Our nearest primate relative, the chimpanzee, shares much of its genome with us. And yet, despite the astounding similarities in our DNA sequences, it's not difficult to discern the face of one species from the other.
Developmental biologist Joanna Wysocka, PhD, researches, among other things, how human faces are formed during early embryonic development. She and graduate student Sara Prescott compared gene expression patterns between humans and chimpanzees in the hopes of identifying not just what makes us recognizably human, but also how human faces also differ among themselves.
They describe their work, which was published today in Cell, as a kind of "cellular anthropology" that can illuminate important genomic tweaks in our recent evolutionary past. In particular, they found that the critical differences between the two species lie not in the DNA sequence of the genes themselves, but in when and where (and to what levels) the genes are made into proteins during development. These changes have led to important, human-specific adaptations. As Wysocka explained in our release:
We are trying to understand the regulatory changes in our DNA that occurred during recent evolution and make us different from the great apes. In particular, we are interested in craniofacial structures, which have undergone a number of adaptations in head shape, eye placement and facial structure that allow us to house larger brains, walk upright and even use our larynx for complex speech.
Those features are first formed, however, shortly after conception, making the steps particularly difficult to study in both humans and chimps. As I describe in our release:
To conduct the study, however, Prescott and her colleagues had to obtain a specialized type of cell present only in very early primate development. The cells, called cranial neural crest cells, originate in humans within about five to six weeks after conception. Although they first appear along what eventually becomes the spinal cord, the neural crest cells then migrate over time to affect facial morphology and differentiate into bone, cartilage and connective tissue of the head, and face.
The researchers used induced pluripotent stem cell technology to create iPS cells from easily obtained tissue samples like hair and skin from humans and chimpanzees. They then cultivated the cells under specific growing conditions to cause them to develop into neural crest cells to study in a laboratory dish.
More from Wysocka:
Of course, humans and chimps are very closely related. Most of the regulatory elements are the same between the two species. But we did find some differences. In particular, we found about 1,000 enhancer regions that are what we termed species-biased, meaning they are more active in one species or the other. Interestingly, many of the genes with species-biased enhancers and expression have been previously shown to be important in craniofacial development or associated with normal intrahuman facial variation.
Previously: Nomadic cells may hold clues to cancer's spread, Stanford researcher wins Vilcek Prize for Creative Promise in Biomedical Science and My baby, my… virus? Stanford researchers find viral proteins in human embryonic cells
Illustration © 2015 Mesa Schumacher