Darwin was sufficiently aware of the importance of the concept of speciation to his theory to include a schematic diagram of his tree of life. (Figure 3 shows the original drawing from his 1837 notebook.) In fact, this is the only figure in the entire book. Fascinatingly, Darwin included the caveat “I think” at the top of the page!

In many cases, evolutionary biologists have been able to identify most of the intermediate steps involved in speciation: from pairs of species that have probably recently split from a single species, to pairs that are just about ready to be pushed into separation. At the more detailed level, a combination of molecular and fossil data has yielded, for instance, a relatively well-resolved and well-dated phylogenetic tree for all the families of living and very recently extinct mammals.

I cannot refrain at this point from digressing to note that from my own personal perspective, there is another aspect of the notions of a common ancestor and of speciation that makes Darwin’s theory truly special. About a decade ago, while working on the book The Accelerating Universe, I was trying to identify the ingredients that make a physical theory of the universe “beautiful” in the eyes of scientists. In the end, I concluded that two of the absolutely essential constituents were simplicity and something that is known as the Copernican principle. (In the case of physics, the third ingredient was symmetry.) By “simplicity,” I mean reductionism, in the sense that most physicists understand it: the ability to explain as many phenomena as possible with as few laws as possible. This has always been, and still is, the goal of modern physics. Physicists are not satisfied, for instance, with having one extremely successful theory (quantum mechanics) for the subatomic world, and one equally successful theory (general relativity) for the universe at large. They would like to have one unified “theory of everything” that would explain it all.

Figure 3