Spongebob is a child of snowball Earth

Image from commons.wikimedia.org

Although looking a little like plants, sponges (Phylum Porifera) are animals. Admittedly, they area very different kind of animal from the ones we see around us in everyday life. Unlike most others they lack bodies constructed from properly organized tissues, they are instead more like giant colonies of single cells. Indeed a famous experiment demonstrated that sponges that have been completely disaggregated into its constituent cells (by forcing them through a fine-mesh screen*) will begin to reconstitute themselves. Sponges lack any sort of gut, they instead live by sieving fine organic particles out of seawater. The food is absorbed directly by specialized ‘collar cells’ (more formally choanocytes) that line the canals and chambers that run through the sponges body. Sponges are also remarkable for their unique skeletons, which are generally made of tiny to microscopic spicules of silica or calcite or a lacy network of elastic organic material known as spongin (as in the original bath sponges). As you might expect with animals that lack a gut, nervous system or indeed organs of any sort, sponges are a very early branch of the animal tree. Indeed the tiny little disc of cells known as Trichoplax adhaerens may be the only living animal that branched away before sponges and all other animals split. Actually I’m oversimplifying here, because the evidence is looking good for sponge paraphyly. That means some of the different sponge groups are more closely related to tissue-grade animals (a clade called Eumetazoa) than to other sponges, so it wasn’t a single sponge-eumetazoan split. The remarkable conclusion that leads to is that we are directly descended from an animal that we would call a sponge.
Given their early divergence and simple construction, one would expect sponges to have a venerable fossil record. Indeed they do, but not quite as long as one might expect. Sponge spicules seemed to appear in the fossil record at the beginning of the Cambrian Period along with a great number of eumetazoan groups (now dated to 542 million years). Some spicules have been reported from older strata but these are not without controversy. In contrast eumetazoans clearly had an older fossil record (e.g. the 545-565 million year old Ediacaran fauna). The mid 1990’s saw the publication of Palaeophragmodictya, the first probable whole body fossils of sponges from the Ediacaran fauna. Nevertheless without spicular preservation (like other ediacaran macrofossils Palaeophragmodictya are preserved as impressions on the base of sandstone beds) there may always be some doubt as to its identity. What remains unusual is that it took so long for such Ediacaran sponges to be found and that they remain a very rare component of Ediacaran faunas.



Small individuals of the putative Ediacaran sponge Palaeophragmodictya. From Gehling and Rigby (1996)



Close up of a large Palaeophragmodictya showing what might be the impression of a spicular mesh.From Gehling and Rigby (1996)

Furthermore divergence dates calculated using molecular clock methods suggested a sponge- eumetazoan divergence of 650 million years. If the common ancestor was itself a sponge-grade organism we should expect the record of sponges and sponge-grade animals extending back to pre-Ediacaran times. This is well illustrated in the following diagram that teases apart the so-called ‘Cambrian explosion’(image from www.snowball.org). An interesting aspect of this molecular date is that it extends the range of animals back into a Period known as the Cryogenian, or just after it.



What is special about the Cryogenian? It was period in Earth history lasting from 750-620 million years where the Earth went through several severe glaciations events known as ‘snowball earths’. Although the exact severity of the snowball-earth glaciations is a contentious topic, there is convincing evidence that the Earth was cold enough to support sea-level glaciers in the equatorial belt. So an important question is: had animal divergence begun in the Cryogenian as the molecular divergence dates suggest? That question has been conclusively answered this year in a Nature paper by Gordon Love and colleagues.
Love et al. found convincing sponge fossils in sediments securely dated to the age of the Marinoan glaciation, the last snowball earth glaciations event of the Cryogenian. As a small aside the Marinoan is named after the seaside suburb of Marino, on the southern coast of Adelaide, my home town. My first ever geological field trip for my degree was looking at the Marinoan rocks of Marino. Anyway enough reminiscing, onto the Cryogenian sponges fossils. What were they? Spicules? Whole body impressions? No, Love et al. found molecular fossils, in particular 24-isopropylcholestanes. These particular hydrocarbons are, according to the authors, the degraded products of C30 sterols, a class of molecules only produced by members of the sponge class Demospongiae (here I have to accept the author’s word, I know far too little about organic chemistry to have any way of assessing the veracity of this statement). An interesting feature of snowball Earth Glaciations is that they are usually covered by a thin but continuous layer of carbonate rock: the 'cap carbonates'. These are thought to have precipitated out under extraordinarily hot conditions bought on by the retreat of the glaciers leaving behind an atmosphere dense in CO2 (which would accumulate while terrestrial weathering was essentially shut down underneath the ice-cover). Anyway these molecular sponge fossils are found in rocks below the cap carbonates, that is during the glacial period itself.
The implications are pretty huge. It means that multicellar animals arose and first diversified in frigid seas largely covered by ice, and not during the flush of warmth that suffused the planet immediately after the glaciers lost their grip. Where could animals exist in such a sea? There were probably numerous little oases of light where cracks in the relatively thin equatorial sea ice would allow local blooms of bacteria and algae. These little patches may well have been the birthplace of multicellular animal life.


A modern day equivalent of the oases that was the birthplace of animal life? Image from www.snowball.org.

References
Gehling, J.G. and Rigby, J.K. (1996) Long Expected Sponges from the Neoproterozoic Ediacara Fauna of South Australia. Journal of Paleontology 70: 185-195.

Love, G., Grosjean, E., Stalvies, C., Fike, D., Grotzinger, J., Bradley, A., Kelly, A., Bhatia, M., Meredith, W., Snape, C., Bowring, S., Condon, D., & Summons, R. (2009). Fossil steroids record the appearance of Demospongiae during the Cryogenian period Nature, 457 (7230), 718-721 DOI: 10.1038/nature07673

*They were the luckiest of all