Life, underground

A new study finds that creatures beneath the Earth’s surface dominated life on Earth for most of our planet’s history – and could hold the key to the search for life on other worlds

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The thin Martian atmosphere as viewed by the Viking 1 orbiter (NASA)

For much of Earth’s history, life hasn’t always been what it seems. Until the diversification of land plants around 400 million years ago, life on Earth was dominated by tiny creatures living beneath the surface – still home to a diverse ecosystem of ancient life.

A new study, published this month in the Journal of the Geological Society, uses data on the current make-up of life on the planet to work out how this balance has changed over billions of years. The research was supported by the European Union’s Horizon 2020 Research and Innovation Programme.

Between 2 billion and 400 million years ago, organisms living beneath the Earth’s surface weighed around 10 times as much as all other life on the planet combined, according to the researchers from the Universities of Edinburgh and Aberdeen. The results offer a new perspective on the history of life on Earth, with important implications for the search for life beyond our planet.

From ‘The deep history of Earth’s biomass, Sean McMahon and John Parnell, Journal of the Geological Society, 12 July 2018’

‘The extensive evidence for a subsurface biosphere on Earth has raised the possibility that other planets may also support life in a subsurface biosphere’ say the authors, who suggest that even the most ‘Earth-like’ planets might be more likely to feature a biosphere dominated by subsurface life.

‘The constraints of surface water, surface irradiation spectrum and surface temperature used to characterize planetary habitability do not apply to a subsurface biosphere’ they add. ‘Therefore, the number of habitable planets around other stars may be substantially greater than is commonly supposed on the basis of surface habitability.’

Life underground today

Fossil of Eophasma jurasicum, an extinct nematode (Ghedoghedo, Wikipedia)

Today, life on Earth is dominated by plants, in terms of their combined weight of carbon – about 500 billion tonnes. Once dominant, underground bacteria is now the second most abundant life form, with a combined weight of about 100 billion tonnes of carbon. Recent studies have revealed life existing far deeper within the Earth’s crust -and in far more complex forms – than was once thought. Nematode worms, for example, are capable of slowing their metabolism to the point where they can survive the breakup and reentry of space shuttles, life in hot springs and deserts, and within mines drilled to depths of more than two miles below the Earth’s surface.

Much of the life that exists beneath the surface is ancient – dominated by prokaryotic phyla with evolutionary origins in the Archaeon – 4,000 – 2,500 million years ago. Researchers hope that their work will help develop new techniques to study microscopic fossils from these ancient underground regions – a record which is still largely unexplored.

‘Prehistoric life on Earth was like an iceberg’ says Dr Sean McMahon, of the University of Edinburgh’s School of Physics and Astronomy. ‘Most of it was found below the surface. The total mass of life on the planet was far smaller before plants took over.’