Varki and colleagues Elaine Muchmore and Nissi Varki , for example, recently identified a genetic mutation that produces the first known biochemical difference between humans and great apes. In genetic terms, the difference between humans and chimps is just 1.6 percent. This comparatively tiny difference obviously produces some divergent results. But until Varki's discovery, no one had identified a single biochemical difference. Varki and colleagues found a difference within a gene ultimately responsible for the production of a cell-surface sugar called sialic acid.
Humans are the exception.
No one yet knows, but Varki offers some intriguing guesses, including one that may eventually help explain some of the processes that helped humans evolve bigger, smarter brains than their closest biological kin. Strangely, though, sialic acid hydroxylase isn't uniformly distributed. It is plentiful in the bodies of mammals, but present only in small quantities in their brains. In humans, of course, the enzyme is completely absent because the gene that produces it does not exist. Varki can't say, but he's looking for answers through experiments in which laboratory mice are genetically manipulated to express either an abundance of the enzyme or none at all.