That would explain why we have yet to make contact with any.
Maybe you’ve never really thought about it, but humans are pretty special. Life on Earth managed to evolve on a planet where the conditions were extremely unstable for the first billion years or so due to fluctuating temperatures and atmospheric conditions. Lucky for us, it is possible that early life forms actually had a stabilizing effect on the planet.
"Most early planetary environments are unstable," Dr Aditya Chopra from the ANU Research School of Earth Sciences and lead author on a new paper on alien life published in Astrobiology. "To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable."
The paper she published with Dr. Charles H. Lineweaver suggests that the reason we have yet to make contact with any alien life forms is that none of them evolved enough to survive for very long.
"The Universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," said Chopra, "Early life is fragile, so we believe it rarely evolves quickly enough to survive."
SEE ALSO: How Did Life Begin?
In 1950, Enrico Fermi, an Italian physicist came up with the Fermi Paradox. If our sun has nine planets and there are billions of stars in the universe, then there must be billions of planets out there. Statistically, at least some of them should be habitable, he thought, which lead to the question of “Where are they all?” Even at a fairly slow speed, it would be possible for a planet that develops the right technology to cross the entire milky way in as little as a million years. Thus, if they are out there, they should have visited us by now.
Many answers have been hypothesized — they are communicating but we don’t know how to listen, they came and left already, they have no interest in us, we are their descendants and we just don’t know, etc. — but Chopra and Lineweaver’s research leads to a new possibility.
The researchers simulated their hypothesis using a model they call the Gaian bottleneck. According to their paper, the model suggests that “extinction is the cosmic default for most life that has ever emerged on the surfaces of wet rocky planets in the Universe” and that “rocky planets need to be inhabited to remain habitable.”
In our own solar system, planets like Mars and Venus both used to be habitable, but the onset of inhospitable conditions would have caused any lifeforms to die off.
"In extremely rare cases — like on Earth — the relatively rapid evolution from single- to multicellular organisms to complex life forms did not produce enough greenhouses gases to cause runaway negative feedback and heat the planet enough to evaporate all its liquid water," explains Campbell Simpson at Gizmodo. "It’s that particular and so far unique quirk that has kept us alive, if the Gaian bottleneck explanation is accurate."
If their hypothesis is faulty, it would lead to a bleaker conclusion — that life forms eventually go through a bottleneck where they self-destruct, a phase that humans have yet to go through: “If we find [life on Mars that has emerged independently from life on Earth], Bostrom argues that the biggest bottleneck — the self-destruction bottleneck — would then be in front of us. This would be ‘‘by far the worst news ever printed on a newspaper cover.’’ As a plausible alternative to such catastrophic logic, we introduce the concept of a Gaian bottleneck, a bottleneck that life on Earth has already passed through.”
I sure hope they’re right, because we do a great job destroying our planet.