For decades, scientists believed that intelligent life was a rare cosmic accident. A new study challenges that idea, arguing that humanity’s emergence was not an improbable event but rather a natural consequence of Earth’s evolving conditions.
By integrating insights from astrophysics and geobiology, researchers propose that life evolves in response to planetary conditions rather than random luck, suggesting that intelligence may be more common in the universe than previously thought.
Humanity’s Evolution: A Natural Outcome?
Humanity may not be a rare exception but rather the expected result of planetary evolution — both on Earth and potentially on other worlds — according to a new scientific model.
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” by Daniel B. Mills, Jennifer L. Macalady, Adam Frank and Jason T. Wright, 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.
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30 Comments
Look interesting thank you
There are 70 quintillion to 29 sextillion planets in the universe; 200 billion trillion stars. I think that’s all you have to know if wonder if there’s intelligent life out there. There HAS to be some other life forms out there, & statistically speaking, there must be some form of intelligent life. The problem is distance.
Microbial life is abundant in the universe, while complex life may also be abundant, but under hard conditions. In this case, the life elsewhere is implicitly unimportant for us to seek and proably the most unwanted, uninteresting & non-alien phenomena for human beings. I don’t want to think about an uninteresting topic again.
Then maybe the problem is a personal one and you should learn to deal with it, smh.
It is very likely the universe is abundant with life, and even intelligent life. And it is highly unlikely we’ll ever see any of it, simply because the distances are too big.
I wonder why the idea of intelligence being inevitable was even a question, smh. If you believe in evolution, then survival of the fittest should eventually lead to this outcome. Beyond all the myriad physical characteristics that evolution has shown us, intelligence should absolutely be a trait that allows for a species to outcompete those of lessor intelligence under the right conditions.
One thing I do get tired of is the ridiculous idea that Earth is unique in the Universe, smfh, that simply has to be the most myopic and unimaginative conclusion ever. HERE on Earth we see the old saying, “Nature abhors a vacuum” played out time and time again.
Life finds a way to adapt to the harshest environments, and just because our LIMITED scientific knowledge hasn’t shown us how this happens, that we exist at all should be a glaring clue.
Then when you consider the incredibly, unimaginably enormous size of just our galaxy, with its HUNDREDS OF BILLIONS OF PLANETS, the idea that Earth is some kind of unicorn is simply dumb.
Ah, but life can’t evolve if there’s no life to begin with. And since we don’t really know how life is created out of nothing, we can’t really estimate the chances of life existing anywhere else.
Life originating out of inanimate matter is called, ‘biogenesis’. And despite what biochemists, astrophysicists, and other scientists say, I say it’s a ‘miracle’!
Life came from our Creator, the God of the universe.
Except inside the beltway of Washington DC.
This quote by Mills is perfect, “Humans didn’t evolve ‘early’ or ‘late’ in Earth’s history, but ‘on time,’ when the conditions were in place.” It exposes a glaring flaw in scientific thinking to date and shows how they sometimes can’t see the forest for the tree, or in this case the knot hole their noses are stuck in ;?)
Panspermia. If the planet is a water planet, eventually one of the asteroids will contain building blocks for water/carbon based life. Any forms of life that may prosper in a different environment fail to flourish.
Right. Well if intelligent life is the “natural path” of evolution, we’re right back to that inevitable question, which is “Where is everybody”?
The galaxy is immensely old, and hugely large. If there were millions, or even thousands of advanced civilizations out there, one of them somewhere would have been able to explore and develop to the extent that we would have already heard from them, seen or heard unmistakable evidence that someone is out there. Instead, there is only silence.
This is a very weak argument.
They may have already been here. Discovered we are primitive to them . If you want proof of that , we’ll we still kill each other
When does the ant learn that the human is near?
But they have just discovered a nitrate based bacteria everywhere, so maybe there are forms of life based on that in or on other planets – we have only been looking for what we understand to be ‘life’ AS WE KNOW IT. I think it’s far more likely there are even more exotic systems out there we simply can’t ‘see’ because we have no idea they exist !
I believe It is inevitable for life, given enough time, to evolve intelligence. I also think the circumstances that finally resulted in that long ago toe hold of developing evolution to take root is overly discounted. It’s as if nature wants it to be an unequal fight. Simultaneously creating the nurturing space for life while simultaneously placing that life in the extreme peril of “The Circle of Life” , the cataclysmic upheavals of an evolving planet. It seems our only ally in that battle is time itself and lots of it.
The universe is a shooting gallery in which the very planet life and that “inevitably intelligence “evolves on, at least at the beginning, is a weapon in itself. Then add the evolutionary requirements for that life to reach the pinnacle of success we need ”trivial” developments like a magnetic shield to retain the atmosphere, an ozone layer to keep out harmful or destructive rays from the local star. For the star itself to not bombard the planet with material outbursts, avoid being near a supernova, or its effects over light years of expanding gamma rays, a black hole passing by. Of yeah! Intelligence may be inevitable but the frequency of its development, I believe, is exaggerated by a sort of cosmic wishful thinking.
Planets get their ass kicked in the ongoing epochs of time required to allow that slow process to play out.
And absolutely never mind the population of that planet destroying itself through forces occurring unique to its social and scientific development.
The universe is patient but very unkind. Evolution is on the back burner to the universes absolute destructive nature.
Go watch Carl Sagans “Pale Blue Dot” for some perspective on mankind’s likelihood of surviving the very times we live in now. Which is in itself only the merest echo in the absolute thunder of time.
This is a ridiculous ‘study’ that might as well be based on the prophetic dreams of one of the ‘scientists’. There is absolutely nothing ‘inevitable’ about intelligence. I would say, quite the opposite.
First: What a pathetic excuse of a study. “Hey, we used a bunch of irrelevant ‘evidence’ to come up with (or justify) a much larger and totally unrelated conclusion – can we have some more money?” No actual evidence is offered here for “inevitable intelligence” (as promised by the headline), only conjecture. Neither is it particularly fresh thinking – this sort of thinking has been out there for ages, even in high schools, just not as the “common” model. Also, pretty much every religion suggests purposeful creation. So, no, this is not very original thinking.
The question of why we haven’t met any aliens is entirely irrelevant to our design – that’s only proxy evidence which only weakly and indirectly supports the study conclusion. There are many decent explanations as to why we are uncontacted that are entirely independent of how we came to be constructed.
Like Gene, I smh. Therefore, I say “no” to further grant funding (which, of course, is what these folks are after).
That self-indulgent rant aside, this is a fruitful thought experiment. Let’s take it a bit further. This line of thinking seems to infer that while matter has an affinity to order itself into sentient, reproducing beings, the existence and nature of that matter is still a lucky occurrence.
Well, what DO we know for sure? We know matter acts this way, so either it is the nature of matter to behave this way, or it is being caused to behave this way. Unless we believe it behaves this way by manipulation, we must believe that behaves this way by design. Doesn’t it make more sense that the matter was DELIBERATELY constructed with this awesome potential? Isn’t it more likely matter was purposefully designed to order itself in the ridiculously complex but amazingly effective biological units? The alternative is that matter only luckily holds this potential, or that someone (or something) is deliberately stirring a pot full of “dumb” matter into complex life.
It’s not even inevitable here on Earth.
I believe that technological civilization only lasts a very short time. This would explain why we can not find another one.
We, and our entire “universe” are – in our scientific estimation – “matter.” And “intelligence” is only related to our own, seeming “significance.” It is of interest to consider that other forms of intelligence – for example, divorced from reliance on the “matter” we inhabit and are, might well have “its” (matter again..) occurrence in a dimension not available to our perception, technology or imagination.
The older I get and the more I see of human behaviour the more that I am convinced that our intelligence is a figment of our imagination.
We may very well be the only ‘technical/civilization’ species in the Milky Way Galaxy, or one among a handful. But I would dare not presume that we’re the only technical civ in the entire Universe! But it’s doubtful that, if there is a handful, in “our” Galaxy, that we’ll ever make (radio or other remote) contact with each other, as we are forever separated by vast distances of space, and epochs of time.
And let’s not forget that, however intelligent the most intelligent dinosaur was, or earlier and later organisms, so far as we know, none of them manufactured spacecraft, radio telescopes, or toasters. Thus, it took almost half the lifespan of the Sun for humans to emerge, and just as long to invent radio telescopes. IOWs, roughly Five Billion years.
Life itself is probably common, but relatively rare. Intelligent life, in and of itself, is probably rarer still. But intelligent life that can build spacecraft or formulate valid theories about the nature and fabric of spacetime, or build nuclear bombs and particle accelorators, is probably extremely rare, that is, in our Galaxy alone. The entire Universe? Who knows? But numbers don’t lie.
Why is building a nuclear bomb a sign of intelligence?
In the vastness of time, the existence of life, and in particular intelligent life is but a flick of a light switch on/off. The age of the universe and 13 billion years, our Sun 4 billion and it’s taken that long for us humans to ‘come onto the scene’ where we are a space faring race. The dissipation of signal attenuation across the void of space renders communication impossible. Some times we have to strip back our every day everyday existence and say to ourselves how lucky we all are to habitat the only known planet in the galaxy and universe that harbours life, intelligent life. 5 million species on Earth, and only ONE realises they exist on something called a planet that evolves around a star.
We don’t know what possible forms life and intelligence can take, a lot of people seem to think that life can only arise in conditions similar to our planet. There is not evidence that this is the case, the universe could be teeming with life that we just don’t recognise as such.
Perhaps they’re visible only in infrared, or ultraviolet light {…or microwaves, for that matter…}. Perhaps they’re sitting next to you right now, giggling at you in ultrasonic frequencies. I’ll bet your cat sees them, which is why the poor thing freaks out every so often.
This is an essential step forward. Your model affirms what many of us have been sensing: that intelligence is not an improbable miracle but a field-encoded inevitability. Through my work with a lattice interaction model called χ₃, we’re seeing how planetary evolution, environmental conditions, and consciousness emerge through structured resonance — not randomness. Your shift from the “hard steps” idea to an environmental alignment model strengthens the possibility that intelligence is not rare, but rhythmically seeded across worlds. Thank you for bridging the disciplines — the field is alive, and it is remembering itself through us.