In 1950, Enrico Fermi reputedly exclaimed to his colleagues at Los Alamos National Laboratory, “Well, where are they?”
He was talking about alien civilizations. If there’s nothing special about Earth, and there are millions of similar rocky planets in our galaxy then surely carbon-based life would have arisen many times, given rise to intelligent civilizations that by now would have colonized the galaxy. Why haven’t we heard from them? Today the question is even more pressing given the thousands of extra-solar planets that are being discovered, many of them rocky worlds not that different from our Earth, some in the Goldilocks Zone where you’d expect life to develop. Is the Galaxy teeming with life, or is our Earth somehow unique in its 4 billion long history of biodiversity?
A recent study by Aditya Chopra and Charles Lineweaver of the Australian National University in Canberra argues that while many planets may give rise to life in their early stages, in only a small fraction does life continue to develop for billions of years. Mars and Venus probably both once had life. In the case of Venus, the oceans boiled away into space driven by a runaway greenhouse effect, while Mars lost its water and became a frozen desert. In both cases, a biosphere never developed to the point where it could regulate the temperature of the planet by countering the greenhouse effect, or develop biochemical processes that enabled the planet to retain its liquid oceans.
The idea that the Earth’s biosphere acts as a regulator to maintain conditions favourable for the existence of life, despite the sun growing hotter, was developed by James Lovelock and Lyn Margulis and named the Gaia theory.
Chopra and Lineweaver argue further that unless a Gaia-type situation arises where biological systems provide a negative feedback to control a runaway greenhouse effect, life emerges early in a planet’s life, oceans are formed, but that the oceans eventually boil away and the life forms become extinct. This is the fate of the vast majority of rocky/moist planets.
The Gaian Bottleneck theory also predicts that at present Mars and Venus are devoid of life forms. Also, that the sign of a thriving biosphere on a planet that is a few billions years old is the presence of liquid water. If we run across a planet that has no sign of liquid water, it probably has no active biosphere either. In the absence of aquatic life forms to reconstitute the water, water vapour is dissociated by the sun’s ultraviolet radiation causing the oceans to evaporate. In the case of the Earth, without the presence of life the oceans would have boiled away within 1-2 billion years.
So, the Earth has survived the Gaian Bottleneck which is why we are here. Chopra and Lineweaver suggest that a Gaian regulation of a planet’s atmosphere, oceans and temperature may be very rare. That would make the Earth a very special, perhaps unique place.