Intelligent life may not be that extraordinary in the universe

For decades, scientists have debated whether humanity’s existence is a rare cosmic accident or the natural outcome of planetary evolution.

A new study challenges long-standing assumptions, suggesting that intelligent life may be an expected result of planetary development rather than an improbable occurrence. The findings, published in the journal Science Advances, provides fresh insight into the search for extraterrestrial life and redefine humanity’s place in the universe.

In 1983, physicist Brandon Carter proposed the “hard steps” model, arguing that the emergence of human-like intelligence was an extremely unlikely event. He based this on the idea that if intelligence were common, it should have evolved much earlier in Earth's history. Instead, humans appeared relatively late, suggesting that evolution required rare, improbable steps. This reasoning led Carter to conclude that intelligent civilizations beyond Earth must be exceedingly rare.

Carter’s model assumed that key evolutionary milestones—such as the emergence of complex cells, multicellular life, and intelligence—were “hard steps” that had a low probability of occurring within a planet’s habitable window. If each step took an extraordinarily long time, then the chance of all the steps occurring before a planet became uninhabitable would be small. Under this model, the probability of intelligent life elsewhere in the universe was expected to be low.

However, this interpretation has been questioned. A team of researchers from Penn State and the University of Munich argue that Earth's environment was not always suitable for complex life, meaning evolutionary steps had to wait until conditions were right. This suggests that intelligence is not a matter of sheer luck but of timing.

Rather than viewing evolutionary milestones as improbable obstacles, the researchers propose that life develops in synchrony with planetary changes. The study emphasizes that Earth underwent multiple “windows of habitability,” where conditions such as oxygen levels, temperature, and nutrient availability became favorable for life to advance.

For example, complex animal life depends on sufficient atmospheric oxygen. Photosynthesizing microbes increased oxygen levels over billions of years, eventually reaching a threshold that allowed more complex organisms to emerge.

According to Dan Mills, lead author and postdoctoral researcher at the University of Munich, this suggests that life evolved not at random but in response to planetary changes.

“We’re arguing that intelligent life may not require a series of lucky breaks to exist,” Mills said. “Humans didn’t evolve ‘early’ or ‘late’ in Earth’s history, but ‘on time,’ when the conditions were in place.”

This perspective challenges Carter’s assumption that the timing of intelligence relative to the sun’s lifespan is a meaningful metric. Instead, the researchers propose that evolutionary timelines should be compared to planetary timescales. Jason Wright, a co-author and professor of astronomy and astrophysics at Penn State, explained that this approach better accounts for life’s dependence on environmental factors.

“If life evolves with the planet, then it will evolve on a planetary time scale at a planetary pace,” Wright said. “Rather than basing predictions on the lifespan of the sun, we should use geological timescales, because that’s how long it takes for the atmosphere and landscape to change.”

The study represents a rare collaboration between astrophysicists and geobiologists, two fields that traditionally operate independently. Jennifer Macalady, a professor of geosciences at Penn State and co-author of the paper, emphasized the importance of this interdisciplinary approach.

“Our fields were far apart, and we put them on the same page to get at this question of how we got here and whether we are alone,” Macalady said. “There was a gulf, and we built a bridge.”

The researchers plan to test their model by investigating biosignatures on exoplanets—chemical traces that indicate biological activity. They also propose studying the conditions necessary for evolutionary milestones, such as the emergence of complex cells, to determine whether these steps are truly as improbable as Carter’s model suggests.

Additionally, the team suggests revisiting evolutionary innovations such as oxygenic photosynthesis and multicellularity to see whether similar processes could have occurred multiple times throughout Earth’s history but left no trace due to extinction or other factors.

“This new perspective suggests that the emergence of intelligent life might not be such a long shot after all,” Wright said. “Instead of a series of improbable events, evolution may be more of a predictable process, unfolding as global conditions allow.”

If intelligence emerges naturally from planetary evolution, then the likelihood of finding extraterrestrial civilizations increases. Rather than being an improbable outcome, intelligence may be an expected result of a planet reaching suitable conditions. This would mean that Earth is not an exception but part of a broader pattern in the universe.

While the study does not confirm the existence of alien life, it provides a new framework for searching for it. By focusing on planetary environments rather than random evolutionary luck, researchers may better identify worlds where intelligence has had time to develop.

“Our framework applies not only to Earth but also to other planets, increasing the possibility that life similar to ours could exist elsewhere,” Wright said.

As scientists continue to explore the cosmos, this new perspective may reshape the way humanity understands its origins and its place among the stars.

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