Sunday, 29 May 2016

Macroscopic Eukaryotes From The Boring Billion

1.56 billion year old eukaryotic body 
fossils from the Gaoyuzhuang Formation
Fossil and genetic evidence pushes the origin of eukaryotes close to two billion years ago. This allows the event to be reliably linked to the increase in atmospheric oxygen and the swathe of environmental changes accompanying it.

Single celled and microscopic multicellular forms are well documented from 1.5 billion years onwards. Macroscopic eukaryotes, however, have only been found in rocks several hundred million years younger still. The reasons for this delay are somewhat unclear, but can be correlated to a billion year period of geochemical stagnation following the Great Oxidation Event. 

Now a recent fossil discovery in China, from the Gaoyuzhuang Formation has pushed back the record of macroscopic eukaryotes much closer to the origin of complex cells themselves. Previous reports of macroscopic eukaryotes of comparable ages were dismissed as inorganic artifacts of preservation or as colonies of prokaryotes with complex morphologies. The 1.56 billion year old Chinese discoveries, however, are undeniably eukaryotic. Microscopic sheets of cells with an organised, complex eukaryotic character were found in the same beds as carbonaceous impressions, a third of which fell into four discrete morphological categories. At 30 centimetres long and 8 centimetres wide, the largest specimen was far larger than any of the previously reported (and debunked) oldest macroscopic eukaryotes.

'Our discovery pushes back nearly one billion years the appearance of macroscopic, multicellular eukaryotes compared to previous research,' said Maoyan Zhu from the Nanjing Institute of Geology and Palaeontology.

The 1.56 billion year old microscopic cell
fragments from the Gaoyuzhuang Formation
The researchers characterised the fossils as similar in form to thalli, the singular form is thallus. A thallus is a sheet of cellular material which does not display differentiation into specific structures. Thalli are found in plants, fungi and a multitude of algal groups.

The fossils lack enough detail to be tied to any one of these groups. The base of the structures possessed holdfasts, suggesting at least a primitive form of differentiation in some of the cells in these creatures - further evidence of their eukaryotic nature.

The specimens have already attracted controversy, with a number of researchers suggesting that they are simply bacterial mats. Until conclusive evidence of this is put forward, however, they remain the oldest macroscopic eukaryotes in existence.