Are aliens hard to find because they're extinct?

Evidence of extraterrestrial life may be hard to find because alien species have all died out, a new study suggests.

Researchers Aditya Chopra and Charles Lineweaver, both of the Australian National University, recently published a paper in the journal Astrobiology suggesting that extinction could be the main cause of humanity’s fruitless search for extraterrestrials.

“The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens,” said Mr. Chopra in a release from the university. However, Chopra and Dr. Lineweaver say that, despite the presence of livable sites, alien life may not have evolved quickly or well enough to sustain itself on various worlds.

“Early life is fragile, so we believe it rarely evolves quickly enough to survive,” Chopra said. “Most early planetary environments are unstable. To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable.”

This conjecture, which Chopra and Lineweaver call a “Gaian bottleneck,” offers a different view on alien existence than previous similar ideas and provides an answer to the Fermi paradox – the conflict between the apparently high probability that alien life could evolve and the lack of any hard evidence demonstrating alien development.

The Gaian bottleneck conjecture submits that the overlap between a species' habitable conditions and its planet's actual conditions is usually very brief, and that most species die out before they can evolve or change their environment enough to match with both.

The new study also offers a different view on a concept first proposed by George Mason University economist Robin Hanson that suggests that science’s current perception of the process of basic evolution that eventually leads to intelligent life and cosmic colonization is wrong.

Prof. Hanson hypothesized that there is a step, which he calls the Great Filter, in the universal “evolutionary path” that almost no species can successfully complete. “[S]o far nothing among the billion trillion stars in our whole past universe," he writes, "has made it all the way along this path.” 

The Great Filter suggests that humanity may be doomed to be filtered out at some point in the future, but Chopra and Lineweaver’s study goes in a different direction in saying that early life is somewhat responsible for making its environment habitable and in a way providing for its own evolution - indicating that humanity already passed through its filter.

This idea means that in most instances where life has existed, it has died; “extinction is the cosmic default,” according to the researchers. It also means that if there was no life originally on a planet after its formation, it is very unlikely that place would ever see the presence of anything living; “planets need to be inhabited to remain habitable.” These two arguments are Chopra and Lineweaver’s answer to Fermi’s paradox and would explain why life is apparently so rare in the universe.

So it is possible that alien species existed all over the universe in environments conducive to life, but if humanity ever finds traces of them it’s likely to be in the form of fossilized, basic life forms that simply didn’t evolve rather than as evidence of sentient aliens with advanced civilizations.

“One intriguing prediction of the Gaian Bottleneck model is that the vast majority of fossils in the universe will be from extinct microbial life, not from multicellular species such as dinosaurs or humanoids that take billions of years to evolve,” said Lineweaver.