A new study from Penn State challenges the long-held belief that intelligent life is rare, suggesting instead that it may be more probable than previously assumed.
In 1961, astrophysicist Frank Drake introduced the Drake Equation, a mathematical framework designed to estimate the likelihood of technologically advanced civilizations in the Milky Way and their potential for communication. This equation marked a major shift in how humanity perceives its place in the cosmos.
By 2023, advances in exoplanet science, astrobiology, and SETI research led to refinements of Drake’s original formula. Among these, Canadian astrophysicist Sara Seager introduced an alternative equation focusing not just on intelligent life but also on detecting biosignatures—chemical indicators of biological activity.
Previously, theoretical physicists had approached the question differently. In 1983, physicist Brandon Carter proposed the “hard steps” model, which argued that life is extremely rare due to a series of improbable evolutionary leaps—or “hard steps”—necessary for complex organisms to emerge. Carter suggested that Earth’s sequence of evolutionary milestones was exceptionally unlikely. The new Penn State study revisits this model, offering a fresh perspective on the idea that life may be more common than Carter originally proposed.
Beyond mathematical formulas, scientists have long explored planetary habitability using the concept of Goldilocks Zones—regions around stars where conditions are “just right” for liquid water and, potentially, life. While the term gained widespread use in the 1990s and 2000s, it was first introduced by Stephen H. Dole in his 1964 book Habitable Planets for Man, published by the RAND Corporation.
Dole examined factors like temperature, atmosphere, and liquid water in determining planetary habitability. More recently, Sara Seager expanded on these ideas by introducing Hycean worlds—a class of exoplanets with hydrogen-rich atmospheres and vast ocean-covered surfaces that could potentially support life.
Now, in their recent paper, the research team composed of Daniel B. Mills, Jennifer L. Macalady, Adam Frank, and Jason T. Wright is reassessing the “hard steps” model by taking a critical look several of its core assumptions with a view rooted in historical geobiology.
“This is a significant shift in how we think about the history of life,” said Jennifer Macalady, professor of geosciences at Penn State and co-author of the new paper.
The study “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” Macalady says.
The research team, composed of astrophysicists and geobiologists, explored the idea that early Earth was initially too harsh for many forms of life. Evolutionary milestones only occurred once the global environment became suitable. For example, the oxygenation process, driven by photosynthetic microbes and bacteria, was a key event in Earth’s evolution. It created atmospheric conditions necessary for complex life to emerge, explained Dan Mills, a postdoctoral researcher at the University of Munich and lead author of the paper.
“We’re arguing that intelligent life may not require a series of lucky breaks to exist,” said Mills, who worked in Macalady’s astrobiology lab at Penn State as an undergraduate researcher. “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.
The study introduces the idea that Earth’s habitability followed a sequence of “windows of habitability”—periods when conditions became favorable for life. These shifts were controlled by factors such as nutrient availability, sea surface temperature, ocean salinity, and atmospheric oxygen levels. The researchers concluded that Earth only recently stabilized into a state that could support complex life.
“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,” said Jason Wright, professor of astronomy and astrophysics at Penn State and co-author on the paper.
“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.”
Looking ahead, the researchers plan to test an alternative model that challenges the uniqueness of the proposed evolutionary “hard steps.” Their next projects will involve studying the atmospheres of exoplanets for biosignatures such as oxygen, as well as examining how environmental factors—including lower oxygen levels and varying temperatures—affect the evolution of life. By studying how single-celled and multicellular organisms adapt to these conditions, they aim to determine how difficult key evolutionary transitions truly are.
“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. Our framework applies not only to Earth but also other planets, increasing the possibility that life similar to ours could exist elsewhere.”
The paper was originally published in Science Advances.
Chrissy Newton is a PR professional and founder of VOCAB Communications. She hosts the Rebelliously Curious podcast, which can be found on The Debrief’s YouTube Channel. Follow her on X: @ChrissyNewton and at chrissynewton.com.