A new study contradicts the long-held belief that intelligent life is a cosmic anomaly, instead suggesting that the development of humanity was a natural result of Earth’s evolving environment—and that other similar life forms may exist elsewhere in the universe.
Directed by a research team at Penn State, the new work redefines the “hard steps” hypothesis, which states that advanced life on Earth was extremely unlikely to happen. The scientists propose that there were particular stages in evolution—affecting aspects like the emergence of advanced life—rather caused by the Earth environment hitting the right stage at the right time.
“Nothing like this has ever come out of the geologic record before,” said Penn State geosciences professor Jennifer Macalady, a co-author on the paper published in Science Advances on February 14. “It tells us that evolution is not simply a function of chance but is highly linked to planetary conditions. That has huge implications for our search for life off Earth.”
Rethinking the Odds of Alien Life
The traditional “hard steps” theory, proposed by physicist Brandon Carter in 1983, argues that intelligent life is desperately rare. The argument? Humans didn’t develop on Earth until around 4.5 billion years ago—essentially half a sun’s lifetime—so that life like ours wouldn’t have had time to emerge elsewhere before the star that nurtured it went out.
But this new study has a different theory. The scientists think that evolution was not held back by blind luck, but by when the Earth’s environment became “permissive” enough to allow for major biological innovations.
For example, complex life in animals relies on some level of oxygen. Only when photosynthetic microorganisms oxidized the atmosphere of Earth did the requirements exist for further advanced life forms to emerge. That would mean intelligent life did not appear “early” or “late” in the planet’s history—it appeared on time when the time was right.
“We’re arguing that intelligent life doesn’t need a series of lucky breaks,” said lead author Dan Mills, a postdoctoral researcher at the University of Munich. “If the right conditions develop, life will naturally progress. Some planets might reach those conditions faster than Earth did, while others could take longer.”
What This Means for the Search for Life
The study dispels the myth that intelligent life is extremely uncommon. Instead, the scientists opine that life evolves in increments as planetary circumstances change over billions of years.
“If life evolves in concert with a planet, it will do so on a planetary timescale,” said Penn State astronomy professor and study co-author Jason Wright. “That is to say we need to be thinking about planetary timelines—instead of the lifetimes of stars—when looking for where life will be.”.
This is an approach that would revolutionize the search for extraterrestrial life. The scientists suggest investigating if key evolutionary landmarks—such as the development of complex cells or intelligence—are truly one-time occurrences or happened repeatedly but left very little evidence, due to extinction.
They also suggest searching the atmospheres of exoplanets for biosignatures, such as oxygen, to determine if similar evolutionary processes are underway elsewhere.
This new vision is suggesting that intelligent life is not as unthinkable,” Wright continued. “Instead of infrequent, unlikely limits, evolution may have a natural, previsible direction. And if so, we may not be alone after all.”.
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