Scientists have identified the protein and genetic regulations that make human and chimpanzee faces different, despite nearly identical genetic backgrounds of the two species.
The key lies in how genes involved in facial development and human facial diversity are regulated - how much, when and where the genes are expressed - rather than dissimilarities among the genes themselves.
In particular, the researchers found that chimps and humans express different levels of proteins known to control facial development, including some involved in jaw and nose length and skin pigmentation.
"We are trying to understand the regulatory changes in our DNA that occurred during recent evolution and make us different from the great apes," said senior author Joanna Wysocka, from the Stanford University School of Medicine.
"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," said Wysocka.
The researchers coined the term "cellular anthropology" to explain how some steps of early primate development can be mimicked in a dish, and thus used to study gene-expression changes that can shed light on our recent evolutionary past.
For their comparison, the researchers focused on areas of DNA known as enhancer regions in human and chimpanzee genomes.
These regions contain chemical tags and proteins bound to the DNA that control how nearby genes are expressed.
For the study, researchers had to obtain a specialised 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.
"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," said lead author Sara Prescott, graduate student at Stanford University.
To obtain this elusive cell type, the researchers used induced pluripotent stem cells, or iPS cells, made from chimpanzees. IPS cells, which are made from easy-to-obtain skin or blood samples, can be coaxed to become other tissues.
Researchers coaxed human and chimpanzee iPS cells to become cranial neural crest cells by growing them in the laboratory under a specific set of conditions.
The researchers found that two genes, PAX3 and PAX7, known to affect snout length and shape in laboratory mice, as well as skin pigmentation, were expressed at higher levels in chimpanzees than in humans.
The study was published in the journal Cell.
