No need for oxygen (Image: Brian Larossa)

Oxygen is supposed to have driven the evolution of complex life – but the discovery of animals that thrive without it tells a different story

GO WEST, young man! More specifically, go about 200 kilometres west of Crete, then straight down to the bottom of the Mediterranean Sea 3.5 kilometres below. There you will find a lake with some extraordinary inhabitants.

Around 6 million years ago when the Mediterranean nearly dried up, vast amounts of salt were deposited on the sea floor. Some of these deposits were exposed about 30,000 years ago. As this salt dissolves, extra-salty, dense water is sinking to the depths, forming a brine lake up to 60 metres deep. Even more surprising than the existence of this lake beneath the sea, however, is what lives in it.

The water in the brine lake does not mix with the water above and so ran out of oxygen long ago. Instead, the toxic gas hydrogen sulphide oozes from the black mud. It’s the last place you would expect to find animals. But that’s exactly what has been discovered: the first animals, as far as we know, that can grow and reproduce without a whiff of oxygen.

These tiny mud-dwellers are far more than a curiosity. They could be the best pointer yet to the origin of complex cells: the basis of most life on Earth, from amoebae to oak trees.

Radical prediction

“The ecology is interesting, but the real significance of these critters is what they say about evolution,” says Bill Martin, an evolutionary biologist at the University of Dusseldorf in Germany. For Martin, the discovery is …