Plate tectonics, oceans and continents could be the secret ingredients for complex life on Earth. And if these geological features are rare elsewhere in the universe, that may explain why we have not yet discovered intelligent alien life. New research from American and Swiss geoscientists suggests that these ingredients represent missing variables in the famous Drake equation, developed over half a century ago to estimate the probability of finding advanced civilizations in our galaxy. Incorporating these new variables could completely redefine the probability of discovering intelligent life in the Milky Way.
The impetus for this research, with its galaxy-spanning implications, came from a mystery all our own: Why did life take so long to evolve beyond simple organisms?
“Life has been around on Earth for about 4 billion years, but complex organisms like animals didn't appear until about 600 million years ago, not long after the start of modern plate tectonics,” said Robert Stern of the University of Texas at Dallas. “Plate tectonics really gets the evolutionary machine going, and we think we understand why.”
Stern and his colleague Taras Gerya of the Swiss Federal Institute of Technology in Zurich assume that plate tectonics – the grinding movement of the upper layers of the planet over long geological periods – helped accelerate the transition to complex life.
In Earth's early history, simple organisms formed in the ocean, but humanity – a highly advanced civilization capable of communicating across space – could not have existed if early life had not moved to land. Huge, resource-rich continents were therefore an indispensable prerequisite for the development of what Stern and Gerya call active communicative civilizations (ACCs) like humanity. But that alone was not enough: the continents had to move.
Earth's geological record suggests that plate tectonics accelerated evolution on land through five distinct processes: it increased nutrient supplies, accelerated oxygenation of the atmosphere and ocean, moderated climate, caused a high rate of alternation in the creation and destruction of habitats, and exerted non-catastrophic environmental pressures that forced organisms to adapt.
The end result of all this environmental pollution: us.
If Stern and Gerya are right, plate tectonics was a prerequisite for later innovations such as the bicycle, the smartphone and the Apollo program.
And for other civilizations in the galaxy to develop similar technological marvels, their planets may also need plate tectonics. But as far as we know, it's rare.
Earth is the only planet in our solar system that has plate tectonics. Volcanism exists on a few other worlds, such as Venus, Mars, and Io, but these worlds have a single fixed shell rather than multiple moving plates. Likewise, ocean worlds such as Enceladus and Europa are surrounded by a layer of ice that prevents any hypothetical life from moving onto land.
We don't know for sure whether there are planets with plate tectonics in distant solar systems – the resolution of today's space telescopes isn't good enough for that. But knowing that they might not exist allows for a more accurate version of the Drake equation.
The revised equation proposes two key factors: the fraction of habitable exoplanets with large continents and oceans, and the fraction of those whose plate tectonics last longer than 500 million years.
This version is much more sophisticated than the original Drake equation, which only took into account the proportion of habitable planets on which intelligent life had evolved.
The Drake equation, a mathematical formula for the probability of finding life or advanced civilizations in the universe. Image credit: University of Rochester
“The original formulation assumed that this factor was close to 1 or 100% – that is, evolution on all planets with life would proceed and, given enough time, lead to an intelligent civilization,” Stern said. “In our view, that is not true.”
Indeed. Their calculations reduce the percentage of these planets developing ACCs to just 0.003% at the minimum and 0.2% at the maximum – a far cry from the original 100%.
When you take into account all the other factors in the Drake equation: the number of stars forming each year, the number of stars with planets, the number of habitable planets, the number of habitable planets with life, the number of civilizations on those planets emitting detectable signals, and how long they emit those signals—well, the chances of finding intelligent alien life drop considerably.
The implications of the original Drake equation were that ACCs are common and we should see them everywhere. But when you add plate tectonics into the equation, the result changes and it becomes clear that it is perfectly understandable why we don't see ETs everywhere in the galaxy.
So intelligent extraterrestrial life may be rarer than we thought. And Earth may be more special than we thought. And all thanks to our planet's fragmented, unruly, and ever-changing top layer.
Learn more:
Amanda Siegfried: “Geoscientists explore why we may be alone in the Milky Way.” University of Texas at Dallas.
Robert Stern and Taras Gerya, “The importance of continents, oceans, and plate tectonics for the evolution of complex life: implications for the discovery of extraterrestrial civilizations.” Nature Scientific Reports.
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