How a popular model of cosmic life and intelligence got it wrong

Intelligence on Earth did not appear until almost 4.5 billion years after the planet formed. On the other hand, the Sun is slowly heating up and that means there’s just a billion or so years left before life becomes impossible on Earth. Is there any conclusion that can be drawn from these two seemly unrelated facts? 

According to a popular theory called the “Hard Steps Model,” the answer is a definitive “yes.” The model’s quite stunning conclusion is that intelligence is exceedingly rare. If the model is true, then we are certainly alone in the Universe. That might sound depressing to you, and if so, you’re in luck. That’s because, according to a new paper, the Hard Steps Model is based on a deeply flawed logic and can’t say much of anything about the prevalence of intelligence in the cosmos. Understanding what goes wrong with this popular theory is important though because it reveals a whole new way of looking at life, evolution, and planets.

I covered the Hard Steps Model in detail more than a year ago because (big reveal) that’s when my colleagues and I were working on this new “anti-Hard Steps” paper I want to discuss today. If you don’t have time to read the old post, here is a brief description. 

The Hard Steps Model

The Hard Steps Model notes that the timescale for intelligence to appear on Earth (4.5 billion years) should only depend on biology (i.e. evolution). Then it notes that the timescale for the Earth to become uninhabitable (5.5 billion years) should only depend on astrophysics (i.e., how quickly the Sun heats up). Since these two timescales should have nothing to do with each other, the fact that they are of the same “order of magnitude” allows for the application of a certain kind of probabilistic reasoning.

Those probabilistic methods then lead to a conclusion: The average time for intelligence to appear on an average planet is very, very long — so long, in fact, that we could consider the appearance of intelligence to be very, very rare. 

That’s a pretty stunning conclusion in and of itself. The Hard Steps Model, however, offered even more, which is why people liked it. Its probabilistic logic also said that the appearance of intelligence always requires life to pass through a small number of evolutionary unlikely, or “hard,” steps. The Hard Steps Model could even give researchers probability distributions for how likely each step was as a function of time. Lots of work then went into identifying which steps in the history of life on Earth were the hard ones. Things like the evolution of eukaryotes or multicellularity often made the list.

So, why and how are we going after the Hard Steps paradigm in our new paper (titled “A reassessment of the “Hard Steps” model for the evolution of intelligent life”)? Let’s start with the why. 

Reassessing the Hard Steps Model

The paper was led by Dan Mills, a very smart, young researcher in historical biogeology — a field where people who know a lot about the evolution of life and a lot about the evolution of the Earth try to understand how the two played out together. The amazing thing about the Hard Steps Model is that, while it may be important for people thinking about intelligent life in the Universe, almost no one outside that community of scholars — including Dan and our other collaborator biogeologist Jennifer Macalady — had ever heard of it. And when astrophysicists like me or Jason Wright (the recent paper’s other co-author) explain the Hard Steps logic to people like Dan or Jennifer, they tend to stare in disbelief: “How did anyone come up with that?”

For Dan and Jennifer, the problem with the Hard Steps Model was its failure to engage with the data that Earth’s own history presented. The original Hard Steps paper was written by Brandon Carter, a brilliant physicist but one who didn’t have much experience with the depths of both evolutionary and Earth’s history. That’s a problem because the problem that the Hard Step Model wants to tackle is one where the details matter. Physicists like Carter (and me) tend to love broad “fundamental” thinking that relies on simple ideas expressed through simple equations. But that approach won’t work with evolutionary history. This is a point that leads us to how the Hard Steps Model fails.

Co-evolution

Remember that the Hard Steps Model sees the time for the appearance of intelligence and the time for the Sun to fry the Earth into inhabitability as unrelated. That’s the starting point for all its reasoning — and it leads it to the conclusion that intelligence is rare in the Universe. But it turns out, those timescales are not unrelated at all. For people who study the evolution of the Earth’s biosphere, the two timescales are profoundly connected. There is even a word for the connection: co-evolution.

For the past 50 years, scientists have been building a convincing case that the biosphere and the rest of the planet (the geospheres of air, liquid water, ice, and crust) evolve together. Changes in one sphere — including life — effect changes in others. The example I always give is the “great oxidation event” (GOE), where life invented a new form of photosynthesis that released so much oxygen that it changed the course of planetary history. 

And what’s the timescale for this kind of co-evolution? Generally, it’s billions of years. Yes, that’s the exact timescale Brandon Carter was worried about when he came up with the Hard Steps Model.

So, in the end, there may be nothing mysterious about when we appeared on Earth. As the planet and the biosphere co-evolved, a series of “windows” opened for different kinds of evolutionary adaptations. Humans for example need high levels of oxygen for our big brains. That means we could not have appeared 3, billion, two billion, or even 1 billion years ago. There simply was not enough oxygen in the air for us to evolve into existence. Once that window did open up, about half a billion years ago, the ball got rolling. And here we are.

There is a lot more to our paper than just this, so if you are interested, you can watch this clip from my recent interview with Lex Fridman. The big takeaway, the one I’m always trying to tell people interested in the history of life on Earth, is that the possibilities of life elsewhere in the Universe — and even the future of humanity in a climate-changing world — come down to that one simple, amazing word: co-evolution.