Atmospheric oxygen concentration on earth first started increasing about 2.45-2.32 billion years ago. So when did the first multicellular organisms that lived in aerobic (oxygen present condition) environment appear?
The earliest known eukaryote is a coil-shaped fossil Grypania spiralis which existed about 2 billion years ago (2 Gyr). Now the record has been pushed further back by 0.10 billion years.
The discovery of centimetre-sized structures of “highly organised and spatially discrete populations of colonial organisms” in black shales from southeastern Gabon (west central African country) has been reported today (July 01) in Nature .
The structures are up to 12 cm in size, and most importantly they have a characteristic shape with a simple but distinct ground pattern of flexible sheets. Carbon and sulphur data further corroborate that they are indeed biogenic objects and not relics.
But the most important feature of the organisms is the cell-to-cell signalling and co-ordinated responses. These two characteristics are commonly associated with multicellular organisms.
If oxygenated conditions had existed at the time of deposition of the organisms, it would in turn mean that they lived and required oxygen for sustenance. Geochemical analysis indicates that the sediments were indeed deposited in an oxygenated condition.
This is the first time ever that multicellular eukaryotic aerobic organisms that survived around 2.1 billion years ago have been recovered. It also means that the first eukaryotic organisms came into being as early as about 200-250 million years after significant rise in oxygen concentration in the Earth's atmosphere.
The soft-bodied organisms are 7-120 mm and 5-70 mm wide. They are 1-10 mm thick. The organisms occur together and have a density of about 40 specimens per sq. metre. Not all organisms have the same shape and size. The most characteristic shape is the fold pattern seen at the centre, and radial fabric at the outer edges.
Going by their characteristic shape and size, the authors conclude that the structures seen “fulfil the general criteria of biogenicity applied to fossil-like forms in the early rock record.”
They also conclude that they “most likely represent fossilised colonial organisms.” Basing on the behaviour of colonial bacteria both in nature and those grown in the laboratory, the scientists conclude that the organisms are typical “colonial eukaryotic organisms.”
Another interesting feature is the co-ordinated growth behaviour. This requires a certain level of organisation. Such organisation is seen even in mat-forming communities. But what distinguishes the Gabon organisms from their mat-forming counterparts is the regular fabric pattern arising out of co-ordinated growth behaviour.
The most basic requirements for co-ordinated growth behaviour are cell-to-cell signalling and co-ordinated responses. These are generally seen and associated with multicellular organisms. The fact that they are seen in the Gabon organisms simply means that they are multicellular organisms.
The most basic requirement for multicellularity is the presence of sufficient oxygen for the eukaryotes to survive and thrive. Together with evidence of sediment deposition in an oxygenated condition, the authors conclude that the multicellular eukaryotic organisms thrived in an aerobic condition (requiring oxygen for survival).
The organsisms are soft bodied and have been pyritized (pyrite is a mineral and is commonly called a ‘fool's gold' as it resembles the noble metal gold).
The authors note that pyritization of “soft tissue is rare but typically results in faithful replication.”
If excellent preservation of soft-bodied organisms is rare, their preservation for billions of years is simply remarkable.