Leonard David's INSIDE OUTER SPACE

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

It’s no wonder we haven’t heard from extraterrestrials. That’s because they are long-gone, extinct.

Astrobiologists from Australian National University (ANU) call it the “Gaian bottleneck” – life on other planets would likely be brief and become extinct very quickly.

“Early life is fragile, so we believe it rarely evolves quickly enough to survive,” contends Aditya Chopra from the ANU Research School of Earth Sciences and lead author on a new paper published in the journal, Astrobiology.

Early microbial life on Venus and Mars, if there was any, failed to stabilize the rapidly changing environment, adds co-author Associate Professor Charley Lineweaver from the ANU Planetary Science Institute.

Different bottleneck scenarios and their fossil predictions. (A) Emergence Bottleneck. Life rarely emerges even on
wet rocky planets. Few planets will have life or even fossils of extinct life. On the few planets where life does emerge, it
persists for billions of years. (B) No Emergence Bottleneck. Life emerges with high probability and usually persists for billions
of years. Thus, life will be abundant on planets throughout the Universe. There will be many planets where life persisted for
billions of years and then went extinct. On the oldest uninhabited planets, fossils of complex life will be abundant. (C) Gaian
Bottleneck. Life emerges with some probability (possibly quite high), but it goes extinct within a billion years (green).
Alternatively, some small fraction of inhabited planets successfully pass through the Gaian bottleneck (light green). The Gaian
bottleneck model predicts that the vast majority of the fossils in the Universe will be from extinct microbial life.
Graphic: Astrobiology

Sliding into oblivion

They propose a potentially universal sequence of events on initially wet rocky planets, a kind of sliding scale into oblivion:

• Hot, high bombardment, uninhabitable.
• Cooler, reduced bombardment, continuous volatile loss.
• Emergence of life in an environment with a tendency to evolve away from habitability.
• Inability to maintain habitability, followed by extinction.

The scientists suggest new life would commonly die out due to runaway heating or cooling on their fledgling planets.

“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,” suggests Lineweaver.

Riding a wild bull

Chopra and Lineweaver conclude in their paper:

“Between the early heat pulses, freezing, volatile content variation, and runaway positive feedbacks, maintaining life on an initially wet rocky planet in the habitable zone may be like trying to ride a wild bull. Most life falls off. Life may be rare in the Universe, not because it is difficult to get started, but because habitable environments are difficult to maintain during the first billion years.”

To read this intriguing Astrobiology paper, go to:

http://adi.life/pubs/ChopraLineweaver2016.pdf