The oldest fossils appear to date from about 3.5 million years ago. Fossils of this age have always been controversial, but there are two kinds, microfossils and stromatolites. The first truly ancient fossils were reported in the 1950s, and the pressure to find ever-older specimens is intense. Mistakes have often been made, and that is no surprise because the oldest fossils are bound to be from extremely simple organisms, and microscopic ones at that. So it's no wonder that great experts have often been caught out over-interpreting a chance bubble or mineral fragment in a microscope slide, even a bit of fluff or a modern plant spore.
It is probably unexpected that the most convincing truly ancient fossils are large structures called stromatolites. These are mounds made partly from living organisms and partly from sediment, and they still exist today. Stromatolites (Fig. 5a) are made from many thin layers that apparently build up over many years or hundreds of years to form irregular mushroom- or cabbage-shaped structures. They are built from microbial mats composed of some of the simplest of living organisms called cyanobacteria, and these have sometimes been called, rather misleadingly, blue-green algae. Algae, like seaweeds, have advanced cells with nuclei, whereas cyanobacteria, like ordinary bacteria, are made from the simplest of cells, without a nucleus.
Typical cyanobacteria photosynthesize, so they live in shallow e water, near the water's edge. Today, they are found generally in J highly saline waters, often in tropical regions, where pools of f seawater have partly evaporated. In less saline waters, herbivorous e animals eat them up. The thin microbial mat may sometimes then be swamped by fine grains of mud, and the cyanobacteria grow up through the sediment to keep in touch with the sunlight. Over time, extensive layered structures may build up. In most fossil examples, the constructing microbes are not preserved, but the layered structure remains. Many early examples have proved controversial, but the oldest that are generally accepted come from Australia, and are dated as 3.43 billion years old.
Perhaps the oldest currently accepted microfossils other than stromatolites date from 3.2 billion years ago. They were reported in 2000, from a massive sulphide deposit in Western Australia. The fossils are thread-like filaments (Fig. 5b) that may be straight, sinuous, or sharply curved, and even tightly intertwined in some areas. The overall shape, uniform width, and lack of orientation all tend to confirm that these might really be fossils, and not merely inorganic structures. If so, they confirm that some of the earliest life may have been thermophilic ('heat-loving') bacteria that lived near a hot, sulphur-producing structure under the sea, as predicted by Euan Nisbet and Norman Sleep's model for the origin oflife.
There is a long gap in time after the 3.4-billion-year-old stromatolites and microfossils before more convincing fossils are found. There are some specimens from rocks dated at 2.5 billion years old in South Africa, and then the famous Gunflint Chert of Canada, dated at 1.9 billion years ago. The Gunflint microfossils include six distinctive forms, some shaped like filaments, others spherical, and some branched, or bearing an umbrella-like structure. These Precambrian cells resemble in shape various modern bacteria, and some were found within stromatolites. Most unusual is Kakabekia, the umbrella-shaped microfossil; it is most
5 like rare micro-organisms found today at the foot of the walls of o t Harlech Castle in Wales. These modern forms are tolerant of o
| ammonia (NH3), produced by ancient Britons urinating against
5b. Filamentous microfossils in a 3,235-million-year-old massive sulfide from Australia the castle walls. So were conditions in Gunflint Chert times also rich in ammonia?
Strange things were happening on the Earth 2 billion years ago, apart from the ammonia-loving Kakabekia. The atmosphere suddenly seemed to carry oxygen, there are organic traces of quite diverse life, and new kinds of microfossils appear, some of them with nuclei. If this is true, these mark the origin of the eukaryotes, and so the origin ofsex.
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