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. So why are we so different?
The answer lies not in our genes as such. It is in how our genes work and interact, including those we have in common. This difference is particularly evident in the brain, where human genes are linked more closely in networks than the same genes in monkeys. Norwegian and US scientists uncovered this difference by studying a large number of genes from both species.
The scientists took the previous results of tissue samples from five chimpanzees and six humans. Here, the gene activity in the heart, kidneys, liver, testicles and brain of both species was analysed using what are called DNA microarrays, a tool that enables a single experiment to detect the level of activity for tens of thousands of genes in cell or tissue samples.
The researchers compared the activity levels in a total of 21 000 individual genes that were identical for both species in the 55 samples. They were all measured at the mRNA stage. This is an intermediate stage where a replica of the gene is on the way out of the cell to produce a protein, and reflects the activity level of the gene.
It turned out that about every fifth gene behaved differently in humans and chimpanzees in the samples that were taken from the brain, heart, kidney and liver, while almost half of the genes behaved differently in testicular tissue. In all, the group was able to identify 90 transcription factors that were particularly different in humans and chimpanzees, says Eivind Almaas, a professor of systems biology at NTNU, who participated in the genetic study conducted at the University of Illinois.
A gene itself has no other function than to be a working drawing for the essential building blocks in an organism’s cells. Organisms use genes like a recipe to build a molecule, usually a protein. The molecule then carries out what we commonly refer to as the gene’s function.
Different proteins are required for construction and maintenance of cells and molecules, and play important roles in all bodily functions. In the genes that are called transcription factors, however, the protein has a special function: It is sent exclusively to control other genes – which means it controls the production and function of other proteins.
Transcription factors turn other genes on and off, dampen or strengthen them, coordinate and regulate them. They can be compared to an advanced traffic lights system at a complicated and congested intersection. The researchers suspected that the difference between us and chimpanzees could be traced to coordinated changes in select transcription factors, and how they are used.
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 ʙᴏɴᴇ, 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 ʙᴏɴᴇs, 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 ᴅᴏᴍɪɴᴀɴᴛ 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.