difference, potentially explains some of the more unusual differences between humans and apes,

“Chimpanzees do not seem to suffer from heart disease, cancers, rheumatoid arthritis or bronchial asthma – common conditions in humans. Nor do they get sick from the human malaria parasite, which uses sialic acid to latch on to our blood cells.”

He’s found that some people produce antibodies that react to Neu5Gc. When an antibody targets a foreign molecule, it triggers inflammation. This observation, seeing how Neu5Gc elicits an immune reaction to create anti-Neu5Gc antibodies, further points to some strong selection against Neu5Gc. Varki believes that the mutation that prevents processing Neu5Ac into Neu5Gc helped shrug off a particular disease.

There’s one glarring thing that this news article gets wrong. It quotes that, Varki and team,

“estimates that the genetic change first appeared up to three million years ago, which coincides with the emergence of Homo erectus, the first of our ancestors to venture out of Africa.”

Uhh, there were no Homo erectus around 3 million years ago. There were ausrtalopithecines then… but the genus Homo didn’t emerge until 500,000 years later. Regardless, there was most definately a mutation that occurred after our last common ancestor with great apes. The irony is that what may have protected our ancestors, is now partially responsible for many diseases, such as malaria and even cancer.

    Tangvoranuntakul, P. (2003). Human uptake and incorporation of an immunogenic nonhuman dietary sialic acid. Proceedings of the National Academy of Sciences, 100(21), 12045-12050. DOI: 10.1073/pnas.2131556100