Charles Darwin was no theoretical physicist, and I am no biologist. Yet, as a theoretical physicist, I have found much to think about in Darwin’s legacy — and in that of his fellow naturalist Alfred Russell Wallace. Darwin’s style of science is not usually thought of as theoretical and certainly not mathematical: he was a careful observer of nature, kept copious notes, contributed to zoological collections; and eventually from his vast repertoire of observation deduced the idea of natural selection as the origin of species. The value of theorizing is often dismissed in the biological sciences as less important than observation; and Darwin was the master observer.

I think that view misses something essential, namely the great formal beauty and almost mathematical inevitability of Darwin’s ideas. Like Einstein’s greatest ideas, the theory of evolution is based on a simple gedankenexperiment: start with a very simple reproducing organism, add Mendel’s laws of heredity and mutability, and follow the system as it inescapably branches out into a tree of life.

Darwin was not particularly interested in astronomy or physics, yet his impact on cosmology was enormous but in a way subconscious. In successfully explaining the origin of species, he eliminated superstition and set a new standard for what an explanation of nature should be like. As I wrote in my book The Black Hole War (Little Brown, 2008), Darwin’s masterstroke was to have “ejected God from the science of life”.

True, Darwin was not the first scientist to cast out supernatural beliefs. Two centuries earlier, Newton — another great Cambridge scientist — had done so more than anyone before his own time. Inertia (mass), acceleration and a universal law of gravitation replaced the hand of God, which was no longer needed to guide the planets. But as historians of 17th-century science never tire of reminding us, Newton was a Christian and a passionate religious believer at that. He spent more time, energy and ink on Christian theology than on physics.

For Newton and his peers, the existence of an intelligent creator must have been an intellectual necessity: how else could you explain the existence of man? Nothing in Newton’s vision of the world could explain the creation, from inanimate material, of so complex an object as a sentient human being. Newton had more than enough reason to believe in a divine origin.

But what Newton failed to do, two centuries later the ultimate (and unwilling) subversive Darwin succeeded at. Darwin’s idea of natural selection — combined with the subsequent discovery by James Watson and Francis Crick (also at Cambridge) of the double-helix structure of DNA — replaced the magic of creation with the laws of probability and chemistry.

In other words, before Darwin, even the greatest physicists had little alternative to a supernatural explanation of the origin of life, and therefore of nature itself. It was the success of Darwinism that forced the issue and set the standard for future theories of origins, whether it be it of life or of the universe. Explanations must be based on the laws of physics, mathematics and probability — and not on the hand of God.

Rejecting the watchmaker

Early in his life, Darwin was deeply impressed by the arguments of the Reverend William Paley (1743–1805), a cleric who had argued for what we would today call “intelligent design”. Paley imagined finding a pocket watch lying on the ground, perhaps while walking in the woods. He might have wondered how such a complex, fine-tuned object came into existence. One possible answer is that it might have been the result of a random accident; a large number of molecules of various types combining by good fortune to form the watch. Paley rightly said that this was too unlikely to be taken seriously. There must be another explanation. The only one that made sense was that the watch had been made for some particular purpose by a skilled craftsperson — the watchmaker.

Paley pursued the thought further. We find in nature certain incredibly complex mechanisms, called human beings, that are capable of far more complex operations than the pocket watch. By analogy Paley argued that accidental creation is too unlikely and that human beings must have been created by an intelligent creator for some purpose.

How and why Darwin came to reject Paley’s compelling argument is well known, but what is less noted is that physics and cosmology pose very similar questions, such as why the universe seems so incredibly fine-tuned for the existence of life. The only explanation, if we can call it an explanation, is that if it were less fine-tuned, intelligent observers like ourselves would have been impossible. I am, of course, referring to the cosmological constant, L. Theoretically, one would expect L to be unity in natural Planck units. But if it were anything bigger now than it is known to be — 10^–123 — it would have prevented the evolution of galaxies, stars and us. Like Paley, we encounter what appears to be an extremely unlikely occurrence.

Most physicists reject a supernatural explanation — a cosmic watchmaker — to account for this fact of fine-tuning. But if not a watchmaker, then what? Until recently, most physicists would have said that it was accidental, a numerical coincidence. The ambition of theoretical physics was to discover a unique mathematical explanation, having nothing to do with our own existence, for all the constants of nature. It would be just a lucky accident that they happened to fall into the narrow range where intelligent life can exist. But as Paley might have complained, accidents involving 123 decimal places are too unlikely.

Enormity of the landscape

Over the last decade a new view has been taking shape, a view that in certain ways has common features with biological evolution. Darwin and Wallace emphasized mutability and natural selection as the main drivers of evolution, but there is something even more basic. Mutability and natural selection would have been powerless to create a human being if it were not for one central fact: the enormity of the landscape of biological designs.

Biological designs are encoded in DNA molecules, which contain two polynucleotide chains twisted around each other to which with four different based pairs (A, G, C and T) are attached. In a complex creature each of these DNA molecules can contain many millions of base pairs. The possible arrangements of those base pairs define the biological landscape, and the number of possibilities is tremendously large. One hundred million base pairs, for example, can be arranged in 4^{100 000 000} ways.

Suppose for a moment that there were only a thousand possible designs, or even a million. What would be the likelihood that any of them would make an intelligent life-form? Completely negligible. But even if such fortunate designs are extremely rare, given 4^{100 000 000} combinations there will be a very large number of them. The first principle of biological evolution — even more fundamental than natural selection — is the enormity of the landscape of biological designs.

The second principle is mutability: the fact that while reproducing, the instructions coded in DNA can discretely jump to new configurations. Natural selection is of course important, but without the mutable landscape nothing interesting would come of it.

The emerging paradigm for explaining the special properties of our universe is, in a sense, an attempt to live up to the standard set by Darwinian evolution: to provide a natural (as opposed to supernatural) non-accidental explanation for the apparently very unlikely specialness of the universe and its laws. Surprisingly, it involves the same two central principles: an enormous landscape of possibilities and random mutation. It even involves a mechanism similar to DNA.

Darwinian standards

Let us begin with the DNA of a universe. What is it and why do we believe such a thing makes sense? String theory is the key. It supposes that at extremely small distances space is a complicated higher-dimensional manifold with many — typically six — tiny “extra” dimensions in addition to the three we see in everyday life. If we could look at the universe through a super-powerful microscope, we would see that it is composed of “Tinkertoy” elements called fluxes, branes, moduli, orientifolds (and more) all arranged on a tiny knot of higher-dimensional space called a Calabi–Yau manifold. The Calabi–Yau manifold is like the basic spine of the DNA molecule, and the other elements can be arranged and rearranged in a huge variety of ways; perhaps as many ways as a real DNA molecule.

Just as the details of DNA determine the biological details of a living organism, so the details of the fluxes, branes and other elements determine the properties of the universe. Again, the numbers are so staggering that even if the world as we know it seems extremely unlikely, there will be many ways of arranging the elements to make the constants of nature consistent with life. In particular, there will be many configurations in which the cosmological constant will be fine-tuned to 123 decimal places.

What about reproduction and mutability? Here is where the inflationary theory of cosmology comes into play. There is much evidence that during the earliest epoch of the universe space itself expanded exponentially. Inflation was a process in which space grew like the surface of an inflating balloon, but instead of thinning out, as the rubber of the balloon would, new bits of space were created to fill the gaps.

For the most part, the new bits of space had the same DNA as the regions surrounding them, but every so often a mutation occurred. A bit of space with new properties, new constants and a new value for the cosmological constant was created. According to standard general relativity, that tiny bubble grew and eventually became a new inflating universe, reproducing and mutating. This whole process is called eternal inflation and it produced a grand multiverse as rich and varied as the tree of life, each with its own laws of physics, constants of nature and elementary particles. Here and there a very rare branch was created that had the special properties that would allow complex life.

Whether string theory with its huge landscape, and eternal inflation with its reproducing pockets of space, will prove to be correct is for the future to decide. What is true is that as of the present time, they provide the only natural explanation of the universe that lives up to the standard set by Darwin.