Thursday 16 March 2017

very very old

In graduate school for one semester, I sat at the feet of the late, great, Lynn Margulis for every minute of her BI504 Evolution course. Her position was that whales, yeast and daffodils were 'essentially the same' and so not worth getting into fine details about. If Evolution was to be an investigation of life's marvellous diversity then we really needed to look for/at life in the remote edges of the earth. We spent quite a lot of time working through stromatolites. These layered conglomerations of bacteria and their residue can be found today in Shark Bay in Western Australia and grow outwards in pillars and columns and mushrooms as photosynthetic cyanobacteria over-grow each other fighting for the light. You don't find stromatolites in Dublin Bay or the Côte d'Azur because cyanobacteria is richly delicious and get eaten by molluscs, crustaceans and starfish before they accumulate in lumps big enough to see. In Shark Bay the sea is so rough that the herbivorous animals get swept clean off the surface twice a day, so the cyanobacteria can make another 2 micron thick layer of greeny-brown. Margulis and her collaborators were fascinated by the fact that fossil stromatolites could be found in great abundance when you looked in pre-Cambrian rocks. The base of the Cambrian is more or less the base of the Phanaerozoic [=manifest destiny animals] aeon when life got sufficiently big and complex to be seen with the naked eye: trilobites, worms, arthropods, chordates and some weird-and-wonderful creatures that are long since extinct. It's called the Cambrian >!explosion!< because in a few inches of the fossil record pretty much all life as we see it appeared, lived, reproduced and died. Before that [let's say 600 mya] life was there but it was much simpler, smaller and less obvious. Hence the interest in fossil stromatolites: they, at least, were Precambrian and bigger than a bread-box.

Margulis tried hard to convince us, and herself and the peer-reviewed scientific literature, that some dots 1μm = a millionth of a metre across were once living creatures, rather than grit or air-bubbles from some purely chemical process. I remember being shown a grainy picture [like R the cluster is about 5μm across] of four little bolupes - one is hidden behind the others as the point of the tetrahedron. Four spoke, and speaks, potently of meiosis and a pair of cell divisions . . . life itself.
You think that 700,000,000 is old? That's nothing compared to microfossils that have just been discovered in Nuvvuagittuq on the Eastern shore of Hudson's Bay in Canada. These aren't tetrads, they are tubes about 20 microns across, and there is an outside chance that they are chemically driven crystal formation with no input from DNA. But the smart money is on them being the earliest, 3.7 billion years ago, evidence of life on Earth. They are close to the Nastapoka Islands if you want to GoogleMap their location. The local concentration of iron suggests that these creatures lived on or in close proximity to hydrothermal vents at the bottom of the sea. They were able to capture the heat and chemical energy and convert that into more of themselves. Photosynthesis, on which almost all the 'obvious' life on Earth today depends, comes waaaaay later. I'd call this pretty cool if it wasn't so smokin' hot down there back then at the dawn of time.

Some of the Nuvvuagittuq greenstone is the oldest rock we know about at 4.3 billion years old. Considering that the consensus for the age of the Earth itself is about 4.6 billion years, that is quite primordial. The rest of solid crust from those dim distant times has been rechurned: subducted into the molten interior to lose all of its layered and crystalline structure. These fossil tubes push back the origins of life; not least because we have to allow them additional time to get from tiny barely detectable - but still autonomously dividing - micron sized dots.

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