Years ago, scientists noted with surprise that widely different organisms appear remarkably similar during the early stages of their development. Experts in embryology couldn't distinguish, for instance, between the early embryos of a bird and a human. Why should embryos so closely resemble each other?

This question has led to an appreciation of evolution as a master tinkerer. A tinkerer does not start from scratch each time, but instead makes use of suitable old bits and pieces to build new things. Once a program works well for building a fish, keep it and use it later as a foundation for building a person.

Over and over in our study of genes and proteins, we discover the effects of tinkering during evolution. The Hox genes of fruitflies are very similar in nucleotide sequence to the genes that perform the same body-shaping function in many other animals, although each species has its own unique sequence. Clearly, these varied genes evolved from a single gene in a remote common ancestor.