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.

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.