Figure 19.1 Living things may be single-celled or complex, multicellular organisms. They may be plants, animals, fungi, bacteria, or archaea. This diversity results from evolution. (credit wolf: modification of work by Gary Kramer; credit coral: modification of work by William Harrigan, NOAA; credit river: modification of work by Vojtěch Dostál; credit fish modification of work by Christian Mehlführer; credit mushroom: modification of work by Cory Zanker; credit tree: modification of work by Joseph Kranak; credit bee: modification of work by Cory Zanker)

Evolutionary medicine is an emerging field that applies evolutionary theory to modern medicine. Rather than just seeking answers to how illness occurs, evolutionary medicine also asks why illness occurs. This approach to medicine has led to many important advances. For example, endogenous retroviruses (ERVs) are pieces of retroviruses that began invading mammalian genomes over 100 million years ago. While studying why smaller mammals tend to get cancer more frequently than larger mammals, scientists discovered that larger mammals have had fewer ERVs invade their genome. Because retroviral integration is associated with cancer, results from this research suggest the possibility that larger mammals are able to control EVR replication until they reach post-reproductive age. More on this research can be found on the PLOS Pathogens website.