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Researchers Say The Aliens Are Silent Because They Are Extinct (theconversation.com)
HughPickens.com writes: The Conversation reports that according to research by Dr. Charles Lineweaver and Dr. Aditya Chopra, a plausible solution to Fermi's paradox is near universal early extinction of life on exoplanets, which they have named the Gaian Bottleneck. "The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," says Chopra. "The mystery of why we haven't yet found signs of aliens may have less to do with the likelihood of the origin of life or intelligence and have more to do with the rarity of the rapid emergence of biological regulation of feedback cycles on planetary surfaces." According to the researchers, 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. About four billion years ago, Earth, Venus and Mars may have all been habitable. However, a billion years or so after formation, Venus turned into a hothouse and Mars froze into an icebox. Even if wet rocky Earth-like planets are in the "Goldilocks Zone" of their host stars, it seems that runaway freezing or heating may be their default fate. Large impactors and huge variation in the amounts of water and greenhouse gases can also induce positive feedback cycles that push planets away from habitable conditions. The difference on Earth may be that as soon as life became widespread on our planet, the earliest metabolisms began to modulate the greenhouse gas composition of the atmosphere. "The emergence of life's ability to regulate initially non-biological feedback mechanisms could be the most significant factor responsible for life's persistence on Earth, conclude Lineweaver and Chopra. "Even if life does emerge on a planet, it rarely evolves quickly enough to regulate greenhouse gases, and thereby keep surface temperatures compatible with liquid water and habitability."
One thing to bear in mind is that life here existed in the form of anaerobic bacteria for a staggeringly long time. Photosynthesis began as a way to split hydrogen sulphide into useful hydrogen ions and a useless waste product of elemental sulphur, which was also usefully inert. Early photosynthesis therefore didn't require much in the way of biochemical sophistication to operate; the waste sulphur is where some large sulphur deposits originated.
That changed with a mutation which let the photosynthesis split not hydrogen sulphide, but water into useful hydrogen and (to anaerobic bacteria) highly toxic and dangerous oxygen. That initially wasn't all that big a problem to early water-splitters; the oceans they were in were rich in iron-II salts which readily absorbed oxygen to become insoluble iron-III salts (this is where the banded iron rock formations come from).
Everything changed when most of the iron-II in solution in the early earth's oceans was used up. Oxygen levels slowly rose, and virtually all bacterial species either adapted or went extinct. Oxygen is toxic to most bacteria.
I would hypothesise that most alien worlds either never make the switch from anaerobic atmosphere to aerobic one, or fail to establish a homeostatic oxygen-nitrogen atmosphere quickly enough and effectively enough to become self-regulating.
Unfortunately, these geek acronyms tend to be English-specific. Some others you will encounter here:
RTKBA - Right to keep and bear arms;
TEOTWAWKI - The end of the world as we know it;
DYKWIA - "Do you know who I am?"
SJW - Social justice warrior