Evolving intelligent life took billions of years − but it may not have been as unlikely as many scientists predicted

A popular model of evolution concludes that it was incredibly unlikely for humanity to evolve on Earth, and that extraterrestrial intelligence is vanishingly rare.

But as experts on the entangled history of life and our planet, we propose that the coevolution of life and Earth’s surface environment may have unfolded in a way that makes the evolutionary origin of humanlike intelligence a more foreseeable or expected outcome than generally thought.

The hard-steps model

Some of the greatest evolutionary biologists of the 20th century famously dismissed the prospect of humanlike intelligence beyond Earth.

This view, firmly rooted in biology, independently gained support from physics in 1983 with an influential publication by Brandon Carter, a theoretical physicist.

In 1983, Carter attempted to explain what he called a remarkable coincidence: the close approximation between the estimated lifespan of the Sun – 10 billion years – and the time Earth took to produce humans – 5 billion years, rounding up.

A man wearing a suit with gray hair and a beard.

He imagined three possibilities. In one, intelligent life like humans generally arises very quickly on planets, geologically speaking – in perhaps millions of years. In another, it typically arises in about the time it took on Earth. And in the last, he imagined that Earth was lucky – ordinarily it would take much longer, say, trillions of years for such life to form.

Carter rejected the first possibility because life on Earth took so much longer than that. He rejected the second as an unlikely coincidence, since there is no reason the processes that govern the Sun’s lifespan – nuclear fusion – should just happen to have the same timescale as biological evolution.

So Carter landed on the third explanation: that humanlike life generally takes much longer to arise than the time provided by the lifetime of a star.

A diagram showing the life cycle of the Sun, from its birth to its growth into a Red Giant around ten billion years.

To explain why humanlike life took so long to arise, Carter proposed that it must depend on extremely unlikely evolutionary steps, and that the Earth is extraordinarily lucky to have taken them all.

He called these evolutionary steps hard steps, and they had two main criteria. One, the hard steps must be required for human existence – meaning if they had not happened, then humans would not be here. Two, the hard steps must have very low probabilities of occurring in the available time, meaning they usually require timescales approaching 10 billion years.