Your Inner Fish

Neil Shubin

The genes affect that affect the head segments lie next to those that affect the segments in the middle of the fly, the part of the body that contains the wings. These bits of DNA, in turn, lie adjacent to the ones that control the development of the rear part of the fly. There is a wonderful to the way the genes are organized: their position along the DNA strand parallels the structure of the body from front to back. (p. 108)

Comment: Read more about correspondence between genotype and phenotype later.

Jawless fish reveal another very important point: they have a very small number of odor genes. Bony fish have more, and still more are seen in amphibians and reptiles. The number of odor genes has increased over time, from relatively few in primitive creatures such as jawless fish, to the enormous number seen in mammals. We mammals, with over a thousand of these genes, devote a huge part of our entire genetic apparatus just to smelling. Presumably, the more of these genes an animal has, the more acute its ability to discern different kinds of smells. In this light, our large number of odor genes makes sense --- mammals are highly specialized smelling animals. Just think of what effective trackers dogs can be.

But where do all our extra odor genes come from? Did they just pop out of the blue? How this expansion happened seems obvious when we look at the structure of the genes. If you compare the odor genes of a mammal with the handful of odor genes in a jawless fish, the “extra” genes in mammals are all variations on a theme: they look like copies, albeit modified ones, of the genes in jawless fish. This means that our large number of odor genes arose by many rounds of duplication of the small number of genes present in primitive species.

This leads us to a paradox. Humans devote about 3 percent of our genome to odor genes, just like every other mammal. When geneticists looked at the structure of the human genes in more detail, they found a big surprise: fully three hundred of these thousand genes are rendered completely functionless by mutations that have altered their structure beyond repair. (Other mammals do use these genes.) Why have so many odor genes if so many of them are entirely useless?

… Primates that develop color vision tend to have large number of knocked-out smell genes. The conclusion is clear. We humans are part of a lineage that has traded smell for sight. We now rely on vision more than on smell, and this is reflected in our genome. In this trade-off, our sense of smell was deemphasized. In this trade-off, our sense of smell was deemphasized, and many of our olfactory genes became functionless. (p. 147)

Comment: The entropy law provides a more natural explanation. When there is no or little selection pressure, genes will deteriorate.