Wheeler Schale fossils from the Cambrian period, when life on Earth exploded (wiki commons)

The explosion of life on Earth 520 million years ago was a result of a "cascade of events" rather than one underlying cause, researchers have said.

A study by Paul Smith of Oxford University and David Harper of Durham University suggests that a combination of interlinking factors led to the rapid diversification of animal life during the Cambrian period.

In the Cambrian period, ecosystems developed, animals evolved and the first appearance of behaviours such as burrowing and swimming emerged.

Over the last few decades, a number of scientists have put forward theories as to why life expanded so fast. All fell into one of three categories - geological, geochemical and biological.

Research published in Current Biology found that the Cambrian explosion fits in with Darwin's theory of evolution - despite appearing to contradict it.

The team said that physical changes such as the ability to swim opened up a huge range of avenues for evolution and that a similar surge in evolution is seen when animals move into new environments.

Smith and Harper spent four years in a remote area of Greenland looking at fossils and concluded that a series of interlocking events led to the explosion of life.

They believe animal life began with a rise in the sea level, which generated a large increase in the area of habitable seafloor, which, in turn, led to an increase in animal diversity.

The scientists say the early events in the Cambrian period then led to a complex interaction of biological, geological and geochemical processes that have been put forward by researchers in the past.

A kernel of truth

Smith, lead author of the report, said: "This is a period of time that has attracted a lot of attention because it is when animals appear very abruptly in the fossil record, and in great diversity. Out of this event came nearly all of the major groups of animals that we recognise today.

"Because it is such a major biological event, it has attracted much opinion and speculation about its cause.

"Work at the Siriuspasset site in north Greenland has cemented our thinking that it wasn't a matter of saying one hypothesis is right and one is wrong. Rather than focusing on one single cause, we should be looking at the interaction of a number of different mechanisms.

"Most of the hypotheses have at least a kernel of truth, but each is insufficient to have been the single cause of the Cambrian explosion. What we need to do now is focus on the sequence of interconnected events and the way they related to each other - the initial geological triggers that led to the geochemical effects, followed by a range of biological processes."

Harper added: "The Cambrian Explosion is one of the most important events in the history of life on our planet, establishing animals as the most visible part of the planet's marine ecosystems.

"It would be naïve to think that any one cause ignited this phenomenal explosion of animal life. Rather, a chain reaction involving a number of biological and geological drivers kicked into gear, escalating the planet's diversity during a relatively short interval of deep time.

"The Cambrian explosion set the scene for much of the subsequent marine life that built on cascading and nested feedback loops, linking the organisms and their environment, that first developed some 520 million years ago."