Change and Constraints in Animal Evolution

Of all the differences between ourselves and our amoeba‑like ancestors a billion years ago, one is paramount: we have bodies. Instead of a single cell, we are made of trillions. This vast collective isn’t just a bag of identical copies, but a menagerie of dozens of types of cells, organized into hundreds of body parts from spleens to eyelashes to skeletons to brains. And most remarkable of all, each of our bodies is built out of a single original cell. As that cell multiplies into an embryo, genes begin producing proteins that control its development. Some of the proteins switch on other genes or shut them down; some leave the cell where they were made and spread away, to act as signals to neighboring cells, which respond by taking on new identities, or by crawling through the embryo to find a new home. Some divide madly, while others commit suicide. And when this dance is over, our bodies have taken shape.

There are millions of other kinds of bodies on Earth, from tentacled squids to quilled porcupines to mouthless tapeworms. They are marvels to behold; their origins are a grand challenge to comprehend. All animals descend from a common single‑celled ancestor, but scientists are still learning how it was that they diversified into so many different bodies. The answer lies both within animals and outside them, in their genetic history and the ecology in which they lived.

Scientists have only just begun exposing the ways that genes build animals, but their results have already proved to be revolutionary. Most animals, including us, use a standard tool kit of body‑building genes. It contains tools for marking off the coordinates of an animal’s body–front and back, left and right, head and tail. It also contains a set of genes that control the development of entire organs such as eyes and limbs. The tool kit is remarkably unchanged from one species to another–a gene that controls the growth of eyes in a mouse can be donated to a fly and build its eye instead.

Judging from the fossil record, this tool kit must have gradually evolved in the millions of years that preceded the Cambrian explosion. It gave animals an extraordinary flexibility for evolving new forms. Simply by making a few changes–altering the timing of a gene’s activity, for example, or the places where it became active–the tool kit could produce dramatically new body plans. On the other hand, as diverse as animals have become, they have obeyed certain rules. There are no six‑eyed fish or seven‑legged horses. It appears that the tool kit shuts down certain paths of evolution.

The diversification of animals has also been controlled by the environment in which animals evolve. Any new kind of animal has to find a place in its ecosystem where it can survive. Otherwise it simply disappears. The fate of any new kind of animal is far from predictable, often depending on random strokes of luck and accidental fortune. Consider vertebrates that live on land. They all have four limbs with digits (or, in the case of snakes, descend from ones that did). But that doesn’t mean that this design evolved simply because it was the best possible way to walk on land. In fact, legs and toes evolved on fish millions of years before they left the water. Only later did they turn out to allow vertebrates to move on dry land. The great transformations that animals have gone through all contain the same lesson: evolution can only tinker with what the history of life has already created.