Boost for alien hunters? Earth life may not be so improbable, study suggests
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The concept of the "great filter" to explain why so far we seem to be alone in the universe is based on erroneous assumptions, according to a new model that describes how life on Earth evolved in step with changing geobiological conditions rather than through a series of improbable events.
"We're arguing that intelligent life may not require a series of lucky breaks to exist," said lead author Dan Mills of the University of Munich in a statement. "Humans didn't evolve 'early' or 'late' in Earth's history, but 'on time' when the conditions were in place."
It was the Australian physicist Brandon Carter who first popularized the notion that life on Earth was the result of a sequence of unlikely events, which he described as "hard steps" in a 1983 paper.
A black hole theorist, from time to time Carter also dipped his hand into more existential matters, specializing in drawing assumptions from probabilistic and anthropic (i.e. the argument that conclusions about the nature of the cosmos have to be constrained by the fact that we exist) reasoning to say something about our existence in the universe.
Related: The search for alien life
This is no better seen than in his Doomsday argument, in which Carter posits that we, as individuals, are more likely to exist at a time when the greatest number of humans are alive. For example, imagine every human who ever lived is given a number based on the order in which they were born, and then these numbers are pulled from a pot like the numbers in a lottery — you're more likely to pull a higher number than a very low number if the total number of humans who have lived and will ever live is large. Since population growth can be modeled as exponential, the fact that we exist now with a relatively low birth number compared to all the hundreds of billions to trillions of people who will follow us suggests that something catastrophic could be about to happen to the human race that will curtail future population numbers. At least, that's the argument; philosophers and statisticians have been arguing about it ever since Carter proposed it.
Carter's "hard steps" model of our evolution on Earth is similarly probabilistic in nature. The sun is nearing the halfway point of its approximately 10-billion-year lifespan, and yet it's taken us — Homo sapiens — nearly all of that time to arrive on the scene. Carter could not see any reason why it would take so long for human-like life to evolve on Earth if complex life is common in the universe. This suggested to Carter that the development of human-like life must be difficult, passing through a series of evolutionary bottlenecks for which the chances of life succeeding are so remote that we would not typically expect those evolutionary transitions to occur in the lifetime of Earth. Life on our planet would therefore be a complete fluke, unlikely to be repeated elsewhere in the universe.
The hard steps idea has subsequently morphed into the concept of the "great filter," the idea that something in the history of all life inevitably brings that life to an end. Suggested great filters have included the origin of life in the first place, the evolution of technological life and the ability of said life to wipe itself out. The existence of the great filter would certainly help explain the apparent "great silence" in the universe that SETI (search for extraterrestrial intelligence) researchers have encountered, with no confirmed evidence of alien life in all the decades that we have been searching.
However, like the Doomsday argument, the "hard steps" model has its critics, and now adding to them are the authors of a new paper that highlights what they say is a fallacy in Carter's reasoning.
Carter specifically assumed that the age of the sun, and therefore the Earth, should have no bearing on how quickly complex life evolved. However, the new paper by Mills (a geomicrobiologist), along with Penn State University co-authors Jennifer Macalady (a professor of geosciences), Adam Frank and Jason Wright (both astrophysicists), points out that the age of the sun and therefore the Earth very much have something to do with it.
The team selected five of the more universally agreed-upon "hard steps:" the origin of life, the evolution of eukaryotes (organisms with cells made from a nucleus containing genetic information surrounded by a membrane), the oxygenation of Earth's atmosphere, the development of complex multicellular life and the arrival of Homo sapiens. They then looked at how geological and atmospheric changes to Earth might have affected when these supposedly hard steps occurred. If Earth were initially hostile to these supposed hard steps, it would naturally explain why they took so long to pass — because they had to wait for Earth to reach the point where they could be possible.
Take, for example, the oxygenation of Earth's atmosphere. For over two billion years after its formation, Earth's atmosphere was mostly carbon dioxide. It was only about 2.1 to 2.4 billion years ago that Earth's atmosphere began to fill with oxygen. This was thanks to the onset of photosynthesis, brought about by the evolution of microbes called cyanobacteria. In turn, the development of cyanobacteria relied on certain climactic and environmental conditions. In some models, the oceans of this era were hot, and the water would have had to cool below 70 degrees Celsius (158 degrees Fahrenheit) for cyanobacteria to evolve. In other models, conditions were milder and the development of cyanobacteria then depended upon the availability of freshwater and how much of Earth's landmass was above sea level. Either way, cyanobacteria's evolution and the onset of photosynthesis and the oxygenation of the atmosphere was delayed until these conditions were met; it couldn't have happened any sooner.
And even once cyanobacteria were ingesting carbon dioxide and exhaling oxygen via photosynthesis, it took time for oxygen levels to build up. Multicellular life requires a certain abundance of oxygen, with more complex life in general requiring more oxygen. The oxygen abundance in the atmosphere suitable for the evolution of Homo sapiens didn't occur until 400 million years ago — meaning that for 91% of Earth's history, there wasn't enough oxygen in the atmosphere to support human life.
In other words, Mills' team propose that these were not "hard steps" as Carter saw them, but that life simply had to wait until Earth could facilitate them — that Earth and life had to co-evolve together.
Related: Fermi Paradox: Where are the aliens?
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Other variables that may have had an effect on how soon the different stages of life's evolution could occur include atmospheric ozone levels, nutrient availability, decreasing sea surface temperatures, decreasing ocean salinity, snowball Earth periods in which the planet completely iced over, and the development of plate tectonics.
"This is a significant shift in how we think about the history of life," said Macalady. "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."
We know from geological evidence that life existed on Earth as early as 3.7 billion years ago, and possibly even earlier. The initial development of life on Earth is known as the "habitability boundary." As different windows of habitability subsequently opened up, life would have been able to evolve in bursts. And if this is the way it happens on Earth, it could be the way it happens on other worlds, too — and perhaps faster or more slowly, depending upon how the geology of those worlds develops.
There is a caveat, in that evolutionary biologists still do not understand how life originated on Earth. This moment of genesis is currently lost in the mists of time, and we cannot yet say whether it was a fluke one-off event or whether it was an easy step. One possibility is that life developed on multiple occasions on Earth, but all the other lineages went extinct, leaving only ourselves — the descendants of LUCA, the last universal common ancestor, from which all known life on Earth evolved — as the only ones left standing. This would give the illusion that life originated only once when it could have had several independent origins.
Other mysteries include how biological cells first evolved, and what caused the dramatic onset of complex life during the Cambrian explosion 540 million years ago.
It is still entirely possible that these were unique and rare events, but the new paper is not arguing that life is common in the universe, only that the concept of hard steps in evolution is not necessarily true and that the development of planetary environments has a big role to play, counter to Carter's original model.
Another caveat is that, so far, astronomers have not yet found another world like Earth, so geologists cannot yet say whether the way in which Earth's geology and atmosphere developed is typical or not. It could yet be that creating a habitable world is where the hard steps really lie.
Until we discover true extraterrestrial life, whether that be microbes on Mars or bonafide little green men, we will continue to grapple with the possibility that Earth and its life are unique. For now, it's a lonely universe out there.
The Mills et al paper was published on Feb. 14 in the journal Science Advances.
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