The First Invasion Of The Land

In most accounts of evolutionary history, the first invasion of the land was by plants, closely followed by animals. Yet it is becoming increasingly clear that the first true invasion actually occurred many hundreds of millions years earlier. Even before the first slimy, leafless plants basked on some rocky shore of Gondwana or Laurentia, a thin microbial scum spanned the otherwise barren waterways of these continents. How far back the terrestrial fossil record extends, however, has been less clear. Microbes are certainly hardy, yet the Earth's continents were a much harsher place billions of years ago; virtually devoid of nutrients and blasted by radiation.

The Barberton Mountains contain some of the most ancient rocks on Earth

Recently, however, evidence has shown that in spite of these hostile conditions, there were living organisms on Earth an incredible 3.22 billion years ago. Some of the oldest rocks on Earth can be found in the Barberton Belt in South Africa.

In 2013, fieldwork conducted by an international team of researchers recovered rock samples from an ancient braided river system and the surrounding soils - palaeosols. Contained within these samples were microscopic grains of iron sulphide which showed a layered structure. By examining the ratio of sulphur isotopes in the mineral, the researchers found that exterior rim ratio matched that found in modern environments inhabited by sulphur bacteria. In addition the cores of the grains bore a different ratio.

This suggested that the fluctuating braided rivers periodically exposed the grains to either dry or wet conditions. During the latter, sulphur bacteria colonised the grains, leading to the different composition of the rims. At 3.22 billion years, the rocks constitute the oldest evidence for terrestrial life. The conditions the sulphur bacteria endured were undoubtedly harsh, including periodic desiccation and constant exposure to ultraviolet radiation. The fact that they were able to colonise such an environment, shows that they must have developed already the genes required to make them the hardy survivors they are today.

Three billion year old rocks from Pilbara
which preserve an ancient microbiome

A second study, this time by researchers from the University of Oregon, has shown that not only was there life on the land during the late Archaean eon, 3 billion years ago, but that complex ecosystems also formed.

Using advanced imaging techniques and isotopic analysis on three billion year old palaeosols from Pilbara, Australia, they found a microbiome consisting of five distinct species. The largest were the spindle-shaped actinobacteria, which are an important component of soils. Smaller spherical forms were similar to purple sulphur bacteria, whose metabolic activities were likely responsible for the sulphate minerals detected within the soil.

'With cell densities of over 1,000 per square millimetre and a diversity of producers and consumers, these microfossils represent a functioning terrestrial ecosystem, not just a few stray cells,' said Gregory Retallack, a professor in the Department of Earth Sciences and director of paleontology collections at the Museum of Natural and Cultural History, University of Oregon. 'They are evidence that life in soils was critical to the cycles of carbon, phosphorus, sulphur and nitrogen very early in the history of the planet.'

The researchers went on to say that these palaeosols may be a useful guide for the search of life on other planets. When we peer further back in time, the degree of difference between the rocky planets in our solar system regresses significantly. As such, to look at the earliest ecosystems is not just a view into the past, but possibly one across space as well.