Gabon, the cradle of macroscopic life?
9/10/10

The discovery of macrofossils dating back 2.1 billion years attests to the appearance of complex organisms very early after the oxygenation of the oceans and the atmosphere. The complexification of life thus took place a lot quicker than had been believed.

virtual reconstructionThe oldest macroscopic fossils have just been discovered in Gabon. Dating back 2.1 billion years, 250 of them have been brought to Europe to be analysed. Their forms, very varied, are each stranger than the next, even though a common pattern takes shape: a central creased mass surrounded by radial divisions. Their nature remains unknown: are they colonies of eukaryotes or bacteria? These relics have brought together an international team composed of around twenty or so scientists from diverse disciplines, who desired to extract their secrets from them…of which the first sections have been revealed in the journal Nature (1). Amongst the researchers, Professor Emmanuelle Javaux, a micro-palaeontologist at the ULg’s Department of Geology.

Before studying these fossils, an understanding of their geological environment is indispensable. They come from ancient rocks found in a quarry situated close to Franceville in Gabon. They have been well preserved and dated, which is a first indispensable step. Sedimentology and mineralogy have then enabled these rocks and their composition to be characterised: they came from a pretty calm marine environment, whipped up from time to time by the tides. Finally, geochemistry has provided clues as to the chemical conditions which prevailed in the oceans and the atmosphere during the epoch we are interested in. In particular iron geochemistry has brought to light the presence of oxygen in the water in which these organisms lived.

After the analysis of the environment, that of the fossils themselves can begin. They are not record holders as far as age is concerned, as fossils of multicellular organisms older than 2.1 billion years have already been discovered, such as those of cyanobacteria. Thus what is striking about these new relics is not their age, but rather their macroscopic character given their age. In effect, whilst carbon compressions of macroscopic algae have existed since 1600 (or even 1900) millions of years ago, it was only 600 million years ago, at the end of the proterozoic period, that animals and algae started to multiply and diversify on our planet. Before that, in other words for a large part of its history, life was almost exclusively microscopic, although microbial communities have built stromatolites, macroscopic limestone structures, since probably 3.5 billion years.

 

(1) El Albani A. et al. 2010. Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago. Nature vol. 466 : 100-104 (1 July 2010). 

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