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https://phys.org/news/2025-02-planetary-evolution-favor-human-life.html
Does planetary evolution favor human-like life? Study ups odds we're not
alone
by Pennsylvania State University
A new model upends the decades-old "hard steps" theory that intelligent
life was an incredibly improbable event and suggests that maybe it
wasn't all that hard or improbable. The team of researchers said the new interpretation of humanity's origin increases the probability of
intelligent life elsewhere in the universe. Credit: NASA
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 was published 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.
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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.
More information: Daniel Mills, A reassessment of the "hard-steps" model
for the evolution of intelligent life, Science Advances (2025). DOI: 10.1126/sciadv.ads5698. www.science.org/doi/10.1126/sciadv.ads5698
Journal information: Science Advances
Provided by Pennsylvania State University
Explore further
Can the 'hard steps' in the evolutionary history of human intelligence
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