The Copernican principle derives its name from that of the Polish astronomer Nicolaus Copernicus, who in the sixteenth century removed the Earth from its privileged position at the center of the universe. Theories that obey the Copernican principle do not require humans to occupy any special place for these theories to work. Copernicus taught us that the Earth is not at the center of the solar system, and all the subsequent findings in astronomy have only strengthened our realization that, from a physics perspective, humans play no special role in the cosmos. We live on a tiny planet that revolves around an ordinary star, in a galaxy that contains hundreds of billions of similar stars. Our physical insignificance continues even further. Not only are there about two hundred billion galaxies in our observable universe, but even ordinary matter—the stuff that we and all the stars and gas in all the galaxies are made of—constitutes only a little over 4 percent of the universe’s energy budget. In other words, we are really nothing special. (In chapter 11 I will discuss some ideas suggesting that we should not take Copernican modesty too far.)

Both reductionism and the Copernican principle are the true trademarks of Darwin’s theory of evolution. Darwin explained just about everything related to life on Earth (except its origin) with one unified vision. One can hardly be more reductionistic than that. At the same time, his theory was Copernican to the core. Humans evolved just like every other organism. In the tree analogy, all of the youngest buds are separated from the main trunk by a similar number of branching nodes, the only difference being that they point in different directions. Equivalently, in Darwin’s evolutionary scheme, all the present-day living organisms, including humans, are the products of similar paths of evolution. Humans definitely do not occupy any exceptional or unique place in this scheme—they are not the lords of creation—but an adaptation and development of their ancestors on Earth. This was the end of “absolute anthropocentrism.” All the terrestrial creatures are part of the same big family. In the words of the influential evolutionary biologist Stephen Jay Gould, “Darwinian evolution is a bush, not a ladder.” To a large extent, what has fueled the opposition to Darwin for more than 150 years is precisely this fear that the theory of evolution displaces humans from the pedestal on which they have put themselves. Darwin has initiated a rethinking of the nature of the world and of humans. Note that in a picture in which only the “fittest” survive (as we shall soon discuss in the context of natural selection), one could argue that insects have clearly outclassed humans, since there are so many more of them. Indeed, the British geneticist J. B. S. Haldane is cited (possibly apocryphally) as having replied to theologians who inquired whether there was anything that could be concluded about the Creator from the study of creation, with the observation that God “has an inordinate fondness for beetles.” Today we know that even in terms of genome size—the entirety of the hereditary information—humans fall far short of, believe it or not, a fresh water ameboid named Polychaos dubium. With 670 billion base pairs of DNA reported, the genome of this microorganism may be more than two hundred times larger than the human genome!

Darwin’s theory, therefore, amply satisfies the two applicable criteria (which admittedly are somewhat subjective) for a truly beautiful theory. No wonder, then, that The Origin has elicited perhaps the most dramatic shift of thought ever brought about by a scientific treatise.

Returning now to the theory itself, Darwin was not content with merely making statements about evolutionary changes and the production of diversity. He regarded it as his main task to explain how these processes have occurred. To achieve this goal, he had to come up with a convincing alternative to creationism for the apparent design in nature. His idea—natural selection—has been esteemed by Tufts University philosopher Daniel C. Dennett as no less than “the single best idea anyone has ever had.”

One of the challenges that the concept of evolution posed concerned adaptation: the observation that species appeared to be perfectly harmonized with their environments, and the mutual adaptedness of the traits of organisms—body parts and physiological processes—to one another. This created a puzzle that confounded even the evolutionary minded among the naturalists that preceded Darwin: If species are so well adapted, how could they evolve and still remain well adapted? Darwin was fully aware of this conundrum, and he made sure that his principle of natural selection provided a satisfactory solution.