Biology Reference
In-Depth Information
shows a five week-old ectopic human embryo with a clearly visible tail arrowed.
The tail is located at the end of the coccyx, a series of four fused vertebrae at the
base of the spine that persists in the adult human. The coccyx is the remnant of the
embryonic tail that is left after the tail has been destroyed. There are rare reports of
some human babies being born with tails that can be up five inches in length. Such
tails result because the destruction of the embryonic tail fails to go to completion,
probably because of mutations in the developmental or programmed cell death path-
ways. The genes that control tail development in mice have been identified. Variant
mice that do not develop tails have a mutation in one of these genes that reduces
its activity. So a plausible explanation of the rare examples of human tails is that
another mutation has increased the activity of this gene in these individuals. A sim-
ilar explanation would account for occasional reports of whales with external hind
legs.
Vestigial structures at the molecular level are called pseudogenes. These are
defined as genes that have lost their original protein-coding ability. They can be
identified because enough of the original gene sequence remains to enable its orig-
inal function to be recognised by comparison with similar genes that have retained
their function. About 19,000 pseudogenes have been identified so far in the human
genome. This number is almost the same as the number of protein-encoding genes
so far identified, which is 19,042. Good examples of pseudogenes are found in the
large family of genes encoding the proteins that enable animals to smell.
Most animals possess cells that enable them to detect chemicals in the environ-
ment. In mammals the sense of smell is created by membrane proteins located at
surface of cells lining the nasal passages. These proteins are called olfactory recep-
tors because they are capable of binding a very large range of different chemicals
present in the air. Each protein detects only one type of such chemical, so there are
many such proteins. They can be recognised by two features; they all fall into one
class of proteins with very similar amino acid sequences, and hence similar con-
formations, and they are found only in the cells lining the nasal passages. These
cells are called olfactory neurons because they are nerve cells, whose other end is
located in the olfactory centre in the brain where the signals coming along the nerve
fibres are interpreted as distinct smells. There are more than one thousand olfactory
receptor genes in the human genome, making it the largest family of related genes
in this species - a superfamily. But more striking still, is that about 60% of these
genes occur as pseudogenes in humans - that is, their sequence has mutated in such
a way that the complete proteins can no longer be made.
How can we account for the occurrence of some many pseudogenes in this human
superfamily? The suggested explanation is that the animal ancestors of humans
relied on their sense of smell to a greater extent than humans do. In support of this
evolutionary explanation is the finding that in mice only 20% of this superfamily
occurs as pseudogenes, while in chimpanzees and gorillas the fraction of pseudo-
genes is around 30%. As human ancestors became more reliant on vision than on
smell, mutations inactivating the olfactory receptor genes became less susceptible
to natural selection, and so they persist today in the form of pseudogenes. It is possi-
ble that some of these pseudogenes have acquired new functions, because although
