In a post-apocalyptic future, what might happen to life if humans left the scene? After all, humans are very likely to disappear long before the sun expands into a red giant and exterminates all living things from the Earth.

Assuming that we don’t extinguish all other life as we disappear (an unlikely feat in spite of our unique propensity for driving extinction), history tells us to expect some pretty fundamental changes when humans are no longer the planet’s dominant animal species.

So if we were given the chance to peek forward in time at the Earth some 50m years after our disappearance, what would we find? Which animal or group of animals would “take over” as the dominant species? Would we have a Planet of the Apes, as imagined in popular fiction? Or would the Earth come to be dominated by dolphins, or rats, or water bears, or cockroaches or pigs, or ants?

The question has inspired a lot of popular speculation and many writers have offered lists of candidate species. Before offering any guesses, however, we need to carefully explain what we mean by a dominant species.

Let’s Stick To The Animal Kingdom

One could argue that the current era is an age of flowering plants. But most people aren’t imagining Audrey Two in Little Shop of Horrors when they envision life in the future (even the fictional triffids had characteristically animal features – predatory behaviour and the ability to move).

Then again, maybe not. Triffid by Shutterstock

So let’s keep the discussion to animals. This is for practical rather than philosophical reasons: by some standards the world is now and always has been dominated by bacteria despite the nominal end of the “age of microbes” some 1.2 billion years ago. This was not because bacteria ceased to be, or declined in prevalence, but rather because in our myopia we tend to place more importance on the large multi-cellular organisms that came after.

By some accounts four out of five animals is a nematode (a roundworm), so from all these examples it’s clear that neither prevalence, abundance nor diversity is the prime requisite for being a “dominant” form of life. Instead our imaginations are captured by large and charismatic organisms.