One reason scientists are skeptical is that Hedges’ clocklike pattern conflicts with the traditional picture of how evolution unfolds. “The classic view of evolution is that it happens in fits and starts,” Benton said. A change in the environment, such as a rise in temperatures after an ice age, might spark a burst of speciation as organisms adapt to their new surroundings. Alternatively, a single remarkable adaptation such as flight in the ancestors of birds or hair in mammals might trigger a massive expansion of animals with those characteristics.

Hedges argues that while such bursts do occur, the vast majority of speciation is more prosaic and evenly timed. To start, two populations become separated, driven apart by geography or other factors. New species emerge every 2 million years, on average, in a metronomic rhythm tapped out by the random nature of genetic mutations. He likens the process to radioactive decay. It’s impossible to predict when an individual radioactive nucleus will decay, but a clump of many atoms will decay at a highly predictable rate known as the material’s half-life. Similarly, mutations strike the genome randomly, but over a long enough time the accumulation of mutations follows a pattern. “There is a kind of speciation clock ticking along,” Hedges said.