The evolution of life on Earth

Abstract

Figure 5.1 is the tree of life, which shows the relationship between the various life forms on Earth. As we go down the tree we are reaching further back in time. This is a highly simplified version, but one that is perfectly adequate for our purposes. The full version, from which Figure 5.1 has been derived, has been established by comparing the biological dissimilarity (distance) between different species of organisms by comparing their RNA and DNA. For example, domestic cats are a species close to the various wild species of cat (e.g., leopards) in that they have almost the same DNA and RNA. By contrast, there is a greater difference between domestic cats and dogs, and an even greater distance between domestic cats and any species of oak tree. Distances can be measured between all organisms, unicellular and multicellular.