Credit: NASA

For most of our planet’s history the major life forms on it were tiny wee creatures that lived underground.

This is the finding of researchers in a collaborative project by the Universities of Edinburgh and Aberdeen.

Dr Sean McMahon,School of Physics and Astronomy, University of Edinburgh, said:

“Prehistoric life on Earth was like an iceberg – most of it was found below the surface.

“The total mass of life on the planet was far smaller before plants took over.“

The researchers estimated how changes to the chemical composition of the atmosphere and oceans through time – which are recorded in rocks found around the world – would have affected the ability of different life forms to thrive.

Life on Earth is thought to have begun around 3.8 billion years ago with single-celled organisms.

Dinosaurs first appeared around 230 million years ago, and the earliest mammals are believed to have evolved millions of years later.

Microscopic organisms, including bacteria, were the most abundant forms of life on Earth from about 2 billion years ago until 400 million years ago, when plants began to spread across the land.

During this era, these organisms weighed around 10 times as much as all other life on the planet combined, according to the study, which offers insight into the evolution of life on Earth.

Professor John Parnell, School of Geosciences, University of Aberdeen, said:

“Life underground was the norm on Earth. Until quite recently, the biggest habitat was below ground. “

Underground bacteria are now the second most abundant life form, with a combined weight of about 100 billion tonnes of carbon.

Plants dominate life on the planet today in terms of their combined weight of carbon, which is about 500 billion tonnes.

The researchers hope their work will help develop new techniques to study microscopic fossils from ancient underground regions.

The study, published in Journal of the Geological Society, was supported by the European Union’s Horizon 2020 Research and Innovation Programme.