In the Journals - Chemical evidence of multicellular life 635 million years ago

A paper in the current issue of Nature [Love et al (2009) Nature 457; 718-722] suggests that multicellular life existed about 100 million years before the explosion of bilaterian animals in the Cambrian. The evidence comes from analysis of rocks from the Arabian peninsula, in which geologically preserved derivatives of characteristic chemicals have been detected. Now, this paper interested me because of its message concerning the dating of the origins of multicellular life; I am not a geologist or a chemist, so many of the details escape me.

Identification of the presence of soft bodied animal in the fossil record is always difficult: it's generally the hard parts of the animal that are preserved by fossilisation (there are exceptions). The Cambrian explosion (see timeline diagram- click on it to link to the interactive Wikipedia diagram) resulted in a wide variety of animal forms: what's less clear is from where this abundance of diverse forms arose. This paper takes chemical approach to establishing the presence of animal remains in rock samples.

Evidence of sponges in the Ediacaran is that of siliceous spicules in rocks (Australia and Mongolia) dating from 543–549 million years ago, and from putative siliceous demosponge spicules dating from 600 million year old samples from South China. Molecular analysis of extant sponges indicates the divergence of the silicisponges, the demosponges and hexactinellids was very early. This diagram illustrates the relationship of sponges to other animal lineages.

The authors examined 64 sedimentary rock samples, in particular looking for extractable saturated and aromatic hydrocarbons. The rock samples were included strata laid down in the Ediacaran-Cambrian boundary and earlier, and were derived from 26 different oil well drillings from the petroleum-rich South Oman Salt Basin. Using gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) and MRM mass spectrometry, a variety of hydrocarbon derivatives were examined.  This analysis is a bit far from my area of expertise, so I'll leave that for you to look up in the paper.

Demospongiae, which include most modern sponges, synthesise a unique steroid, 24- isopropylcholesterol and related structures. The geologically stable form is 24-Isopropylcholestane: it's this chemical that the authors identified in their rock core samples, and is interpreted as evidence that these sponges were present at least 635 million years ago.  This dating evidence places multicellular organisms in the Cryogenian period (this predates the Ediacaran period illustrated in the diagram above).  As you might imagine from its name, the Cryogenian period was one in with the Earth was particularly cold - it lasted from 850 to 635 million years ago.  Sponges may have arisen in such harsh conditions, or their origins may be much older I suppose, and if the molecular taxonomies are correct, this would push our common ancestor back into the Cryogenian period.

One does need to bear in mind that the organisms responsible for the synthesis of this characteristic steroid, could actually belong to  group that is ancestral to the demosponges, particularly as the phylogenetic relationships between demosponges and other groups are uncertain in precise detail.  An accompanying News and Views article in the same issue presents this graphically (figure below, subscription may be required).