Are aliens real? The Milky Way could have 40 billion Earth-sized planets, but many evolutionary scientists assume that intelligent life is vanishingly rare. A new study upends that belief, proposing that human evolution was merely a natural planetary process that can and likely has occurred on other Earth-like planets.

“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 on a paper published in Science Advances. “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.”

Aliens And Intelligent Life: The ‘Hard Steps’ Theory

It’s a common belief among scientists that human-like intelligence arose because of improbable events in the evolutionary history of life on Earth. This “hard steps” model, developed by Brandon Carter, an Australian theoretical physicist, in 1983, is based on the fact that stars like the sun exist for about nine billion years. Yet, humans only appeared after 4.6 billion years. Why the delay? Refinements of Carter’s theory define five unlikely steps, including the appearance of photosynthesis that created enough oxygen for the emergence of multicellular animals.

In July, researchers suggested that Earth’s plate tectonics may be the key reason complex life exists here but not elsewhere. The researchers argue that plate movement regulates climate, recycles nutrients, and maintains conditions suitable for life, suggesting that this geological process is rare on exoplanets (planets that orbit stars other than the sun), making Earth uniquely habitable.

Aliens And Intelligent Life: Natural Evolutionary Steps

This new model, developed by astrophysicists and geobiologists at Penn State, suggests that the appearance of intelligent life on Earth wasn’t all that hard or improbable. While the “hard steps” theory posits that the appearance of enough oxygen in Earth's atmosphere to support complex animal life was an unlikely event, this new model ranks it as a natural evolutionary step for Earth.

“We're arguing that intelligent life may not require a series of lucky breaks to exist,” said Dan Mills, lead author on the paper, a postdoctoral researcher at The University of Munich, who worked in the 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."

Why Humans Evolved: ‘Windows Of Habitability’

The study uses not the sun’s lifespan but 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 of 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.”

The paper explores how “windows of habitability” have opened across Earth's history. Key factors include changes in sea surface temperature, how salty the sea is, and how much oxygen there is in the atmosphere. In short, Earth has only become hospitable to humanity in the last 200,000 years, but as a natural result of conditions.

Aliens And Intelligent Life: The Hunt Off-Earth

The takeaway from this new research is that intelligent life on Earth — and any Earth-like planet — is likely inevitable.

“This new perspective suggests that the emergence of intelligent life might not be such a long shot after all,” said Wright, adding that evolution may be 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,” he said. “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.”

NASA’s James Webb Space Telescope and Pandora missions are now searching the atmospheres of planets outside our solar system for biosignatures such as oxygen. If they find them, according to this new theory, they may well find intelligent aliens.

Wishing you clear skies and wide eyes.