Recent research indicates that humanity might not be an extraordinary anomaly but rather a typical outcome of planetary evolution, which could also apply to other worlds.

This groundbreaking model challenges the decades-old “hard steps” theory, which suggested that the rise of intelligent life was a rare occurrence. A research team at Penn State has asserted that this new viewpoint significantly boosts the likelihood of intelligent life existing elsewhere in the cosmos.

“This represents a fundamental shift in our understanding of life’s history,” stated Jennifer Macalady, a geosciences professor at Penn State and co-author of the research published on February 14 in Science Advances.

Macalady emphasized that this perspective highlights the evolution of complex life as being potentially more influenced by environmental interactions than sheer chance, paving the way for captivating new research avenues in our exploration of life’s origins and our place in the universe.

The “hard steps” model, initially proposed by theoretical physicist Brandon Carter in 1983, contended that the emergence of humans was exceedingly improbable relative to the sun’s lifespan and that the chances of similar beings evolving elsewhere were minimal.

However, the Penn State study, which involved a diverse team of astrophysicists and geobiologists, posits that Earth’s early environment was largely unwelcoming to most life forms. It was only when the planet’s conditions became more conducive to life that critical evolutionary milestones could occur.

For instance, the emergence of diverse animal life necessitated a specific oxygen level in the atmosphere, which was facilitated by photosynthetic microbes and bacteria. This process initiated oxygenation, creating opportunities for more complex life forms to thrive, explained Dan Mills, the lead author and postdoctoral researcher at The University of Munich.

“Our research suggests that the existence of intelligent life does not hinge on a series of fortunate events,” Mills noted, recounting his undergraduate experience in Macalady’s astrobiology lab. “Humans didn’t evolve ‘early’ or ‘late’ in Earth’s history; instead, they appeared ‘on time’ when suitable conditions prevailed. It’s possible that other planets could reach these favorable conditions faster than Earth while some might take much longer.”

The original prediction of the “hard steps” theory posited that few, if any, similar civilizations exist across the universe, given that significant steps like the origin of life and the rise of complex organisms are deemed exceedingly improbable based on Carter’s analysis of the sun’s lifespan and Earth’s age.

In their study, the researchers suggested that the timeline of human development can be elucidated by the sequential opening of “windows of habitability,” influenced by factors such as nutrient levels, sea surface temperatures, ocean salinity, and atmospheric oxygen concentrations throughout Earth’s history.

Considering the myriad factors at play, the researchers concluded that Earth has only recently become suitable for human existence, suggesting that humanity is merely a natural consequence of prevailing environmental conditions.

“Rather than basing our predictions on the sun’s lifespan, we propose using geological timelines because these account for the long-term changes in the atmosphere and landscape,” remarked Jason Wright, a professor of astronomy and astrophysics at Penn State and another co-author of the study.

Wright elaborated that the dominance of the “hard steps” model can be attributed to its origins in astrophysics, traditionally the field used to comprehend planet formation and stellar systems.

The collaborative paper emerges from a merger of physicists and geobiologists, incorporating insights from various disciplines to construct a more nuanced understanding of life’s evolution on planets like Earth.

“This paper exemplifies a remarkable act of interdisciplinary collaboration,” said Macalady, who also leads Penn State’s Astrobiology Research Center. “Our two fields were previously quite separate, and we united them to probe fundamental questions regarding our origins and whether we are alone in the universe. We aimed to bridge what previously felt like a considerable divide.”

The researchers plan to further validate their alternative framework, specifically examining the supposed uniqueness of the evolutionary “hard steps.” Their suggested research initiatives encompass endeavors such as analyzing the atmospheres of exoplanets for biosignatures, including oxygen detection.

They also aim to assess the conditions required for the supposed “hard steps” by investigating both unicellular and multicellular organisms in specific environments characterized by lower oxygen and temperature.

Beyond the proposed studies, the team urges the research community to explore the possibility that innovations like the onset of life, oxygenic photosynthesis, and the emergence of eukaryotic cells might not be unique events in Earth’s history. Could such developments have occurred independently, with evidence lost due to extinction or other factors?

“This new perspective implies that the rise of intelligent life may not be as improbable as once thought,” Wright concluded. “Rather than being the result of a series of unlikely occurrences, evolution might follow a more predictable pathway, unfolding as conditions permit. This framework is applicable not just to Earth but to other planets as well, heightening the likelihood that life akin to ours exists beyond our solar system.”

The paper also features Adam Frank from the University of Rochester as a co-author.