Sunday, 7 June 2015

Dispelling The Evidence For Archaean Prokaryotes

1.2 billion year old eukaryote microfossils from Loch Torridon in Scotland
Preserved eukaryote cells can be found dating back to 1.631 billion years ago, nearly four hundred million years older than when genetic studies indicate that they should have appeared. Markings and depressions in still older formations, however, have been taken by some palaeontologists as evidence of still older eukaryotes. Others regard these traces, such as Grypania, as nothing more than bacterial formations or inorganic sedimentary structures created by quirks of deposition or mineralisation.

Yet the oldest putative traces of complex cells are geochemical and not fossils. Sterols are a class of organic molecules which play important structural and metabolic roles in eukaryote cells. Metabolic processes involving sterols can give rise to a class of derived compounds, the steranes. Neither steranes or sterols are produced by simple prokaryotic cells. So when 2.5 to 2.8 billion year old steranes were found in Australia, they were taken as geochemical evidence of eukaryotes from that temporal range or biomarkers. The question, however, was one of contamination.

A possible source of the steranes was from the steel drill head used to collect the sample on account of the chemicals used to treat and harden the metal. A recent study conducted by an international team of researchers confirms that they were most likely modern contaminants rather than cell derivatives. They needed to collect fresh samples from the locality under the cleanest possible conditions.

Drilling into the night: several weeks were required to extract the
samples as the lack of synthetic lubricants restricted the drill speed
The drilling itself was conducted over a period of several weeks. Usually the process is a lot faster but to remove further sources of contamination no synthetic lubricants were used and so a slow grind was required to reach the depth from which the steranes were reported to have come from.

The effort paid off when several highly sensitive mass spectrometers were unable to detect even picogram quantities of steranes. Large amounts of other compounds were detected, including diamondoids and polyaromatic hydrocarbons, which indicated that organic matter within the rock had been altered by pressure and temperature.

'The entire organic material in these samples was modified by pressure and temperature during the course of billions of years, and no biomarker molecules could have survived. We are thus unable to draw any conclusions on the original biological signature of the material', said Christian Hallman, leader of the Max Planck Institute's organic paleobiogeochemistry research group.

This is not to say that there were not eukaryotes over 2.5 billion years ago, but that the steranes originally detected cannot be used as evidence for their existence. Even so, the lack of convincing eukaryote fossils from the time suggests that eukaryotes are still recent evolutionary products compared to the prokaryotes.