Taxonomy

The theory of organic evolution, developed by Charles Darwin in the mid-1800s, revolutionized taxonomy. In Origin of Species, Darwin proposed that "community of descent—the one known cause of close similarity in organic beings" be used as the basis for taxonomic classification systems. Ever since Darwin, taxonomists have tried to represent phylogeny (lines of evolutionary descent) in their classification systems. Thus, modern taxonomists who study plants or animals do not merely name and catalog species, they also try to construct evolutionary trees showing the relationships of different species.

The early evolutionary taxonomists relied on morphological features to classify organisms. Many early animal taxonomists found similarities in the embryos of related organisms and proposed that these similarities indicate evolutionary affinities. For example, early in development, all animals exist as a ball of cells, referred to as a blastula. The blastula of higher animals, called the coelomates, forms an invagination, called a blastopore, which later develops into the digestive cavity. Early taxonomists further divided coelomate animals into two large taxonomic groups, the protosomes, in which the blastopore opening develops into the mouth, and the deuterostomes, in which the blastopore opening develops into the anus. In contrast to the coelomates, the blastula of lower animals does not form a blastopore and these animals are called the acoelomates. When acoelomate animals are fully grown, they lack true digestive systems.

Thus, the early evolutionary taxonomists concluded humans are more closely related to segmented worms than to flatworms, since humans and segmented worms both have coelomate embryos, whereas flat worms have an acoelomate embryo. In addition, humans are more closely related to sea urchins than to segmented worms, since humans and sea urchins are both coelomate deuterostomes whereas round worms are coelomate protostomes.