Survivors: The Animals and Plants that Time has Left Behind. Richard Fortey

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СКАЧАТЬ apparatus as it scrapes away the thin nutritious bacterial layer that paints the rocky surface. This is not inappropriately compared with grazing by herbivores on terrestrial environments. Like grass, the ‘blue greens’ grow back, and the molluscs move on. But these micro-organisms never have the chance to build complex or elaborate structures like mounds or ‘stagshorns’ because the constant assault of herbivores renders their best attempts at architecture futile. Everything is eaten back before it can grow too big. However, in the special, warm world of Hamelin Pool the grazers are kept at bay. No snails sully the sticky surfaces of the stromatolites; the fish there don’t nibble away the ‘blue greens’ for supper; in fact, nothing much ventures into the almost unnaturally limpid seas. Some authorities believe that the very low nutrient levels in the Pool are as important in growing stromatolites as the absence of grazers. Whatever the reason, the simple organisms have it all their own way for once. And when they do, they reconstruct the Precambrian world. This is how life was before marine animals chomped and scraped away ancient biological constructions that had covered much of the sub-aqueous environment since life began. In Shark Bay a prelapsarian age can be restored to view, a time before velvet worms or even vendobionts, or anything that crawled upon its belly in the mud. I have seen dozens of artist’s reconstructions of ancient seascapes that owe a debt to the prospect at Hamelin Pool. So when I saw the living stromatolites I was not unprepared for the experience. However, I recall seeing Picasso’s Guernica for the first time; just because an image is familiar does not diminish the impact of the real thing.

      Cyanobacteria are simple organisms that often make long, green and narrow threads with organic walls which can be as thin as a few thousandths of a millimetre, but which often occur in sufficient profusion to make green slime. Other species are tiny round cells that grow by fission – essentially splitting in two, to double up as identical twins. They are ubiquitous. When a glass of water is left in the light on a window ledge, cyanobacteria will usually appear as a green smudge. They have been wrongly called ‘blue-green algae’ in old textbooks, but as we shall see the algae are altogether more complicated organisms. When raindrops wash over rocks in a desert these tiny organisms will soon take advantage of the opportunity to grow, and the rocks will shortly glisten with microscopic life. In the sea, their numbers occasionally erupt into ‘blooms’ of billions of cells that can poison fish, or even humans, if they eat the wrong kind of shellfish too soon after one of these events. In biological jargon the ‘blue greens’ are described as prokaryotes. They are both the smallest and the simplest-looking cells – often no more than a sphere or a sausage – but there are hundreds of different species. They lack an organised nucleus surrounded by a membrane that is present in every cell in what are termed eukaryotes. Every organism that has been mentioned so far in this book (including the author) is a eukaryote, which is another way of saying that our narrative has now arrived back to a simpler way of organising a living entity. Prokaryotes came before eukaryotes in time, which also means that they are closer to the main trunk of the tree of life. So there was a world before eukaryotes where the cyanobacteria were state-of-the-art and where the prospect before us in the shallow waters in one corner of Shark Bay would have been typical of much of the world, rather than a special survivor. I should flag up at this point that this prokaryote– eukaryote division is itself an over-simplification, and this topic will be revisited in the next chapter.

      Modern seaweeds are both plants and eukaryotes, to emphasise the point again, and do not build stromatolitic mounds. In Shark Bay, the majority of such ‘advanced’ organisms are discouraged by the low levels of nutrients available there; hence they leave the dominant cyanobacteria to cooperate in making different kinds of mounds. In the typical stromatolite the mode of growth is cumulative. The living ‘skin’ is a thin layer of growing threads matted and twined together. The technical term for it is a ‘biofilm’. The cyanobacterial mats are positively attracted to light and grow upwards. Any blown dust and other fine sedimentary material becomes incorporated in the surface layer and maybe provides the modest nutrient required. The slimy surface layer of the bacterium encourages the precipitation of calcium carbonate from its dissolved state in seawater, thus making a thin ‘crust’. A new living layer grows on top of the one beneath, and may be able to extend a tiny bit further laterally: this is why some of the stromatolite mounds are wider at the top than at the base. Naturally, the ‘blue greens’ are only able to grow in the sunlight that gives them nourishment, and are quiescent at night. Some scientists at the University of California even claim to have recognised daily growth increments. The overall rate of growth is extraordinarily slow, however, and certainly less than 1 mm a year (and possibly as little as 0.3 mm). It has been stated that some of the Hamelin Pool structures could be a thousand years old, that is, they grow more slowly than the slowest-growing conifer on land. The life and death of the wool industry would register as no more than a hand-depth on the height of a stromatolite column. Time can be ticked out in microscopic laminations, and history reduced to a measuring stick made by timekeepers invisible to the naked eye.

      Stromatolites vary in form according to where they are found on the shore. It is easy for me to see that ones at the edge of the sea are little more than pimply mats. At least to this unschooled observer, some of them superficially do not look very different from some of the mats that covered sediment surfaces in the Precambrian at Mistaken Point. They are made particularly by one of the spherical, or coccoidal types called Entophysalis, and the internal layering is not well developed. Further down the shore in Hamelin Pool the stromatolites that I tentatively touched represent the dominant kind in the intertidal zone, with a typical columnar-cushion shape. This kind of column is constructed particularly by a filamentous cyanobacterium called Schizothrix, which under the microscope is an intense emerald-green colour. It has lots of apparent partitions that make the organism look something like an old-fashioned tube of circular cough sweets. These particular stromatolites are very well laminated internally, so that the mechanism of being built up layer by layer is particularly patent. It has been proved that the cushions ‘lean’ a little to the north, each component filament attracted preferentially to the sun (but on such a minute scale) in this, the southern hemisphere; the god Ra evidently ruled in the prokaryotic shallows. Further out to sea again, to a depth of a little more than three metres, there live the lumpier, bumpier, lobed, and somewhat rounded stromatolites that are a collaboration of many different microbes. These include cyanobacteria of the genera Microcoleus and Phormidium; the latter is another concatenation of delicately segmented threads, while the former comprises microscopic ‘ropes’ made up of bundles of a kind of entwined green spaghetti. The different species collaborate to grow together, like a confederation of medieval guilds, with each tiny specialist contributing to the function of an integrated community. True algae – diatoms – may chip in as part of the community among the deeper water stromatolites, but this group of eukaryotes probably did not evolve until much later. Beneath the surface skin of the growing mound, bacteria of a different kind from cyanobacteria process waste products and can cope with low, or even no oxygen; they are like artisans that moved the dung from the streets of the medieval village and made it a trade. Life encouraged specialised habits and habitats from the first.

      Stromatolites are the most ancient organic structures, and their recognition as fossils transformed the way we understood the endurance of life on earth and the evolution of its atmosphere. I admit that viewed with complete impartiality when it comes to visual impact, the Shark Bay mounds are not on a par with the Empire State Building or the pyramid of Cheops. But stromatolites are one of the wonders of the world. Rationalists are not permitted to have shrines, but if they were then Shark Bay, where stromatolites were discovered alive, might be high on the list. Although many more living stromatolites have since been discovered, those in Shark Bay have been most thoroughly studied. From their initial recognition in 1954 the fame of these living stromatolites spread, until by the late 1960s they were finding a place in textbooks. As so often happens in science, the discovery of these living mounds happened just when palaeontologists were making major finds of microscopic fossils in rocks of Precambrian age, opening up debates about the biological history of the earth. The strange creatures of the Ediacaran, like Fractofusus and Charniodiscus, took the record of life back tens of millions of years before the great burst of familiar fossils such as trilobites that appeared in the Cambrian, 542 million years ago. But there remained more than three billion years of the history of life on earth in СКАЧАТЬ