Why deep oceans gave rise to the first complex organisms on Earth: Stable temperatures let early life forms make the best of limited oxygen supplies

For billions of years life on Earth was microscopic.

However, around 570 million years ago that all changed as complex organisms including animals with soft, sponge-like bodies sprang to life deep in the ocean.

Scientists have long been baffled as to why organisms first appeared deep in the ocean where light and food are scarce, according to Daily Mail.

A new study has found that stable temperatures were key to survival as these early lifeforms were unable to deal with fluctuations in temperature.

Deep underwater was the only place where the temperature was stable - so it was here they flourished, despite the fact there was limited oxygen.

Researchers from Stanford University believe this might uncover a missing link in the evolution of life - and even how our own species came to exist.

They believe it could also shed light on the kinds of organisms that will be able to survive in different environments in the future.

'You can't have intelligent life without complex life,' explained Tom Boag, lead author on the paper and a doctoral candidate in geological sciences.

Previously scientists theorised that animals had an optimum temperature at which they can thrive with a minimum of oxygen.

According to the theory, oxygen requirements are higher at temperatures that are either colder or warmer than a happy medium.

Scientists tested this theory by looking at the oxygen needs of sea anemones.

Their ability to breathe through the skin closely mimics the biology of fossils collected from the Ediacaran oceans.

'We assumed that their ability to tolerate low oxygen would get worse as the temperatures increased', Mr Boag said.

'That had been observed in more complex animals like fish and lobsters and crabs,' he said.

Scientists weren't sure whether colder temperatures would also strain the animals' tolerance.

As predicted, they found the anemones needed more oxygen when temperatures in an experimental tank veered outside their comfort zone.

This led researchers to the conclusion that like anemones, Ediacaran life would also require stable temperatures to make the most efficient use of the ocean's limited oxygen supplies.

Despite the fact the deep ocean taken oxygen during the Ediacaran period it was also a very stable place. 

On the surface of the water temperatures can change by as much as 10C. However, at one kilometre (0.62 miles) under, seasonal variations are less than 1C.

In a world with low oxygen levels, animals unable to regulate their own body temperature couldn't have withstood an environment that so regularly swung outside their Goldilocks temperature.

'The only place where temperatures were consistent was in the deep ocean,' said Erik Sperling.

In a world of limited oxygen, the newly evolving life needed to be as efficient as possible and that could only be achieved in the relatively stable depths.

'That's why animals appeared there,' he said.