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.