This theory will need to overcome the Fermi Paradox and the existence of Trump voters
By Penn State
This model challenges the long-standing “hard steps” theory, which argues that the emergence of intelligent life was an extremely unlikely event.
Instead, researchers from Penn State propose that the development of intelligence was not a matter of improbable luck but a natural consequence of environmental conditions aligning over time.
Their findings suggest that intelligent life may be more common in the universe than previously thought.
A Shift in Perspective on Life’s Origins
“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 today (February 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.
The Role of Planetary Conditions in Evolution
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.
Intelligent Life: A Matter of Time?
“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.
Windows of Habitability: Earth’s Unique Timing
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.”
Bridging Astrophysics and Geobiology
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.”
Testing the New Model: Next Steps
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?
A Predictable Path to Intelligence?
“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.”
Reference: “A reassessment of the “hard-steps” model for the
evolution of intelligent life” 14 February 2025, Science Advances.
DOI: 10.1126/sciadv.ads5698
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.
