Can the ‘hard steps’ in the evolutionary history of human intelligence be recast with geological thresholds?

What took so long for humans to appear on Earth? The Earth is 4.6 billion years old, and life began about 4 billion years ago, yet humans—the only intelligent, technological species we know of in the universe—have existed only for the last 200,000 years. Why didn’t we come sooner? What factor(s) delayed our appearance? And what can life’s timeline here say about the possibility of other technologically advanced lifeforms in the universe?

One hypothesis of our evolutionary history is the “hard steps” model. In 1983, the Australian physicist Brandon Carter hypothesized that the path to humanity required “successful passage through a number of intermediate steps,” each of which was very improbable and difficult given the time that had been available. Originally, Carter saw only two steps—the origin of the genetic code, and “the final breakthrough in cerebral development.”

Many researchers have since modified Carter’s idea and proposed more than two steps, with the most popular formulation of the model today envisioning five: 1) the creation of single celled life (“abiogenesis”), 2) the appearance of photosynthesis that creates oxygen, 3) the emergence of eukaryotic cells from prokaryotic cells, 4) the emergence of complex life, such as multicellular animals, and 5) the rise of Homo sapiens with an established language.

These steps are “hard” in the sense that they would only have developed once (as opposed to, for example, the evolutionary development of eyes, which occurred many times and which have emerged in various degrees of light utilization).

There were approximately a billion years behind each of these steps. Although the sun has 5 billion years left before it becomes a red dwarf, life on Earth will end in about a billion years when the ever more luminous sun causes a runaway greenhouse effect and the planet will end up like Venus, broiling hot and desiccated.

So the question arises if intelligent, technological animals like us developed on Earth only late in the time available, cutting it close. Would the same fate befall any advanced extraterrestrial life elsewhere in the galaxy? Might they have developed too late in their sun’s timespan to get to Earth? The hard steps model thus predicts that technological species such as humans on Earth are exceedingly rare in the universe.

“The hard steps model argues that humanity was unlikely to evolve before the death of Earth’s biosphere,” said Daniel B. Mills, lead author of the paper. “Our existence represents an exceptional case in the universe where intelligent life managed to evolve before the planetary clock ran out.”

Mills is at the Ludwig-Maximilians-Universität München in Germany and also Pennsylvania State University in the United States, writing with colleagues at Pennsylvania State University and the University of Rochester in New York state. The paper has been released on the preprint server arXiv and has been submitted to a journal.

In effect, Carter’s criteria for hard steps are that they are improbably within the allotted period of time, and essential for the ultimate evolution of humanity. The researchers here challenge the hard steps model by asking whether its steps are truly improbable and truly essential.

Is human intelligence really necessary, or can intelligence of other forms suffice, such as the intelligence seen in the collective behavior of insects or ants, the tool-making intelligence of crows and dolphins, or the intelligence that developed independently in octopuses after they split from our last common ancestor, a primitive flatworm that lived 750 million years ago?

“The basis for seeing ‘human intelligence’ as a ‘hard step’ is uncertain,” Mills and colleagues write.

And according to the Silurian hypothesis—which one of the co-authors, Adam Frank of the University of Rochester helped develop—it’s possible other intelligent species existed on Earth in the deep past, with any material evidence of them now utterly gone, wiped away by Earth’s geological processes before about a half-billion years ago.

They also question whether the “Major Transitions in Evolution” (MTE) framework, first proposed in 1995, proposing eight major transitions in evolution up to humans, are truly necessary for the development of complexity or an advanced lifeform. Is a genetic code really required? Language?

Taking a distinct turn from biological evolution, the work also challenges some key assumptions of the hard steps model by closely examining the role of historical geobiology—how the Earth’s surface environment and life have co-evolved over geologic, deep time. Passing key environmental thresholds would open up new “permissive” environments.

In this scenario, environmental change does not create evolutionary innovation, but “represents the removal of an external constraint that had previously prevented the innovation from evolving.”

It’s possible that the stage of life that was first present in a new environment would thrive and dominate that environmental episode, setting evolution down a new branch instead of the status quo branch that existed prior to the threshold being passed. Global environmental changes may have been controlled when the hard steps (the five above and others added since) candidates evolved.

The paper concludes, “We raise the possibility that there are no hard steps (despite the appearance of major evolutionary singularities in the universal tree of life), and that the broad pace of evolution on Earth is set by global environmental processes operating on geologic timescales (i.e., billions of years). Put differently, humans originated so ‘late’ in Earth’s history because the ‘window of human habitability’ has only opened relatively recently in Earth history.”

Mills said the notion of hard steps has left an imprint on humanity that is not justified. “Many people have taken these conclusions for granted, as if science has actually proven that our existence on Earth depended on chance events with small likelihoods in the available time,” he said.

“Not only are these conclusions unjustified, they are damaging to our collective self-image, contributing to the notion that humans are an accident of Earth’s biosphere rather than a natural expression of it.”

He believes this idea has handicapped human life and is steering us in the wrong direction.

“This attitude of human unlikelihood and fragility contributes to a form of ‘learned helplessness,'” he said, “that sees the climate crisis, for example, as our death sentence, rather than the inevitable challenge any global civilization must face.”

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