Darwin's theory of common descent is a basic theory in modern biology which states that different species of living organisms can ultimately be traced to a single common ancestor. Ultimately, at least in theory, it leads to the conclusion that all forms of life-bacteria, fungi, plants, and animals-can be traced back to a single universal common ancestor. Over hundreds of millions of years, life branches from a single species into multiple ones through natural selection.
Charles Darwin's The Origin of the Species, published in the 1850s, grew out of Darwin's revolutionary observations of the more than one dozen separate species of finches which inhabit the Galapagos Islands in the Pacific Ocean west of Ecuador. Darwin noticed that each finch species differed only slightly from the others, mainly in terms of beak structure, and was adapted specifically to the different food types which were most prevalent on different islands and in different ecological niches. Moreover, all of these variations appeared to be found only in the Galapagos Islands. He speculated that all of the finches in the Galapagos Islands were descended from a single population of common finches which migrated from the mainland.
Essentially, says Lynn Fancher of the College of DuPage, there are two parts to the Darwinian theory of evolution. Natural selection, she says, is "the how part of the theory"-basically, the idea that only the fittest will survive and pass on their genes to the next generation. Common descent, however, is "the 'what' part of the theory." Over millions of years of natural selection, one common ancestor can give rise to populations of completely separate species.
These two parts of the Darwinian approach must operate together for the theory of evolution as a whole to work. The idea that a large population today can all be traced back to a single ancestor is not controversial within a single population: It's simple mathematics. On a statistical basis, for instance, every person of at least partly European descent is probably a descendant of the medieval monarch Charlemagne.
What Darwin added to this, however, is that because of the gradual but steady differences which build up over time as a result of natural selection, populations that share common descent won't necessarily resemble each other. Each population might have been separated into separate regions, or ecological niches, in which they gradually became specialized to their particular surroundings and food sources. Over a lengthy period of time, this would lead them to become separate species, like Darwin's finches.
Over a much longer period of time, the differences would be much greater. For instance, Darwin's theory of common descent states that all mammals share a single common ancestor, the first mammal; that all mammals and reptiles together share a single common ancestor that gave rise to both types of animals; that all animals can be traced back to a single early multicellular organism; and, ultimately, that all forms of life on Earth can be traced back to a single universal common ancestor.
These ideas about the first forms of life are, of course, speculative. However, there are two basic types of data that are commonly cited as evidence for the common descent of related species today, which, according to the theory of evolution, share a somewhat more recent common ancestor. First, it is possible to trace gradual changes over time in the fossil record, as has been done for early hominids-from Homo sapiens to Homo erectus, Homo habilis and the australopithecines.
Secondly, and much more recently, molecular biologists have studied the common appearance of specific genes in the DNA of different animal species as evidence that those separate species actually share a common ancestor. In this case, the hypothesis is that because complex segments of their genetic code match, the most likely explanation is that they are both descended from a single common ancestor that also had that segment of code in its DNA.