Reconstruction of early life on land

The microscopic testate amoebae are thought to have helped plants colonise land hundreds of millions of years ago © Strullu-Derrien et al. 2019/Victor Leshyk

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A tiny fossil amoeba is helping us to understand how plants first bloomed

Single-celled amoebae were an early form of life on Earth that evolved in the sea. Now scientists have uncovered the earliest ever terrestrial species of an important type known as testate amoebae.

The fossil amoebae were found in ancient Scottish rock dating to 400 million years ago, pushing back the origin of the organisms by hundreds of millions of years. They also help us to understand how plants and animals transitioned from water to land.

Unicellular amoebae are microscopic living organisms made up of just a single cell. Within this group the testate amoebae have an enveloping, vase-like shell that can be preserved as a fossil.

Testate amoebae are a significant component in modern ponds, peat lands and soils playing an important role in nutrient cycling. These unicellular creatures move around using cell outgrowths that look like tentacles, hunting down bacteria and other microorganisms. Each gram of soil can contain many thousands of these amoebae.

They may also have helped plants to colonise the land over 400 million years ago.

Dr Christine Strullu-Derrien is a Scientific Associate at the Museum and lead author of a paper describing a new species of testate amoeba.

Christine found the species while researching the origin and evolution of soil-dwelling fungi that live in association with plants.

This discovery of these amoebae, now named Palaeoleptochlamys hassi, pushes back the fossil record of these organisms from the end of the Permian (250 million years ago) to the Early Devonian (407 million years ago). The results have been published in the journal Current Biology.

A partnership between fungi and plants

Known as mycorrhiza, the mutual relationship between fungi and plants is vital for both to survive. While the plants provide the fungi with energy, the fungi supply the plants with water and nutrients.

But the relationship goes deeper than this, as the mycorrhizal network spread out beneath the soil can even be used to pass messages between individual plants, warning each other of hungry arthropods and voracious herbivores.

Testate amoebae fossil

The 'shells' of the testate amoebae are found in the Rhynie chert © Strullu-Derrien et al. 2019

Christine and her colleagues are using a rock formation known as Rhynie chert to find out when this intimate relationship first started.

This sedimentary rock is found near Aberdeen, Scotland, and contains exceptionally well-persevered fossils of some of the earliest plants and animals to have colonised the land. The level of preservation is such that even single-celled organisms can be seen.       

'The main purpose of this work was to look at the environment 400 million years ago and to see what kind of organisms you can find in ponds and soil,' explains Christine. 'I work on the relationship with fungi and other organisms, but in this research I found an organism that I did not know.

'I realised that it was a testate amoeba, which is part of the microorganism community and was very important in the early environment.'

The first colonisers

Dr Paul Kenrick, a researcher at the Museum and co-author of the paper, says, 'The Rhynie chert from Scotland is one of the world's most famous geological sites. It is the earliest site where we have a really well-preserved terrestrial ecosystem.'

At the time when the plants were fossilised, Aberdeen was a vastly different place.

Located just below the equator in the southern hemisphere, it would have experienced a much warmer climate. The plants that had moved onto land were still small - only around 20 centimetres in height and similar looking to moss that would later evolve - and were growing in what would have been a geothermal wetland environment. 

Plants moved onto land in geothermal wetlands

When plants first moved onto land, it would have looked a little like Iceland does today © Dennis van de Water/Shutterstock

This is not the earliest evidence of land plants, as they are thought to have made the transition at least 470 million years ago, but the Rhynie chert contains the earliest complete terrestrial ecosystem ever discovered. This is down to where the plants were growing and the way in which they were preserved. 

'It would have looked a little like a geothermal wetland in Iceland,' explains Paul. 'These plants were growing near hot springs that were flooding the surrounding environment. It is this silica-rich water that covered the ecosystem and preserved the organisms in such detail.'

As the water that submerged the plants cooled the silica was deposited in the form of sinter. This trapped and preserved in exceptional detail not only the plants but also the early animals, fungi and all other associated microorganisms that were living in ponds and soil. This has provided an unusually detailed look into what was happening on land at this early point in time.

Microbial helpers

This discovery is helping scientists to work out how life on land got started and when the complex, interdependent relationships between plants and microorganisms began. The new fossils support the hypothesis that these testate amoebae transitioned from marine through freshwater environments, eventually colonising soil ecosystems at the same time as early vascular plants.

Reconstruction of early plants in the Rhynie chert

A reconstruction of the early terrestrial ecosystem, showing how the first plants were in relationships with early animals and microorganisms © Strullu-Derrien et al. 2019/Victor Leshyk

'Life on land goes way back in time, but mostly in the form of bacterial mats,' explains Paul. It is likely that plants and animals made the transition to land at the same time, and that the microbial mats present effectively primed the ground for their arrival.

'One of the enabling factors for the plants would have been the microbial community that was already there,' says Paul.

This suggests that the symbiotic relationships seen today are as ancient as terrestrial plants themselves and may even have been necessary for the transition to occur in the first place.

From the towering red wood forests of California to the expansive plains of east Africa, the success of all terrestrial plants was probably given a helping hand by a rich and diverse community of tiny microorganisms already making a living in the warm mud beside a geothermal pool half a billion years ago.