New study: Intelligent life is likely very common throughout the universe

Humanity has long wondered whether intelligent life is a rare cosmic accident or an inevitable outcome of planetary evolution. For decades, scientists have debated the probability of intelligent beings arising elsewhere in the universe.

Some argue that our existence depends on a series of nearly impossible steps, making it unlikely that similar beings exist beyond Earth.

Others suggest that evolution follows predictable patterns, meaning life like ours could be common on other planets.

A new study from researchers at Penn State challenges the idea that intelligence is an improbable fluke. Their model suggests that life evolves as planetary conditions shift, making intelligence less about luck and more about timing.

The findings reshape the discussion about our origins and the likelihood of discovering extraterrestrial civilizations.

Is intelligent life rare?

For years, the dominant theory explaining the rarity of intelligent life has been the “hard steps” model.

First proposed by physicist Brandon Carter in 1983, this idea suggests that life’s progression from simple organisms to intelligent beings required multiple, extremely difficult evolutionary steps.

Carter’s model assumes that, because it took billions of years for humans to appear on Earth, the odds of intelligent life arising elsewhere must be extremely low.

However, the new study challenges this assumption. Researchers argue that Earth’s environment was initially inhospitable to complex life, and evolutionary breakthroughs occurred when conditions became suitable.

This suggests that intelligence may not be the result of rare luck but a natural outcome of planetary development.

“This is a significant shift in how we think about the history of life,” said Jennifer Macalady, professor of geosciences at Penn State.

“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.”

Role of environmental conditions

The new model proposes that key evolutionary transitions, such as the emergence of complex cells and multicellular life, were made possible by gradual changes in Earth’s atmosphere and oceans.

Life did not evolve randomly but rather in response to shifts in environmental factors like oxygen levels, temperature, and ocean chemistry.

For example, complex animals require sufficient oxygen to survive. Earth’s atmosphere became oxygen-rich due to the activities of photosynthesizing microbes and bacteria, which created the right conditions for more advanced life to appear.

“We’re arguing that intelligent life may not require a series of lucky breaks to exist,” said Dan Mills, postdoctoral researcher at The University of Munich.

“Humans didn’t evolve ‘early’ or ‘late’ in Earth’s history, but ‘on time,’ when the conditions were in place. 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.”

A different perspective on time

The “hard steps” model is based on the assumption that evolution should be measured against the lifespan of the sun.

With the sun expected to last around 10 billion years, and Earth already 5 billion years old, Carter suggested that intelligent life must be extremely rare since it appeared relatively late.

The new study takes a different approach. Instead of measuring evolution against the sun’s lifespan, researchers suggest using a geological time scale. Evolution follows planetary changes, not the arbitrary ticking of a cosmic clock.

“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,” commented Jason Wright, professor of astronomy and astrophysics at Penn State.

“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.”

Rethinking how intelligent life evolves

One reason the “hard steps” model remained dominant for so long is its astrophysical origins. Astrophysics has traditionally shaped how scientists think about planetary formation and the conditions required for life.

However, evolution is a biological process influenced by planetary chemistry, ocean currents, and atmospheric shifts.

To create their new model, the research team combined insights from astrophysics, geobiology, and planetary science.

This interdisciplinary approach allowed them to challenge long-held assumptions and present a more comprehensive view of how life evolves.

“This paper is the most generous act of interdisciplinary work,” said 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.”

Testing the new model

The researchers are now working to test their alternative model by studying planetary atmospheres and evaluating evolutionary “hard steps” under different environmental conditions.

They suggest searching for biosignatures, such as oxygen, in the atmospheres of exoplanets to determine whether life-supporting conditions develop elsewhere.

They also plan to investigate whether supposed “hard steps” in evolution were truly as rare as previously thought.

By studying single-celled and multicellular life in different environmental settings, they aim to determine whether major evolutionary transitions could occur under a broader range of conditions than previously assumed.

Are evolutionary milestones truly unique?

Another important question the team hopes to explore is whether key evolutionary breakthroughs – such as the emergence of oxygenic photosynthesis, eukaryotic cells, and animal multicellularity – were truly one-time events in Earth’s history.

They suggest that similar evolutionary advances may have occurred multiple times but left no fossil record due to extinction or environmental changes.

If such innovations occurred more than once, it would suggest that evolution follows a predictable path, which increases the chances that life similar to ours exists elsewhere.

Could intelligent life exist on many planets?

The study presents a hopeful view of life’s potential beyond Earth. If intelligence arises naturally when planetary conditions align, then the universe may be filled with civilizations at different stages of development.

“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 that unfolds as global conditions allow.”

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

As scientists continue to refine their understanding of evolution and the role of planetary conditions, new discoveries may bring us closer to answering one of humanity’s greatest questions: Are we alone in the universe?

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The study was co-authored by Adam Frank of the University of Rochester. The research received support from Penn State’s Astrobiology Research Center, the Penn State Center for Exoplanets and Habitable Worlds, the Penn State Extraterrestrial Intelligence Center, NASA‘s Exobiology program, and the German Research Foundation.

The study is published in the journal Science Advances.

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