Each species that lives on Earth today is the product of an evolutionary lineage — that is, it arose from a preexisting species, which itself arose from a preexisting species, and so on back through time. For any two species living today, their evolutionary lineages can be traced back in time until the two lineages intersect. At that intersection is the species that was the most recent common ancestral species of the two modern species. (Sometimes, this common ancestral species is referred to as the common ancestor, but this term refers to a group of organisms rather than to a single ancestor.) For example, the common ancestor of humans and chimpanzees was a species estimated to have lived 6 to 7 million years ago, whereas the common ancestor of humans and the puffer fish was an ancient fish that lived in the Earth’s oceans more than 400 million years ago. Thus, humans are not descended from chimpanzees or from any other ape living today but from a species that no longer exists. Nor are humans descended from the species of fish that live today but, rather, from the species of fish that gave rise to the early tetrapods.
If the common ancestor of two species lived relatively recently, those two species are likely to have more physical features and behaviors in common than two species with a more distant common ancestor. Humans are thus far more similar to chimps than they are to fish. Nevertheless, all organisms share some common traits because they all share common ancestors at some point in the past. For example, based on accumulating fossil and molecular evidence, the common ancestor of humans, cows, whales, and bats was likely a small mammal that lived about 100 million years ago. The descendants of that common ancestor have undergone major changes, but their skeletons remain strikingly similar. A person writes, a cow walks, a whale swims, and a bat flies with structures built of bones that are different in detail but similar in general structure and relation to each other.
Evolutionary biologists call similar structures that derive from common ancestry “homologies.” Comparative anatomists investigate such homologies, not only in bone structure but also in other parts of the body, and work out