Recent studies have identified important genes that direct embryonic development. Specific developmental regulators help define larger body regions, such as heads and tails or the left and right sides of the body. David M. Kingsley explains how many key developmental genes are conserved among animals that look very different. A diversity of body forms can emerge from changing where and when these shared developmental regulators are expressed. Fossils suggest that similar developmental mechanisms were used in animals that evolved millions of years ago. From the Series:HHMI’s Understanding Biomedical Research Series