Darwin's Predictions
Embryology is "the strongest single class of facts in favor of change of forms."
Darwin considered the process that a fertilized egg goes through on the way to becoming a multibillion-celled adult as crucial to understanding how basic anatomical parts of both animals and plants change over evolutionary time. The new science of evolutionary developmental biology, "evo devo," is revealing the truth behind Darwin's assertion quoted above. Until recently, scientists could say that forms do change, but they couldn't say exactly how, because they didn't know which genes were essential for the development of any organism. They now know that, remarkable as it may seem, all organisms on Earth, from microbes to man (a human embryo is seen here), share a common "tool kit" of so-called master genes, which govern how bodies and body parts form. When and which of these genes are turned "on" or "off" during the embryonic stage determines how each organism is built.