A team of researchers from Penn State has developed a new model for the evolution of intelligent life, shifting the odds in favor of human-like life existing elsewhere in the universe. This alternative to the decades-old “hard steps” theory proposes that intelligent life may not be as improbable as previously thought.
According to Jennifer Macalady, professor of geosciences at Penn State and co-author on the paper, the new model suggests that the evolution of complex life is less about luck and more about the interplay between life and its environment. This perspective opens up exciting new avenues of research in our quest to understand our origins and place in the universe.
The original “hard steps” theory, developed by theoretical physicist Brandon Carter in 1983, argued that human-like beings were highly unlikely due to the time it took for humans to evolve on Earth relative to the total lifespan of the sun. However, this new study proposes that key evolutionary steps only became possible when the global environment reached a “permissive” state.
Complex life forms require specific conditions, such as oxygen levels in the atmosphere. The oxygenation of Earth’s atmosphere through photosynthesizing microbes and bacteria created a window of opportunity for more recent life forms to develop.
According to Dan Mills, postdoctoral researcher at The University of Munich and lead author on the paper, intelligent life may not require a series of lucky breaks to exist. Humans evolved “on time” when conditions were in place, suggesting that other planets may be able to achieve these conditions more rapidly or slowly than Earth did.
The researchers proposed that the timing of human origins can be explained by the sequential opening of “windows of habitability” over Earth’s history, driven by changes in nutrient availability, sea surface temperature, ocean salinity levels, and oxygen levels. The team suggests that rather than relying on the lifespan of the sun, we should use a geological time scale to predict the evolution of life.
The study marks an interdisciplinary collaboration between physicists and geobiologists, who learned from each other’s fields to develop a nuanced picture of how life evolves on a planet like Earth.
Source: https://phys.org/news/2025-02-planetary-evolution-favor-human-life.html