One of the enduring puzzles in astrobiology is why we haven’t found any aliens yet — and it’s worth another look, in light of Breakthrough Listen’s latest (and unprecedented) effort to find some in a new survey of millions of stars in our galaxy. Astronomers estimate there are between 200 and 400 billion stars in the Milky Way. The Kepler telescope has proven that many stars have planets, and if the Earth is in any way a typical planet, we should see some evidence of life elsewhere in the galaxy after 13.8 billion years. Except, of course, we don’t. This is known as the Fermi Paradox.
A paper by astrobiologists Aditya Chopra and Charley Lineweaver of the Australian National University has an explanation for why this might be so. It’s a modification of the so-called “Great Filter” theory and of the Gaia Hypothesis, and it argues that life on most planets may become extinct not long after it first evolves. This would partially explain why efforts like Breakthrough Listen have thus far failed to find the results they seek. Before we tackle the Gaian bottleneck theory, let’s discuss the larger Gaian hypothesis.
The Gaian Hypothesis
First proposed by James Lovelock, the Gaia hypothesis proposes that living organisms interact with inorganic material on the planet in a manner that perpetuates or regulates the conditions for life on the planet.
Lovelock proposes three central arguments: 1). The Earth is an extremely favorable habitat for life, 2). Life has greatly altered planetary chemistry and the environment, including both the atmosphere and sea, and 3). Earth’s environment has remained fairly stable over a long period of time.
Of these three points, #2 is absolutely true. We know, for example, that an event known as the Great Oxygenation Event killed most of the life on Earth when oxygen — produced as waste from anaerobic cyanobacteria — could no longer be absorbed by inorganic sources. Life on Earth is known to have existed for over 3.8 billion years, but it wasn’t until some 2.5B years ago that oxygen began to appear in measurable quantities, and the level of O2 in the atmosphere didn’t begin approaching modern concentrations until some 850 million years ago.
Even if the GOE was our only evidence for the potential planetary impact of biological life, the transformation of Earth’s atmosphere is proof that biology can absolutely alter planetary conditions. But the evidence for the Earth being an extremely favorable habitat for life is undercut by what we now know about periods of geologic time where the Earth was almost completely frozen (the so-called “Snowball Earth,”) while point #3 — Earth’s environmental stability — is weakened by the existence of events like the Great Oxygenation Event, which wiped out vast numbers of species on Earth. In fact, the production of oxygen may have actually caused the Snowball Earth. These events rule out strong Gaia, or the idea that life optimizes the biosphere to meet its own needs.
The “weak” Gaia model, on the other hand, has more staying power. In the image above, Coevolutionary Gaia and and Influential Gaia posit that life has a collective impact on Earth’s environment and that the evolution of life and the evolution of the environment are intertwined. These are much less controversial points and are already well-explored and established in literature. There’s debate over whether or not this constitues a “Gaian” hypothetical at all.
It is absolutely possible that the life on a planet could play a role in stabilizing the biosphere and making it more likely that life would continue to exist. But based on what we know of Earth’s history, it seems possible that the effect could also run in the opposite direction. The GOE is thought to have possibly caused the Huronian glaciation that nearly froze the Earth because it led to CO2 replacing methane in the Earth’s atmosphere — and CO2 isn’t as potent a greenhouse gas. This led to the Earth nearly freezing solid as a result.
Now that we’ve covered the Gaia hypothesis in a bit more detail and constrained the mechanism of its operation, let’s look at what these scientists are proposing.
The Great HEPA Filter
The “Great Filter” theory explains the Fermi Paradox by positing the existence of a specific bottleneck that prevents most species (or life in general) from reaching the point where the colonization of space is possible. Life might evolve frequently, but supernovae and gamma ray bursts sterilized many of the planets on which it evolved earlier in our galactic history. Maybe highly intelligent life has a tendency to over-consume the resources of its home planet, and therefore becomes extinct before developing the technology to colonize a galaxy. Life might be abundant, but the evolution of high intelligence and efficient tool use could be extremely rare.
What Chopra and Lineweaver propose is that life may evolve often, but only rarely survives long enough to begin to have an impact on its own planet in a way that creates and maintains an environment amenable to life. They call this the Gaian bottleneck.
The blue area at the bottom of the diagram shows the range of planetary environments that can form, while the yellow cylinder shows the conditions required for the formation of an “abiogenesis habitable zone (ABZ).” If a planet’s initial environment is within this zone, life can begin to evolve. As life evolves (green area) it creates wider zones of habitability, which allow for other life to evolve in turn.
To understand how this particular filter would function, consider the fates of Earth, Venus, and Mars. We now have strong evidence that Mars was once a warmer and wetter place than it is today, with many of the base conditions believed to favor the evolution of life. There’s less information available on Venus’ early history, but what data we do have suggests that it, too, may have once had liquid oceans and even a short-lived magnetic field. (Mars has remnants of such a field today).
The difference between our three planets is that both Venus and Mars may have lost the conditions that made them initially habitable more quickly than life on those planets could evolve to keep them habitable. From the paper:
Liquid water is not easy to maintain on a planetary surface. The initial inventory and the timescale with which water is lost to space due to a runaway greenhouse, or frozen due to ice-albedo positive feedback, are poorly quantified, but plausible estimates of future trajectories have been made. On Earth, dissociation of water vapor by UV radiation in the upper atmosphere is ongoing and will eventually (1–2 billion years from now) lead to the loss of water from the bioshell and the subsequent extinction of life on Earth.
The authors’ argue that early life may perform a vital regulating function on an early planetary environment by impacting both the planet’s albedo (how much light it reflects from the sun) and the greenhouse gases present in its atmosphere. They modeled how quickly a planet lost its water based on either a runaway greenhouse effect (Venus) or runaway glaciation (Mars). In both cases, the amount of water remaining on the planet is a fraction of its total by roughly one billion years old.
The Gaian bottleneck theory offers a model for how life might be distributed within the universe, as shown below:
In an emergent bottleneck scenario, the conditions for life are rare, but once life evolves, it persists for billions of years. This assumption helps create the Fermi Paradox, because if even a handful of civilizations exist, we should have seen some evidence of them already.
If there is no emergence bottleneck, life should already be plentiful — think of this as the Star Trek / Star Wars hypothesis, where hundreds of species exist across the entire galaxy.
The Gaian model predicts an initial surge of life within the first billion years of planetary development, but that the vast majority of this life never survives long enough to advance beyond single-cell organisms. It predicts that the majority of life we’ll find out among the stars will be fossilized remnants of primitive creatures, assuming any evidence exists to be found at all.
Obviously we won’t be able to prove or disprove any of these models until we can actually evaluate the conditions around other planets. But if the Gaian bottleneck theory is accurate, we might find evidence for it on the moons and rocky planets of our own solar system. It’s virtually impossible to build a probe that can survive on the surface of Venus for any length of time, but Titan, Mars, Europa, and Ganymede might contain such life — or fossilized evidence of its original existence.