Science: A spinal column

IT IS amazing what Charles Darwin achieved, considering how little he knew about genetics and embryology.

All evolution of body form is dependent on changes in the genes controlling the development of an embryo. New combinations of genes in the fertilised egg gave rise to these changes. Our own limbs evolved from the fins of our fishy ancestors. With time they became astonishingly successful organs not just for walking, but for manipulation. Meanwhile, snakes gave up their legs. A recent report reveals the mechanisms by which this happened - just the sort of evidence Darwin needed to provide the foundations for his theory.

Pythons have more than 300 vertebrae with ribs on each one, almost all the way back to the tail. These ribs and vertebrae are not unlike those in our chests; it is as if, somehow, the chest region of the lizard ancestor from which they evolved became enormously elongated. There is no sign of any forelimb, but near the tail is a rudimentary leg with a small femur.

How did the chest region become so elongated? This is related to the number of segments into which the body is subdivided during development, and how each is given an identity. During early development, tissue in the main body axis is divided into small blocks of tissue that later give rise to vertebrae. Humans have seven vertebrae in the neck and 12 in the chest. The different character of these vertebrae - we do not, for example, have ribs in our necks or lower backs - is determined by a set of genes known as Hox genes.

Hox genes were discovered in the development of the fruit fly, drosophila. Mutations in particular genes were found to change the antenna of the fly into a leg. It turns out that a certain set of genes is responsible for putting the right structures into place along the body axis. These genes are all on the same chromosome; the order in which they range along it corresponds to the order in which they are expressed along the body axis of the developing embryo.

The development of all vertebrates appears to be remarkably similar to that of the fly. Many key genes controlling the development of the fly also control our own development. It seems that once evolution found ways to generate patterns in the embryo, it stuck to them and used them in different ways, rather as folding a simple piece of paper can give rise to a huge range of shapes.

In us, as in flies, Hox genes pattern our main body axis; they determine where our neck vertebrae will be and where those of our chests will begin and end. Different Hox genes are activated to determine what sort of vertebra each block of tissue will give rise to. In snakes such as the python, it has just been shown that the Hox gene normally associated with chest vertebrae is expressed along most of the body axis, which explains why nearly all vertebrae carry ribs. Only near the tail does the pattern of expression change; that is where the leg bud forms. So a change in the pattern of Hox gene expression could have resulted in a sudden and rapid evolution of snakes. Darwin would have been pleased.

The writer is professor of biology as applied to medicine at University College London