Life on Earth - David Attenborough

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in an organism in a matter of hours, using a device the size of a mobile phone. The millions of DNA sequences that have been established, stored in databases and compared show us unequivocally that, just as Darwin predicted, all life on earth shares a common ancestor. Because parts of our DNA accumulate mutations at a constant rate, like a molecular clock, we can use DNA sequences to estimate when two species split apart. In general, genetic and fossil timings agree with each other, although genetic data do sometimes throw up surprises. Using this method we can estimate that the Last Universal Common Ancestor of all life on earth – commonly known as LUCA, and basically a population of simple bacteria – lived around 4 billion years ago. Everything we can see around us can trace its ancestry back to that group of cells.

      Such vast periods of time baffle the imagination, but we can form some idea of the relative duration of the major phases of the history of life if we compare the entire span, from these first beginnings until today, with one year. That means that, roughly, each day represents around ten million years. On such a calendar, the Gunflint fossils of algae-like organisms, which seemed so extremely ancient when they were first discovered, are seen to be quite late-comers in the history of life, not appearing until the second week of August. In the Grand Canyon, the oldest worm trails were burrowed through the mud in the second week of November and the first fish appeared in the limestone seas a week later. The little lizard will have scuttled across the beach during the middle of December and humans did not appear until the evening of 31 December.

      But we must return to January. The bacteria fed initially on the various carbon compounds that had taken so many millions of years to accumulate in the primordial seas, producing methane as a by-product. Similar bacteria still exist today, all over the planet. And that was all there was, for around five or six months of our year. Then, in the early summer of the year of life, so some time over 2 billion years ago, bacteria developed an amazing biochemical trick. Instead of taking ready-made food from their surroundings, they began to manufacture their own within their cell walls, drawing the energy needed to do so from the sun. This process is called photosynthesis. One of the ingredients required by the earliest form of photosynthesis is hydrogen, a gas that is produced in great quantities during volcanic eruptions.

      Conditions very similar to those in which the early photosynthesising bacteria lived can be found today in such volcanic areas as Yellowstone in Wyoming. Here a great mass of molten rock, lying only a thousand metres or so, down in the earth’s crust, heats the rocks on the surface. In places, the ground water is well above boiling point. It rises up channels through the rocks under decreasing pressure until suddenly it flashes into steam and water spouts high into the air as a geyser. Elsewhere, the water wells up into steaming pools. As it trickles away and cools, the salts it gathered from the rocks on its way up, together with those derived from the molten mass far below, are deposited to form rimmed and buttressed basins, surrounded by tiers of terraces. In these scalding mineral-laden waters, bacteria flourish. Some grow into matted filaments and curds, others into thick leathery sheets. Many are brilliantly coloured, their intensity of hue varying during the year as the colonies wax and wane. The names given to these pools hint at the variety of the bacteria and the splendour of the effects they produce – Emerald Pool, Sulphur Cauldron, Beryl Spring, Firehole Falls, Morning Glory Pool and – a particularly rich one with several species of bacteria – Artists’ Paintpots.

      Hot spring, its water coloured by bacteria, Yellowstone National Park, Wyoming, USA.

      When you wander through this amazing landscape, you can smell sulphurated hydrogen, the unmistakable stench of rotting eggs, produced by the reaction of ground water with the molten rock far beneath. This is the source from which many of the bacteria here obtain their hydrogen, and as long as bacteria were dependent upon volcanic action for it, they could not spread widely. But other forms eventually arose which were able to extract hydrogen from a very much more widespread source – water. This development was to have a profound effect on all life to come, for if hydrogen is extracted from water, the element that remains is oxygen. The organisms that did this are barely more complex in structure than bacteria. They are sometimes called blue-green algae because they appeared to be close relatives of the green algae that are common in ponds, but now we realise they are similar to the ancestors of those algae, and they are referred to as cyanobacteria or, simply, blue-greens. The chemical agent which they contain, making it possible for them to use water in the photosynthetic process, is chlorophyll, which is also possessed by true algae and plants.

      Blue-greens are found wherever there is constant moisture. You can often see mats of them, beaded with silver bubbles of oxygen, blanketing the bottoms of ponds. In Shark Bay, on the northwest coast of tropical Australia, they have developed in a particularly spectacular and significant form. Hamelin Pool, one small arm of this vast inlet, has its entrance blocked by a sand bar covered with eel grass. The flow of water in and out of the Pool is so greatly impeded that evaporation under the grilling sun has made the waters very salty indeed. As a result, marine creatures such as molluscs which would normally feed on blue-greens and keep them in check, cannot survive. The blue-greens, therefore, flourish uncropped just as they did when they were the most advanced form of life anywhere in the world. They secrete lime, forming stony cushions near the shores of the Pool and teetering columns at greater depths. Here is the explanation of those mysterious shapes seen in section in the Gunflint Chert. The blue-green pillars of Hamelin Pool are living stromatolites, and the groups of them standing on the sun-dappled seafloor are as close as we may ever get to a scene from the world of 2 billion years ago.

      The arrival of the blue-greens marked a point of no return in the history of life. In ways we do not fully understand, the oxygen they produced eventually accumulated over the millennia to form the kind of oxygen-rich atmosphere that we know today. Our lives, and those of all other animals, depend on it. We need it not only to breathe but to protect us. Oxygen in the atmosphere forms a screen, the ozone layer, which cuts off most of the ultraviolet rays of the sun.

      Life remained at this stage of development for a vast period. Then, around 2 billion years ago one single-celled life form found itself trapped inside another, in an entirely chance encounter. You can find examples of the kind of organisms it eventually produced in almost any patch of fresh water.

      A drop from a pond, viewed through a microscope, swarms with tiny organisms, some spinning, some crawling, some whizzing across the field of vision like rockets. As a group they are often called the protozoa, or protists – the name means ‘first animals’, although they are now seen as a very disparate group, not all of which have any affinity with animals. They are all single cells, yet within their cell walls they contain much more complex structures than any bacterium possesses. One central packet, the nucleus, is full of DNA. This appears to be the organising force of the cell. Elongated bodies, the mitochondria, provide energy by burning oxygen in much the same way as many bacteria do. Many cells have a thrashing tail attached to them and this resembles a filamentous bacterium called a spirochete. Some also contain chloroplasts, packets of chlorophyll which, like blue-greens, use the energy of sunlight to assemble complex molecules as food for the cell. Each of these tiny organisms thus appears to be a committee of simpler ones. This, in effect, is what they are. The mitochondria are the descendants of the single-celled organism that was trapped some 2 billion years ago, say in June in the year of life, while the chloroplasts are descended from a trapped blue-green.

      Protozoans reproduce by splitting into two, as bacteria do, but their internal structure is much more complex and their division, not surprisingly, is consequently an elaborate business. Most of the separate structures, the members of the committee, themselves split. Indeed, the mitochondria and chloroplasts, each with their own DNA as befits their origins as separate organisms, often do so independently of the division of the main cell. The DNA within the nucleus replicates in a particularly complex manner which ensures that all its genes are copied and that each daughter cell receives a complete duplicate set. There are, however, several other methods of reproduction practised by various protozoans on occasions. The details vary. The essential feature of all the techniques is that a shuffling of genes is involved. In some cases this takes place when two cells join up and exchange genes before breaking apart and then undergoing cell division some time later. In other cases, cells normally contain two

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