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.
Estimated evolution of atmospheric oxygen. The upper red and lower green lines represent the range of the estimates. The stages are: stage 1 (3.85–2.45Gyr ago (Ga)), stage 2 (2.45–1.85Ga), stage 3 (1.85–0.85Ga), Stage 4 (0.85–0.54Ga )and stage 5 (0.54Ga–present). Image by Wikipedia
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.
Image via Slideplayer
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.