Monday, June 4, 2007

Had we but world enough and time

There seems to be a reasonable amount of interest in Precambrian organic matter because of its relevance to oil and gas exploration, but not so much work on what the distribution of organic molecules implies for the nature of life at the time. Nobody seems to have done anything specifically on the Ediacaran organisms to see whether they were chemically related to the later Metazoans or not, but the molecular evidence for a single unified biochemistry on Earth for the last 3.8 billion years seems very strong. I have found a couple of articles by Brock et al. about investigations of a 2.7 billion year old formation in the Pilbara (Science 1999, 285, 5430; Geochimica and Cosmochimica Acta 2003, 67(22), 4289) that report sterols, made today only by Eukaryotes, straight-chain alkanes with a 12C:13C isotopic ratio characteristic of modern cyanobacteria, and isoprenoid compounds, typically made today by Archaea, with a 12C:13C isotopic ratio characteristic of modern methanogen Archaea.

This means that the chemical phenotypes of these classes of organism were well-established at least 2.7 billion years ago. Thus the genetic differences between these classes, which are vast, either:

* Have to be compressed into the time between 2.7 billion years ago and the origin of life on Earth, which has to be after the crust solidified. This is vaguely possible, as there is no plausible reason that the ‘molecular clock’ has run at the same speed all the time.

* Arose, if one extrapolates the molecular clock sphexishally backwards, well before the Earth’s crust solidified.

Isotopic distributions of 12C:13C in organic deposits show no sharp discontinuity as we move further back in time, but are consistent with a biological origin (Schidlowski, Precambrian Research, 2001, 106(1-2), 117-134). Thus the essential biochemistry of today’s methanogens, or something very similar, were already established in the very oldest sedimentary rocks we know about, about 3.8 billion years old.


Marco said...

Just trying to get the time sequence right in my head. What is the approximate time that the moon was formed? Presumably the crust was molten before and after the collision that formed it. Is there any form of extremophile that could find an environment to prosper amongst the chaos?

Chris Fellows said...

The moon seems to have formed some time not long before 4.5 billion years ago. I can't imagine an extremophile 'prospering' on a molten Earth, but I can imagine micro-organisms could have survived in solid fragments blasted off into space that later settled back down on the surface when things were more congenial.

Marco said...

Yes of course, and the collisions and consequent ejections are required for spreading to say underground areas of Mars, or the atmospheres of gas giants or the watery insides of their moons, where some sort of evolutionary process can take over to specialise there.