Читать книгу The Explosion of Life Forms - Группа авторов - Страница 13

Despite laboratory experiments and increasingly sophisticated techniques, we are faced with the absence of traces of the past, irrefutable “evidence” of the first moments. Indeed, down-to-earth, strictly geological considerations make it difficult to explore morphological evidence of the past. Volcanoes, tectonic plates, metamorphism and massive meteorite bombardments occurred during the Hadean period¹ (4.5 to 4 billion years ago), during which warm, iron-laden oceans were formed, the Archean period² (4 to 2.5 billion years ago), and the billions of years that preceded us.

Geological records provide direct evidence of the presence of primitive life on Earth in four main ways: through microfossils, stromatolites (literally “layers of stone”, from the Greek: stroma, “carpet”, and lithos, “stone”), molecular biomarkers, and stable isotope ratios. However, the interpretation of the samples and in situ analyses is still under discussion, as Archean rocks are scarce and poorly preserved.

Traces of ancient life may be hidden in fossil remains, which careful excavations are trying to discover. Fossils are rare because the fossilization process takes a long time, and when traces remain, they must escape decomposition and destruction, only to reappear later through soil erosion. Of the evidence left by ancient organisms, 99% is recent, dating back 545 million years. They include leaves, footprints, shells, bones, teeth, and spores or skeletons of fish or dinosaurs, most often preserved by sedimentation.

However, micropaleontologists refer to persistent traces of microorganisms in the form of stromatolites that existed almost 4 billion years ago (Schopf 1993; Allwood et al. 2006).

So, what happened in the first billion years of Earth’s history that led to the appearance of organisms that are similar to today’s bacteria?

Today, microbial communities build laminated “stone mats” in two ways. The cyanobacteria’s method is to trap fine sediments with a sticky film of mucus that each cell secretes, then cement the sediment grains with calcium carbonate precipitated in water. Because living cyanobacteria are photosynthetic, they move towards the light, so the bacterial mat always remains on the outside of the stromatolite. The cyanobacteria’s second method of constructing stromatolites is the precipitation of their own carbonate structure in water, with the incorporation of sediments (Awramik 1994).

Corrugated forms, resulting from probable laminations, have been observed in the south-west of Greenland at sites dating back 3.8 billion years. In the Isua region, the outcrops of ancient sedimentary rocks show forms in the metamorphosed minerals that resemble layers of stromatolites 1 to 4 cm high, produced by microorganisms that would have developed at that time, as suggested by the presence of yttrium (Y), a rare earth typical of sediments deposited in a shallow marine environment. However, these results have been contested by some, who instead point to formations of tectonic origin (Nutman 2016).

Elsewhere, in the Pilbara region (Warrawoona rock formation) in Western Australia, small sedimentary cushions, or domes, aged at 3.5 billion years old can be found. Early life environments in the Pilbara Craton also included shallow marine sedimentary environments and hydrothermal regions. Other such formations, dating back 2.5 billion years, can be found in the Transvaal in South Africa.

Figure 1.1. Stromatolites dating back 2.5 billion years, observed in the Transvaal in South Africa. Courtesy of Pierre Thomas (2016). For a color version of this figure, see www.iste.co.uk/chapouthier/life.zip

This variety of environments, combined with the variety of stromatolite forms found, would indicate a biosphere that was already diverse 3 to 4 billion years ago.

Biodiversity originates from the origins of life itself.

A major contribution highlights a point that is rarely addressed in the study of the metabolism of early stromatolites. Indeed, the hypothesis that cyanobacteria formed fossil stromatolites by mineralization and lithification of microbial mats assumes that oxygenic photosynthesis is a very old process, one that was active more than 3 billion years ago. However, the surface of the Earth was predominantly anoxic in the Archean period, containing less than 1% of today’s oxygen concentration. The increase in oxygen in the ocean, as a result of cyanobacterial photosynthesis, gradually turned it into an oxidizing environment, whereas it was initially reductive. This means that the oldest stromatolites are the product of phototrophic anoxygenic microorganisms, which do not produce oxygen. Supporting this hypothesis, researchers have recently observed that phototrophic sulpho- oxidizing alpha-proteobacteria are responsible for the precipitation of aragonite, the main calcium carbonate constituting the stromatolites of Lake Dziani Dzaha in Mayotte (Gérard et al. 2018).

In addition, other anaerobic methanotrophic microbial communities were present prior to the oxygenation of the Earth’s atmosphere. Using sulfate ions or organic sulfur to oxidize methane, these microorganisms that lived in sedimentary lake environments left traces in the Tumbiana formation, aged at 2.7 billion years old (Lepot et al. 2019).

The anoxygenic photosynthesis forming the first stromatolites was carried out by phototrophic anoxygenic microorganisms and would therefore have occurred before oxygen photosynthesis.