UNIVERSITY PARK, Pa. — Humanity may not be extraordinary but rather the natural evolutionary outcome for our planet and likely others, according to a new model for how intelligent life developed on Earth.
The model, which upends the decades-old "hard steps" theory that intelligent life was an incredibly improbable event, suggests that maybe it wasn't all that hard or improbable. A team of researchers at Penn State, who led the work, said the new interpretation of humanity's origin increases the probability of intelligent life elsewhere in the universe.
"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 the paper, which published today (Feb. 14) in the journal 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."
Initially developed by theoretical physicist Brandon Carter in 1983, the "hard steps" model argues that our evolutionary origin was highly unlikely due to the time it took for humans to evolve on Earth relative to the total lifespan of the sun — and therefore the likelihood of human-like beings beyond Earth is extremely low.
In the new study, a team of researchers that included astrophysicists and geobiologists argued that Earth's environment was initially inhospitable to many forms of life, and that key evolutionary steps only became possible when the global environment reached a "permissive" state.
For example, complex animal life requires a certain level of oxygen in the atmosphere, so the oxygenation of Earth's atmosphere through photosynthesizing microbes and bacteria was a natural evolutionary step for the planet, which created a window of opportunity for more recent life forms to develop, explained Dan Mills, postdoctoral researcher at The University of Munich and lead author on 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 central prediction of the "hard steps" theory states that very few, if any, other civilizations exist throughout the universe, because steps such as the origin of life, the development of complex cells and the emergence of human intelligence are improbable based on Carter's interpretation of the sun's total lifespan being 10 billion years, and the Earth's age of around 5 billion years.
In the new study, 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 the amount of oxygen in the atmosphere. Given all the interplaying factors, they said, the Earth has only recently become hospitable to humanity — it's simply the natural result of those conditions at work.
"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."
Wright explained that part of the reason that the "hard steps" model has prevailed for so long is that it originated from his own discipline of astrophysics, which is the default field used to understand the formation of planets and celestial systems. The team's paper is a collaboration between physicists and geobiologists, each learning from each other's fields to develop a nuanced picture of how life evolves on a planet like Earth.
"This paper is the most generous act of interdisciplinary work," said Macalady, who also directs Penn State's Astrobiology Research Center. "Our fields were far apart, and we put them on the same page to get at this question of how we got here and are we alone? There was a gulf, and we built a bridge."
The researchers said they plan to test their alternative model, including questioning the unique status of the proposed evolutionary "hard steps." The recommended research projects are outlined in the current paper and include such work as searching the atmospheres of planets outside our solar system for biosignatures, like the presence of oxygen. The team also proposed testing the requirements for proposed "hard steps" to determine how hard they actually are by studying uni- and multicellular forms of life under specific environmental conditions such as lower oxygen and temperature levels.
Beyond the proposed projects, the team suggested the research community should investigate whether innovations —such as the origin of life, oxygenic photosynthesis, eukaryotic cells, animal multicellularity and Homo sapiens — are truly singular events in Earth's history. Could similar innovations have evolved independently in the past, but evidence that they happened was lost due to extinction or other factors?
"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 other co-author on the paper is Adam Frank of the University of Rochester. Penn State's Astrobiology Research Center, the Penn State Center for Exoplanets and Habitable Worlds, the Penn State Extraterrestrial Intelligence Center, the NASA Exobiology program and the German Research Foundation supported this work.
