Can the “difficult stages” in the evolutionary history of human intelligence be reformulated using geological thresholds?

Why did it take so long for humans to appear on Earth? The Earth is 4.6 billion years old, and life appeared about 4 billion years ago, but humans, the only intelligent and technological species we know of in the universe, have only been around for 200,000 years. Why didn’t we arrive sooner? What factors delayed our appearance? And what can the timeline of life tell us about the possibility of other technologically advanced life forms in the universe?

One hypothesis of our evolutionary history is the “difficult stages” model. In 1983, Australian physicist Brandon Carter hypothesized that the path to humanity required “successful passage through a number of intermediate stages,” each of which was highly unlikely and difficult given the time available. Carter originally saw only two: the origin of the genetic code and “the final breakthrough in brain development.”

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

These steps are “hard” in the sense that they would have developed only once (unlike, for example, the evolutionary development of eyes, which occurred many times and arose at varying degrees of light use).

About a billion years have passed since each of these stages. Although the Sun has 5 billion years left before it becomes a red dwarf, life on Earth will disappear in about a billion years when the increasingly luminous Sun causes a runaway greenhouse effect and the planet ends up like Venus, burning and dried out.

The question then arises as to whether intelligent, technological animals like us evolved on Earth only late in the available time, by the skin of our teeth. Would the same fate befall any advanced alien life elsewhere in the galaxy? Could it have evolved too late in the lifespan of their sun to arrive on Earth? The hard-step model therefore predicts that technological species such as humans on Earth are extremely rare in the universe.

“The hard-step model argues that humanity probably would not have evolved before the disappearance of Earth’s biosphere,” said Daniel B. Mills, lead author of the study. “Our existence represents an exceptional case in the universe where intelligent life managed to evolve before the planetary clock went off.”

Mills works at Ludwig-Maximilians-University of Munich in Germany and Pennsylvania State University in the United States. He writes with colleagues at Pennsylvania State University and the University of Rochester in New York State. The article has been posted on the preprint server arXiv and was submitted to a journal.

Indeed, Carter’s criteria for difficult steps are that they are improbable in the allotted time frame and essential for the ultimate evolution of humanity. The researchers here challenge the difficult step model by asking whether these steps are truly improbable and truly essential.

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

“The basis on which human intelligence can be considered a difficult stage 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 is possible that other intelligent species existed on Earth in the distant past, and that any physical evidence of their existence is now completely gone, erased by Earth’s geological processes about half a billion years ago.

They also question whether the “major transitions in evolution” (MTE) framework, first proposed in 1995 and which proposes eight major transitions in evolution up to humans, is really necessary for the development of complexity or advanced life. Is a genetic code really necessary? A language?

The book moves away from biological evolution and challenges some key assumptions of the hard-stage model by looking closely at the role of historical geobiology, that is, how the Earth’s surface environment and life have co-evolved over deep geological time. The crossing of key environmental thresholds would open the way to 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 is possible that the life stage that was first present in a new environment thrived and dominated that environmental episode, putting evolution on a new branch instead of the status quo branch that existed before the threshold was crossed. Global environmental changes may have been controlled as candidates for difficult stages (the five above and others added since) evolved.

The paper concludes: “We raise the possibility that there are no decisive steps (despite the appearance of major evolutionary singularities in the universal tree of life) and that the general pace of evolution on Earth is determined by global environmental processes operating on geological time scales (i.e. billions of years). In other words, the appearance of humans so “late” in Earth’s history is due to the fact that the “window of human habitability” opened only relatively recently in Earth’s history.”

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

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

He believes that this idea handicaps human life and leads us in the wrong direction.

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

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