A trait like an arm is encoded in many genes, which cooperate with one another to build it. Some genes produce proteins that physically join together to do a job. In other cases, a protein encoded by one gene is required to switch on other genes.
It turns out that clusters of these genes — sometimes called modules — tend to keep working together over the course of millions of years. But they get rewired along the way. They respond to new signals, and act to help build new traits.
Homology refers to structural similarities of organisms - e.g., bats and humans have limbs with five digits. The reuse of genetic structure is termed "deep homology".
Another example:
Neurons communicate with each other by forming connections called synapses. The neurons use a network of genes to build a complete scaffolding to support the synapse. In February, Alexandre Alié and Michael Manuel of the National Center for Scientific Research in France reported finding 13 of these scaffold-building genes in single-celled relatives of animals known as choanoflagellates.
We are built from these versatile building blocks that take on different functions depending on context. It is this very property that lies at the heart of why evolution is possible.
