Charles Darwin was not yet 30 when he got the basic idea for the theory of evolution. But it wasn't until he turned 50 that he presented his argument to the world. He spent those two decades methodically compiling evidence for his theory and coming up with responses to every skeptical counterargument he could think of. And the counterargument he anticipated most of all was that the gradual evolutionary process he envisioned could not produce certain complex structures.
Consider the human eye. It is made up of many parts—a retina, a lens, muscles, jelly, and so on—all of which must interact for sight to occur. Damage one part—detach the retina, for instance—and blindness can follow. In fact, the eye functions only if the parts are of the right size and shape to work with one another. If Darwin was right, then the complex eye had evolved from simple precursors. In On the Origin of Species, Darwin wrote that this idea “seems, I freely confess, absurd in the highest possible degree.”
But Darwin could nonetheless see a path to the evolution of complexity. In each generation, individuals varied in their traits. Some variations increased their survival and allowed them to have more offspring. Over generations those advantageous variations would become more common—would, in a word, be “selected.” As new variations emerged and spread, they could gradually tinker with anatomy, producing complex structures.
The human eye, Darwin argued, could have evolved from a simple light-catching patch of tissue of the kind that animals such as flatworms grow today. Natural selection could have turned the patch into a cup that could detect the direction of the light. Then, some added feature would work with the cup to further improve vision, better adapting an organism to its surroundings, and so this intermediate precursor of an eye would be passed down to future generations. And, step-by-step, natural selection could drive this transformation to increased complexity because each intermediate form would provide an advantage over what came before.
Darwin's musings on the origin of complexity have found support in modern biology. Today biologists can probe the eye and other organs in detail at the molecular level, where they find immensely complex proteins joining together to make structures that bear a striking resemblance to portals, conveyor belts and motors. Such intricate systems of proteins can evolve from simpler ones, with natural selection favoring the intermediates along the way.
But recently some scientists and philosophers have suggested that complexity can arise through other routes. Some argue that life has a built-in tendency to become more complex over time. Others maintain that as random mutations arise, complexity emerges as a side effect, even without natural selection to help it along. Complexity, they say, is not purely the result of millions of years of fine-tuning through natural selection—the process that Richard Dawkins famously dubbed “the blind watchmaker.” To some extent, it just happens.
A Sum of Varied Parts
Biologists and philosophers have pondered the evolution of complexity for decades, but according to Daniel W. McShea, a paleobiologist at Duke University, they have been hobbled by vague definitions. “It's not just that they don't know how to put a number on it. They don't know what they mean by the word,” McShea says.
McShea has been contemplating this question for years, working closely with Robert N. Brandon, also at Duke. McShea and Brandon suggest that we look not only at the sheer number of parts making up living things but at the types of parts. Our bodies are made of 10 trillion cells. If they were all of one type, we would be featureless heaps of protoplasm. Instead we have muscle cells, red blood cells, skin cells, and so on. Even a single organ can have many different cell types. The retina, for example, has about 60 different kinds of neurons, each with a distinct task. By this measure, we can say that we humans are, indeed, more complex than an animal such as a sponge, which has perhaps only six cell types.