2015-11-22 15:08:46
Although we may occasionally come across a specimen in the street that appears to have been raised in a zoo, the facial features of human beings are very different from those of chimpanzees. This fact seems the most normal thing in the world, but it is not so when we realize that the chimpanzee is the genetically closest species to ours.
This has its importance. Genetic relationships are essential in determining the body and facial shape of organisms. Thus cats, lynxes, leopards, and even tigers or lions, although different species, have faces very similar in general form. The same is true of horses, donkeys, and zebras, to take another example, and the same is true of many species of primates. Without going any further, you will not deny that chimpanzees and gorillas have a certain air of family. However, despite the fact that chimpanzees are genetically more related to us than they are to gorillas, strange as it may seem, it is not us that Chita’s relatives are most similar to. Why are humans so different from other primates when it comes to face?
As I have said on other occasions, science is not averse to wondering about anything and trying to answer its questions, no matter how far-fetched they may seem. In this case, the question seems unimportant, but answering it may lead us to find out how necessarily similar genes between humans and chimpanzees produce very different morphological results (if not, compare the face of my namesake George Clooney with that of the most handsome gorilla, in case you prefer not to compare it with that of your partner). Finding out this can, in turn, help us understand how changes in some genes can cause diseases or problems with fetal development, and try to avoid them.
Furthermore, the genes involved in shaping our faces have been very important in our evolution. Changes in the jaws and skull that affect the shape of our heads have been essential to accommodate a larger brain, capable of the intellectual feats of our species.
Unraveling which genes generate the shapes of the faces of humans and chimpanzees is not easy. Let us consider that faces are produced from billions of cells of bone, cartilage, skin, muscle…, which can be organized in an infinite number of ways, but that they do so in only one precise way. This organization depends on which genes are involved in generating the interactions and contacts between the cells so that they can reproduce and place themselves during fetal development in the places that correspond to them: nose, forehead, cheekbones, etc.
mothers of the face
Previous research has revealed that facial cells derive from stem cells, which throughout fetal development acquire their final fate by becoming adult cells. These craniofacial stem cells are generated five or six weeks after fertilization, from even more primitive embryonic stem cells, in the structure of the embryo called the neural crest, and from there they migrate to the face and skull.
Of course, extracting these cells from human and chimpanzee embryos for study, killing or damaging them in the process, is out of the question, for obvious ethical reasons. To circumvent this difficulty, the researchers use an interesting strategy. Taking advantage of the fact that adult cells (easily obtained from the skin or blood of animals or people) can become induced embryonic stem cells (called iPSCs) simply by stimulating the activity of four genes, the researchers generate iPSCs and place them in in a nutritious medium that stimulates them to become craniofacial stem cells.
Once converted into human or chimpanzee craniofacial stem cells, the researchers now compare not genes, which we already know are very similar in both species, but changes in the genome that affect how strongly some genes work. Previous research had also shown that changes in the intensity of gene function affect the craniofacial shape of various species.
The researchers thus discover more than a thousand regions in the genome that have been chemically modified differently in humans and chimpanzees. These epigenetic chemical modifications affect the intensity of functioning of the genes adjacent to them. Interestingly, some of the genes close to these regions were already known to affect craniofacial development, although others were unknown until now.
In particular, the researchers find that two genes, called PAX3 and PAX7, known to affect snout length in laboratory mice, function at higher levels in chimpanzees than in humans. In contrast, the BMP4 gene, which is involved in modulating the beak shape of some finch species or the mouth shape of some fish, functions at a higher level in humans than in chimpanzees.
Of course, the system used by the researchers is artificial and it becomes necessary to confirm the findings of unknown genes by other means. However, this study has revealed dozens of new candidate genes, whose operation at different levels of activity could affect the development of the skull and face. With these studies, the researchers claim to have launched a new scientific discipline they call “cellular anthropology,” which we hope will provide a wealth of insights into the genetics of human evolution.
Reference:
Prescott et al., Enhancer Divergence and cis-Regulatory Evolution in the Human and Chimp Neural Crest, Cell (2015),
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