Life on Earth probably began in the depths of the ocean and not on the planet's surface. 

The claim comes from Isabelle Daniel at the University of Lyon and colleagues in Germany and France who have surveyed the current knowledge on the origins of life on Earth. They conclude that when life on Earth began, about four billion years ago, the conditions on Earth's surface would have been unfavourable for life to emerge. 

Only a few modern species can live in the kind of extreme environment that was present on the primitive Earth's surface, explained Daniel. Volcanic eruptions and meteorite impacts created an erratic climate above ground, but volcanic activity regulated the temperature at the bottom of the ocean at a more favourable 20-50°C. Harmful radiation would have been filtered out by the ocean and the high pressure conditions of the ocean could have stabilised essential biological molecules such as DNA and RNA, added Daniel.

The scientists cite evidence that most surface organisms can withstand pressures higher than they are used to without consequence to their life cycle or metabolism. According to Daniel, this may represent one of the most ancestral physical conditions under which life had to emerge. In fact, Daniel proposes that the last universal common ancestor (LUCA) to all living organisms was a piezophile, an organism that prefers to live at high pressures. 

If life did emerge at the bottom of the ocean, it is possible that it could have begun under similar conditions on other celestial bodies, such as in the deep oceans of Jupiter's moon Europa.

But the hypothesis may never be confirmed, said Daniel. Life has erased most traces of its origins, and reinventing life itself would simply take too much time and good fortune, she said.

Elinor Richards