Chimpanzees may be humanity’s closest relative, genetically speaking, but it would be hard to tell by looking at their faces. Side by side, chimps have more prominent brows, bigger ears, stubbier noses and a considerable amount more hair than most humans. Now, researchers at Stanford University believe they are closing in on why people look so different from our close chimp cousins.
Scientists have long known that chimpanzees are closely related to humans and recent genetic sequencing has revealed that humans share 99 percent of their DNA with chimps. But when it comes down to developing facial features, the many difference lie with how that 99-percent-similar DNA is regulated and expressed.
“If we want to understand what makes human and chimp faces different, we have to look to the source — to the cell types responsible for making these early patterning decisions,” study author Sara Prescott said in a statement. “If we were to look later in development or in adult tissues, we would see differences between the species but they will tell us little about how those differences were created during embryogenesis.”
To pinpoint exactly where chimp and human faces start to differ, Prescott’s team compared segments of DNA that determine how specific genes are expressed in “neural crest cells,” a type of cell that eventually develops into bone, cartilage and facial tissue. Prescott watched to see which genetic regions were activated as her samples of neural crest cells grew, eventually determining that there are about 1,000 groups of genes that triggered in different ways during the development of facial features in chimps and humans. The researchers also found that chimps expressed two genes known to affect nose length and shape as well as skin color much more strongly than humans.
“It’s becoming clear that these cellular pathways can be used in many ways to affect facial shape,” Joanna Wysocka, the study’s senior author, said in a statement.
The face isn’t the only place that shows off how apes and humans share a common ancestor: scientists are also looking to the shoulders for clues to why humans and chimps look the way they do. According to new studies of Australopithecus shoulder bones, humans actually have more “primitive” shoulders than chimps or gorillas, Rachel Feltman writes for The Washington Post. In this case, “primitive” means human shoulders have more in common with a monkey’s – the last common ancestor we shared with our ape cousins.
"These changes in the shoulder, which were probably initially driven by the use of tools well back into human evolution, also made us great throwers," evolutionary biologist Neil T. Roach and one of the study’s authors said in a statement. "Our unique throwing ability likely helped our ancestors hunt and protect themselves, turning our species into the most dominant predators on earth."
While scientists are still searching for any sign of that common ancestor, they can still find hints at how apes and humans split by poking around in their genes.