Mystery of how first animals appeared on Earth 'solved' by scientists

Giant glaciers that ground mountains to powder during the 'Snowball Earth' period and then dramatic global warming are believed to have helped complex life overthrow once-dominant bacteria

The Earth was once covered almost entirely by ice and the massive glaciers are believed to have played a key role in the emergence of complex life (Stephen Hudson/Wikipedia)

New research has shed fresh light on how animals first appeared on the Earth.

It is a story spanning hundreds of millions of years, of entire mountain ranges ground to dust by vast glaciers that once covered the planet and of dramatic climate change that ushered in a new biological age.

Life on Earth was dominated by simple bacteria up until about 650 million years ago when more complex forms of life suddenly took over.

The reasons why this happened were something of a mystery.

However scientists in Australia and Germany now claim to have identified the pivotal moment that set life on Earth on course to evolve into a vast array of animal life, from insects to dinosaurs and humans.

This “revolution of ecosystems” began to happen about 700 million years ago when the planet became what is known as ‘Snowball Earth’ as ice covered virtually its entire surface.

Giant glaciers ground down huge mountains to powder, releasing life-giving nutrients such as phosphates. And then, during an “extreme global heating event”, the Snowball melted and rivers washed the nutrients into the ocean.

The newly temperate climate and the sudden influx of food created the perfect conditions for the first complex life – algae – that are the ancestors of us all.

One of the researchers, Dr Jochen Brocks, of The Australian National University (ANU), said they had found evidence of what had happened by crushing rocks from the period, then extracting “molecules of ancient organisms”.

“These molecules tell us that it really became interesting 650 million years ago. It was a revolution of ecosystems, it was the rise of algae,” he said.

The bacteria that once dominated the oceans found themselves being squeezed out by more complex life-forms.

“These large and nutritious organisms at the base of the food web provided the burst of energy required for the evolution of complex ecosystems, where increasingly large and complex animals, including humans, could thrive on Earth,” Dr Brocks said.

The remains of the ancient organisms recovered from the sedimentary rocks found in central Australia were dated to the period just after the melting of Snowball Earth.

Dr Amber Jarrett, also of the ANU, said: “In these rocks we discovered striking signals of molecular fossils.

“We immediately knew that we had made a ground-breaking discovery that Snowball Earth was directly involved in the evolution of large and complex life.”

The research was described in a paper published in journal Nature.

The paper noted that algae had actually evolved much earlier than the period studied, anything from 900 million to 1.9 billion years ago.

However, the researchers wrote: “The corrected record of fossil steroids demonstrates that algae only broke the incumbency of phototrophic bacteria as the principal marine primary producers 659 to 645 million years ago.

“This new timeframe offers a network of explanations for a Neoproterozoic/Paleozoic rise in atmospheric oxygen levels, establishment of more modern nutrient and carbon cycles, and the evolution of an increasingly complex biota [life].”