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Study challenges idea that intelligent life extremely rare in the universe

UNIVERSITY PARK, Pa. — We might have plenty of human-like neighbors among the stars after all, despite decades of scientific thinking about humanity’s place in the universe suggesting otherwise. A thought-provoking review by researchers from Penn State proposes that intelligent life may be a natural outcome of planetary evolution rather than an astronomical accident.

Since 1983, scientists embraced what became known as the “hard-steps” model, proposed by physicist Brandon Carter. This theory suggested that becoming human required clearing several evolutionary barriers so improbable that successfully crossing all of them was like winning multiple cosmic lotteries in succession. This helped explain why we haven’t detected signs of intelligent life elsewhere; perhaps we’re an astronomical fluke.

“This is a significant shift in how we think about the history of life,” says co-author Jennifer Macalady, professor of geosciences at Penn State, in a statement. “It suggests that the evolution of complex life may be less about luck and 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.”

The research team, which uniquely combined expertise from astrophysics and geobiology, presents compelling evidence for a radically different explanation. Instead of requiring an incredibly lucky series of evolutionary accidents, intelligent life may emerge naturally when planets develop the right environmental conditions.

“We’re arguing that intelligent life may not require a series of lucky breaks to exist. Humans didn’t evolve ‘early’ or ‘late’ in Earth’s history, but ‘on time,’ when the conditions were in place,” said lead author Dan Mills, who worked in Macalady’s astrobiology lab at Penn State as an undergraduate researcher. “Perhaps it’s only a matter of time, and maybe other planets are able to achieve these conditions more rapidly than Earth did, while other planets might take even longer.”

The research team identified five major evolutionary innovations previously considered “hard steps”: the origin of life itself, the development of oxygen-producing photosynthesis, the emergence of complex cells, the evolution of multicellular animal life, and the development of human intelligence. Instead of viewing these as improbable accidents, the study suggests each innovation emerged predictably once Earth’s environment could support it.

“We’re taking the view that rather than base our predictions on the lifespan of the sun, we should use a geological time scale, because that’s how long it takes for the atmosphere and landscape to change,” says Jason Wright, professor of astronomy and astrophysics at Penn State and study co-author. “These are normal timescales on the Earth. If life evolves with the planet, then it will evolve on a planetary time scale at a planetary pace.”

This new perspective, published in Science Advances, has profound implications for the search for extraterrestrial intelligence. Rather than asking how likely it is for evolution to produce intelligent life, scientists might focus more on identifying planets that have developed Earth-like environmental conditions over long periods.

The collaboration between physicists and geobiologists was crucial to developing this new understanding. “This paper is the most generous act of interdisciplinary work,” says Macalady, who also directs Penn State’s Astrobiology Research Center. “Our fields were far apart, and we put them on the same page to get at this question of how we got here and are we alone? There was a gulf, and we built a bridge.”

Researchers have outlined several ways to test their model, including searching for biosignatures in the atmospheres of exoplanets and studying how single-celled and multicellular organisms respond to various environmental conditions. They also suggest investigating whether similar evolutionary innovations might have occurred multiple times in Earth’s past, with evidence lost to extinction or other factors.

For scientists searching for alien civilizations, this research suggests focusing more on a planet’s environmental history than calculating evolutionary probabilities. Worlds that developed Earth-like conditions might naturally progress toward complex, human-like life.