Intelligent Life May Be More Likely to Exist on Other Planets Than Previously Thought, Scientists Say

Scientists have adhered to the “hard steps” model of intelligent life for decades. Developed by Australian theoretical physicist Brandon Carter in 1983, the idea suggests a remarkable series of rare events—such as the development of complex cells and the onset of oxygen-producing photosynthesis—led to the occurrence of human life on Earth.

Now, a team of scientists at Penn State University argue that intelligent life isn’t all that improbable. Their critique of the hard steps model was published in the journal Science Advances on February 14.

“In short, our framework shows how hard steps may not actually exist—past evolutionary transitions that needed to happen for us humans to be here may not have been hard or unlikely in the available time,” says lead author Dan Mills, a geomicrobiologist at the University of Munich, to Will Dunham at Reuters.

At the core of this dispute is whether humans on Earth exist because of a major cosmic coincidence. With the hard steps theory, Carter argued that Earth got exceptionally lucky—ordinarily, he figured, intelligent life would take so long to develop on a planet that its host star would burn out before that process of evolution had time to run its course. The reason intelligent life is so slow to form, he said, is because the hard steps must be met.

In their new model, the scientists present alternative explanations for the hard steps described by Carter. They argue that key evolutionary steps only became possible when the Earth became hospitable to them. In their view, the development of intelligent life was less about luck and more about being “on time.” They see intelligent life as the natural result of a hospitable environment. “Carter had not considered these points in 1983,” write the researchers in the Conversation.

“Carter assumed humans could have evolved at any time, but that’s just wrong,” Mills tells Popular Science’s Lauren Leffer. “For the vast majority of Earth’s history, the planet wasn’t supportive of humans.”

But as the Earth surpassed each milestone necessary for intelligent life, the odds that it would reach the next one might have risen. “Life might have originated very quickly once temperatures were appropriate for the stability of biomolecules and liquid water,” explains study co-author Jennifer Macalady, a geoscientist at Penn State, to Reuters. “The Earth has only been habitable for humans since the second rise of oxygen in the atmosphere approximately 0.5 billion years ago, meaning that humans could not have evolved on Earth prior to that relatively recent moment.”

Basically, we evolved as soon as we could. “This is a significant shift in how we think about the history of life,” adds Macalady in a statement. The evolution of species like humans is more “about the interplay between life and its environment, opening up exciting new avenues of research in our quest to understand our origins and our place in the universe,” she says.

Still, more research is needed to confirm how rare intelligent life is in the cosmos. To do that, scientists will have to look both on Earth and in space, according to the paper. On our planet, for instance, scientists could identify when the Earth’s surface environment became conducive to each suggested hard step. And they could test whether a given step really evolved only once, or if it happened a few times independently, making it less rare. Astronomers could continue their observations of other planets to see how many host one of the hard step candidates, like oxygen-producing photosynthesis.

“Understanding the probability of intelligent life emerging helps us understand our own place in the world,” Mills says to Reuters. “Are we humans a cosmic fluke, as the hard steps model predicts? Or are we instead the more expected and typical outcome of a living planet, as our alternative framework suggests?”

Sara Hashemi | READ MORE

Sara Hashemi is a science writer and fact-checker currently based in New York City. Her work has appeared in Sierra, The Body, Maisonneuve magazine and more.