Four billion years ago, eight planets circled the sun. There was potential for life on the second, third, and fourth farthest from the star, but none yet existed. On Venus, an overactive greenhouse effect exterminated that potential. On Mars, low gravity allowed solar wind to wipe away the atmosphere, freezing out the potential. On Earth, life blossomed in the oceans, proliferated and evolved to the point where those in possession of it could ponder their own origins.

But why Earth? A little-known but surprisingly potent theory suggests we are alone or almost alone in the universe — not because life is rare but because co-evolution with planetary forces is so strenuous that most life does not survive. Proponents of this theory, called the Gaian Bottleneck, are drawing from the older Gaian World hypothesis, which suggests the presence of life changes the environment, helping maintain the conditions necessary for life to continue to exist.

The Gaian Bottleneck, is quite controversial in the community of scientists studying the origins of life. Researchers are split on whether Venus and Mars ever hosted life — there’s not definitive proof either way — and there is still no consensus on the nature of early Earth processes. With no clear consensus about the origin of life on Earth itself and no data about life elsewhere theories like the Gaian Bottleneck created by Aditya Chopra, an astrobiology postdoc at the University of Washington, and Charles Lineweaver can’t be exactly proven or disproven. Instead, they become part of a complex web of ideas about our relationship to the rest of the universe that inform how we try to answer one of humanity’s most existential questions: Are we alone?

“What we are predicting is that we will find that most planets are not inhabited,” Chopra tells Inverse, “and we shouldn’t be disappointed.”

The fact that life has proved so difficult to find beyond Earth has long puzzled scientists, most of whom aren’t comfortable leaping to the conclusion that we’re alone. The most famous way of cutting through the knot of contradictions that creates is called a thesis called the Great Filter that posits extinction-level event killing of all living beings occurs on many planets relatively frequently.

Because extinction-level events can take a lot of different forms, there are a huge variety of Great Filter hypotheses, and nine different points at which life could fail. James Kasting, a geophysicist at Penn State University, puts it in stark terms. “I’m worried that the filter is in our future, and not in our past,” he says.

However, with so little data on life, it’s hard to know exactly what the pressure point will be. “The Great Filter could be this Gaian Bottleneck, but it could be the origin of life itself,” Kasting explains. “It could be the origin of eukaryotes, the origin of sex, the origin of intelligent life — it could be that technological civilizations like ours destroy themselves. I think climate change could be a Great Filter for us.”

However, Chopra and Lineweaver look at the lack of life in the universe and come to the opposite conclusion: If life is common and we haven’t found it in the universe, it is possible that we exist on the other side of the filter. If there is one point in the evolution of life that is so difficult it causes most life to go extinct, it is possible earthlings have been there and done that.

A view of nearly 10,000 galaxies are seen in a Hubble Telescope composite photograph released March 9, 2004. The Hubble Ultra Deep Field (HUDF) photograph is a composite of a million one-second exposures and reveals galaxies from the time shortly after the big bang.  (Photo by NASA/Getty Images)
A view of nearly 10,000 galaxies are seen in a Hubble Telescope composite photograph released March 9, 2004. The Hubble Ultra Deep Field (HUDF) photograph is a composite of a million one-second exposures and reveals galaxies from the time shortly after the big bang. (Photo by NASA/Getty Images)

To create the Gaian Bottleneck hypothesis, they turned to a theory on the interaction between life and the planet called the Gaia Hypothesis. In a Gaian world, the existence of life is what makes a planet inhabitable. It’s a controversial idea, and when it was first suggested in the 1970’s, it gained an almost theological aspect — that life works together with non-living processes to create a self-regulating system. The theory has been accepted to a certain extent today — the idea that life effects the environment is generally agreed upon, but the scientific community largely rejects the anthropomorphic and theological aspects.

The controversial part of Chopra’s and Lineweaver’s Gaian Bottleneck is its core premise that early microbial life is what kept Earth from turning out like Venus or Mars. That’s nearly impossible to prove, especially because exactly what effect life has on the modern environment is hard to explain.

Over the last several million years, the Earth’s environment has been pretty much the best place to be if you are an aerobic complex organism, like we are, says James Kirchner, a geochemist at the Swiss Federal Research Institute. “We have to recognize, of course, that there is an element of survivor fallacy there.” Kirchner points out that most of the organisms that have lived on Earth have gone extinct, and lived in climes and biomes that are totally different from what exist on Earth today – “So to turn around and say the climate is ideal, is like: Of course — for us,” he says.

But Chopra points out that the Earth has been habitable for other organisms throughout its history, which is not the case for our sister planets, Venus and Mars. There are theories that both Venus and Mars had some form of microbial life, says Chopra. We know that Mars had vast oceans at one point, and it is likely that Venus had oceans early in its planetary life as well. With environments like those of the early Earth, it would make sense for both to have had microbial life, says Chopra. And the failure of life and the uninhabitable conditions on both planets suggests to Chopra that sustaining that life for billions of years is unusual. But it’s unclear exactly what would make life on Earth special. “It depends on what selection process life on Earth experienced,” Chopra says. “It is plausible that some of those are rare.”

When we look for intelligent life, Chopra points out, we are using an assumption that life goes extinct after a long period. “However, late extinction of life is something that we don’t have any evidence for,” he says. “We must, as rational scientists, turn towards the idea of early extinction of life, which we have some evidence for.”

LONDON, ENGLAND - DECEMBER 03:  A view of the world's most complete Stegosaurus fossil as it is unveiled at a press preview at the Natural History Museum on December 3, 2014 in London, England. The skeleton is 560 centimetres long, 290 centimetres tall, has over 300 bones and is the first complete dinosaur specimen to go on display at the Natural History Museum in nearly 100 years. (Photo by Carl Court/Getty Images)
LONDON, ENGLAND - DECEMBER 03: A view of the world's most complete Stegosaurus fossil as it is unveiled at a press preview at the Natural History Museum on December 3, 2014 in London, England. The skeleton is 560 centimetres long, 290 centimetres tall, has over 300 bones and is the first complete dinosaur specimen to go on display at the Natural History Museum in nearly 100 years. (Photo by Carl Court/Getty Images)

But there is not a lot of evidence to support either hypothesis. We haven’t gotten to the point where we can find fossil evidence of microbial life on Venus or Mars yet, so Chopra’s hypothesis is based on research on the origin of life on Earth and its hypothetical loss on Venus and Mars. One of the big divides between Chopra and skeptics is the rate of hydrogen loss to space, which can lead to planets drying out. Chopra doesn’t agree, and thinks that there is enough evidence in ancient fossils to suggest that the Earth was covered with microbial mats that modified the rate of hydrogen loss, keeping Earth habitable.

Chopra predicts that life would have looked very different in Venus and Mars. Instead of colonies and strong microbial communities, he thinks Venus and Mars had pockets of microbes. On Venus or Mars, “we will find individuals that were not able to control the greenhouse gases and albedo of the planet,” he says. In this theory, the microbes were not cooperative enough to affect the atmosphere or reflectivity of the surfaces of Venus or Mars.

Without cooperative microbes affecting the early atmosphere, most life would go extinct very early on in planetary history, says Chopra. To makes intelligent life even more rare in the universe, and Chopra points out that of all the life on Earth, only humans seem to be intelligent life. “Just because it happened once on Earth, doesn’t mean we should expect it elsewhere.” Still, Chopra says, if we find a planet that has liquid water after billions of years, it might be an indication that such a planet has intelligent life — but he thinks finding such a planet is extremely unlikely.

“The universe is under no obligation to prevent disappointment about us being alone,” says Chopra. “Even if we are not alone in the entire universe, just because of the rarity of planets inhabited for billions of years: It is reasonable to assume that we are alone in our local universe.”

Other scientists have pushed back against this theory, rejecting assumptions about water loss and weathering in the early world, to the existence of life elsewhere. Lee Kump, a biochemical-geologist from Penn State University, says that there are models that show the Earth would still be wet without microbial life. And Kasting from Penn also points out, “we don’t know how widespread life was in the early Earth.” He is skeptical that microbes could have had a world-altering effect because the early Earth didn’t have very much exposed land. There is also the problem that rocks from 3.5 billion years ago are extremely rare. This makes it hard to argue whether life was widespread or just present in pockets around hydrothermal vents and shallow water on the early Earth.

An artist's impression of the planetary system around the red dwarf, Gliese 581, showing what astronomers believe is one of the most earth like planets found outside our solar system.  The planet may have water running on it's surface, and is 20.5 light-years away in the constellation Libra. (Photo by ESO via Getty Images)
An artist's impression of the planetary system around the red dwarf, Gliese 581, showing what astronomers believe is one of the most earth like planets found outside our solar system.  The planet may have water running on it's surface, and is 20.5 light-years away in the constellation Libra. (Photo by ESO via Getty Images) 

However, without strong observational evidence either way, Kump takes a middle ground on the influence of life. “It’s very likely that the climate history for the planet was strongly influenced, if not determined by those interactions with life,” he says. “But that’s not quite the same as saying that the only reason it’s habitable is because of life.” Pushing this hypothesis beyond the origin of life on Earth into the universe at large is difficult. “When you open up the possibility that there are solar systems out there that might have been less challenging for physical processes,” Kump says, “I find it hard to say that life was absolutely essential You have to make an awful lot of shaky assumptions to get any answer at all.”

Finding an answer has been Kasting’s life’s work. “I think life arises frequently, if not all the time,” he says. If researchers can get a good look at 10-20 Earth-like planets in habitable zones, he thinks they will find life. He’s looking for gases like nitrous oxide or methane in an atmosphere with oxygen, as these gases are hard to make without life.

To find these gases, researchers will need a very powerful direct imaging telescope. Reduced gases are likely to be present in the atmosphere in very low concentrations — a few hundred parts per million. “I’d like the chance to test that hypothesis,” says Kasting, pointing out that he doesn’t have much time left in his life to do so.

Whether Chopra is correct, or not, having the discussion about the connection between the origin of life and extraterrestrial life is important, Kasting says. “We need arguments like this because we need the impetus to build big telescopes and do future planetary missions.”

Without experimental evidence for the Gaian Bottleneck, Chopra and Lineweaver’s theory remains unconvincing to the scientific community. Chopra hopes that work from researchers like Kasting will provide answers in the next 30 years. As for whether we are alone in the universe, Chopra says the answer, regardless of whether it’s yes or no, is “profound no matter what the answer may be.”

Photos via Getty Images / NASA, Getty Images / Carl Court, Getty Images / Handout

Dyani Sabin is a science writer from small-town Ohio transplanted to New York City. Former biology researcher and library supervisor, you can also find her writing at Scienceline.