Thanks to the many people who left comments on my recent post about some recent work on the intersection of stem cells and human evolution. I noticed that several people expressed variations on the same theme, one which deserves a response. To recap briefly: a great deal of research indicates that a couple million years ago, our hominid ancestors lost the ability to make one of the main sugars that coat mammal cells, called Neu5Gc. This ancient chapter in our history turns out to have a big effect on current research on embryonic stem cells. When human stem cells are raised on a substrate made of mouse cells or calf serum, they absorb the nonhuman Neu5Gc sugars, which ends up