The origins of life …

I have been reading Richard Fortey’s book “Survivors” (see “”They don’t do much, do they?””) which is a rare and precious thing in the world of natural history writing as he devotes two whole chapters to the algae.   The first of these describes his encounters with living stromatolites in Shark Bay, in Western Australia.   The point he is making as he writes is that he did not just make an extraordinarily long journey to get there (first, get to Perth, then travel a further 800 km north …) but that he is also making a similarly long journey back in time, as stromatolites are survivors from the Precambrian era and, indeed, may have played a vital role in creating the oxygen-rich atmosphere that we take for granted.

I have a polished specimen of a stromatolite, purchased from a fossil shop in Durham, which shows the characteristic fine laminations. Each of these represents a layer of Cyanobacteria (blue-green algae) filaments which have, in turn, trapped sediment particles.   The laminations are, in turn, formed into dome-like structures, reflecting the vertical growth of the filaments in search of sunlight.   This stromatolite is from the Ordovician era (I think), which dates it to between 443 and 485 million years ago.

stromatolite_argentina

A stromatolite from Argentina, possibly from the Ordovician era. The specimen is 10 cm long.

Not only were stromatolites extremely abundant in the Precambrian (approx. 4.6 billion to 540 million years ago) but the organisms from which they were formed appear to be very similar to Cyanobacteria that can still be found today.   They are extremely delicate structures that thrived, at least in part, because there were few other organisms in the Precambrian that could compete with them or graze them and, consequently, had shallow marine habitats to themselves for an extremely long time. During this period, they were busily photosynthesising away, taking carbon dioxide and water and converting it to simple sugars (which they needed to grow) and oxygen (which was, as far as the Cyanobacteria was concerned, a waste product). This oxygen was released as tiny bubbles (see “Ecological yin and yang…”) which, ever so slowly, accumulated in the atmosphere.   Maybe it is no surprise that the Precambrian, the era before fossils of multicellular organisms are common, lasts for about four fifths of the entire lifespan of the earth: it took this long for all those tiny bubbles to add up to enough oxygen to allow more complicated organisms to survive.

And what did those multicellular organisms feed upon?   That’s right: the Cyanobacteria had sown the seeds of their own destruction.   There is evidence not just of a gradual decline of stromatolites through the later Precambrian and into the Cambrian and Ordovician eras, but also of a resurgence of stromatolites after the mass extinction at the end of the Ordovician (which would have removed the multicellular grazers and left our tough little Cyanobacteria behind).   Stromatolites are found sporadically throughout the fossil record and in a small number of locations in the present day, but their heyday lies far in the past.

References

Bengston, S. (2002). The early worm catches the – what?   Pp. 289-317. In: The Fossil Record of Predation (edited by Kowalewski, M., and Kelley, P.H.). The Paleontological Society Papers 8, The Paleontological Society, Boulder, Colorado.

Fortey, R. (2011).   Survivors: the Animals and Plants that Time Has Left Behind.   Harper Press, London.

Sheehan, P.M. & Harris, M.T. (2004). Microbialite resurgence after the Late Ordovician extinction. Nature (London) 430: 75-78.

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