"There are four million different kinds of animals and plants in the world." "Four million different solutions to the problems of staying alive." "This is the story of how a few of them came to be as they are." "The South American rainforest." "The richest and most varied assemblage of life in the world." "Those are howler monkeys up there." "There are around 50 different kinds of monkeys in these forests." "Some of the most beautiful creatures here are hummingbirds." "54 different kinds have been found within a few miles of here, and over 300 have been found in South America as a whole." "Nobody knows how many kinds of animals there are here." "Wherever you look, there's life." "There are several hundred thousand insects that have been named, and, without doubt, many more that haven't." "All these creatures and plants form one complex mosaic." "The orchid needs the bee to pollinate it." "The anteater couldn't have existed before the ants." "So unless the whole complex came about in a flash of instant creation, different organisms must have appeared at different times." "But which came first, and why should there be such an immense variety?" "Such questions obsessed a young 24-year-old Englishman who came here in 1832." "His name was Charles Darwin and he was enthralled to the point of ecstasy by the richness of life he found here." "In one day, in a small area, he discovered 69 different species of beetle." "As he wrote in his journal," ""It's enough to disturb the composure of the entomologist's mind" ""to contemplate the future dimension of a complete catalogue."" "The conventional view of the time was that every species of animal and plant had been individually created by God." "And Darwin was no atheist." "During the next three years, the Beagle sailed round South America and up into the Pacific." "600 miles west of Ecuador, they came to the lonely Galapagos islands." "It was here, on these volcanic islands, that Darwin's doubts about the creation of species were reawakened." "Everywhere, Darwin found creatures that bore a general resemblance to those he had seen on the mainland." "But nearly all were slightly different." "These, for example, were cormorants similar to those he had seen flying along Brazilian rivers." "But in the Galapagos, their wings were so small, with such stunted feathers, that the birds had lost their powers of flight." "And these were clearly iguanas." "He'd seen them climbing trees in the South American forests, but on the Galapagos, with its sparse vegetation, these iguanas fed on seaweed, and they were not the same." "Smaller, darker and with unusually long claws to help them keep a foothold among the crashing breakers." "They also had extraordinary habits, swimming fearlessly out to sea and diving deep to graze on the sea bed." "The Galapagos Islands got their name from the herds of tortoises that live on them, which sailors for centuries had slaughtered for food." "But these too were obviously different from mainland tortoises." "They were many times bigger." "The English vice-governor of the islands told Darwin that he could tell which island a tortoise came from by its shape." "This one, for example, with its deep, rounded shell, comes from a well-watered island where it can feed on vegetation on the ground." "This one has a peak to the front of its shell that enables it to stretch its long neck upwards." "It comes from an arid island, where the tortoises have to crane up to reach the only food there, the branches of trees and cactus." "The suspicion grew in Darwin's mind that species were not fixed for ever." "Perhaps the tortoises were all descended from common ancestors and had changed to suit their particular islands." "The differences Darwin noticed among these Galapagos animals were, of course, tiny." "But if they could develop them, wasn't it possible that over the thousands or millions of years, a whole series of such differences might add up to one revolutionary change?" "Was it not possible, perhaps, that in the past amphibians had developed watertight skins and so turned into reptiles?" "Or that a lizard-like reptile had developed a feathery kind of scale and become a bird?" "And even that man himself might be descended from a group of tree-swinging apes?" "In truth, the idea was not a new one." "Others before Darwin suggested that all life on earth might have a common ancestry, but Darwin went further." "He gave the idea irresistible force by suggesting a mechanism which might have brought that about." "He called the mechanism natural selection." "Put briefly, his argument was this." "Individuals of the same species are not absolutely identical." "Some of these giant tortoise hatchlings may have, from birth, slightly longer necks than others." "In times of drought, they will reach leaves and live, while the shorter-necked ones die." "So those best fitted for the environment will transmit their characteristics to their offspring." "After many generations, tortoises on arid islands will have longer necks than those on well-watered ones." "And so one species will have given rise to another." "In these programmes, we're going to survey the unmeasurable variety of animals produced by natural selection, and look at them not as isolated oddities, but as elements in a long and continuing story that began 1,000 million years ago," "and is still continuing today." "Some creatures, the mammals, such as these sea lions and myself, are relatively recent arrivals on the scene." "Others: birds, reptiles, amphibians, fish, have been here much longer than we have." "In places where conditions have remained unchanged over immense periods of time, there are still creatures living which resemble very closely their early ancestors." "They can tell us a lot." "But to disentangle the story, we shall also have to look for evidence in the rocks." "The bodies of animals fall into the bottom of ancient seas and swamps, sometimes get entombed in the accumulating sediment." "When, after millions of years, those sediments turn to rock, those remains of animals and plants survive as fossils." "Since the discovery of radioactivity, scientists have developed techniques of measuring the age of rocks based on the rates at which some chemical elements decay." "So fossils can be dated to within a few million years." "But there are much simpler ways of establishing the ages of rocks that anyone can use, and there is no more dramatic place to do so than in the Grand Canyon in the American west." "The Colorado river, aided by wind and rain, has cut a gigantic section through the sandstones and limestones of Arizona." "The layers still lie largely undisturbed, so obviously the lower ones were deposited before the upper ones." "So if we want to trace the ancestry of life back to its beginnings here, we have to go deeper and deeper into the canyon." "This is the greatest gash that exists in the surface of the earth." "From the rim to the river at the bottom is a vertical mile." "There are a number of trails down." "The usual way is on the back of a mule." "Here, we're about 500 feet below the lip of the canyon." "Already the rocks are about 200 million years old." "There are no mammal fossils here, but there are some four-legged land animals." "Small reptiles: a little lizard-like creature that has left its tracks along here, which was once the face of a sand dune." "Farther down, there are no signs of any reptiles, but in limestones 400 million years old, the bones of strange armoured fish have been found." "The trail winds on through rocks formed on the bottom of ancient seas." "With every 20 feet we descend, we go back a further million years." "The Grand Canyon is really two canyons, one inside the other." "For a while, the trail flattens out as it approaches the rim of the inner canyon." "Here, I'm about two thirds of the way down, 3,500 feet below the rim." "The rocks here are about 500 million years old." "These rocks have no backboned animals in them at all, no fish." "The only creatures are those without backbones, including a whole lot of worms which have left this delicate tracery of trails in what was mud on the bottom of a shallow sea." "At last, the bottom and the Colorado river." "It's taken nearly a day, going fairly easily, to get this far." "We've ridden seven miles of trail and have descended a mile into the earth's crust." "The rocks here are getting on for 2,000 million years old." "For the past 700 or 800 feet of our descent, they've had no signs of any fossils in them." "For years, it was thought that all rocks of this age had no fossils." "Why was this?" "Was it because they were so unimaginably old that they'd had all life crushed from them?" "Or did life begin with creatures as big as a worm?" "For many years, this was a great puzzle." "And then, 20 or 30 years ago, people realised they'd been looking in the wrong rocks and in the wrong way." "These are the right rocks." "They're a kind of flint called churt, and they're on the shores of Lake Superior, in Canada, about 1,000 miles east and north of the Grand Canyon." "They were well-known during the last century because the pioneers used them in their flintlock guns." "And scientists have recognised for a long time that they were ancient rocks." "We now know they are about the same age as the rocks at the bottom of the Grand Canyon, about 2,000 million years old." "But these strange rings in them..." "For a long time, these were a subject of great controversy." "Some scientists maintained they were signs of very early life." "Others, that they were no more than the result of the ordinary chemical processes during the rocks' formation." "But in the 1950s, scientists started looking at them in the right way." "First of all, you have to cut a wafer-thin slice of the gunflint rock." "This is then ground down further for several hours until the slice is translucent." "When scientists first prepared churt to look at under the microscope, many doubted that primitive life forms, even if they existed 2,000 million years ago, could be preserved as tiny fossils." "And then scientists saw this." "Marks in rocks can be deceptive." "They may just be the result of mineral action." "But these filaments were almost identical to primitive algae growing today." "The search continued." "Soon, the fossilised remains of other kinds of primitive life were found that had once lived in those early seas." "Since those discoveries, other micro-fossils have been found elsewhere in rocks that are even more ancient, some over 3,000 million years old." "These immense periods of time baffle the imagination." "But perhaps we can get an idea of the relative lengths of the stages if we condense the history of life on earth into one year." "Then 10 million years become one day." "On that calendar, I'm talking in the last moment of December 31st, and primitive man will have appeared only a few hours ago, in the early afternoon." "The first backboned animal will have crawled up onto land during the last week of November, and these churts will have been formed on June 15th." "Now let's go back way, way, to the beginning of January." "To the beginning of life." "Over 3,500 million years ago, our planet was radically different in almost every way from the one we live on now." "Erupting volcanoes built up islands of lava and ash in the global seas." "The atmosphere was filled with gases such as ammonia, methane, hydrogen and steam." "There was virtually no oxygen." "In consequence, there was no ozone layer." "So ultraviolet rays in strengths that would be lethal to us bathed the young planet." "The ultraviolet light, together with heat, electrical and radioactive discharges, brought about chemical changes in the waters." "Complex carbon compounds were formed, including amino acids, the building blocks of protein." "For millions of years, the chemical soup thickened and changed." "Possibly some compounds were added to it from outer space." "Some carbon compounds aggregated in droplets, with a membrane through which other chemicals could pass." "Eventually, unusually large molecules appeared which had extraordinary characteristics." "They caused amino acids to form around them, and so built proteins." "They could also produce copies of themselves." "Such a molecule, known as DNA, is at the centre of every life cell." "Its shape is a double spiral, linked by chemical units of just four kinds." "Their arrangement acts as a code for the production of proteins, and a group of them in a section of DNA is called a gene." "On occasion, the DNA unzips." "Each half then attracts the correct chemical units and forms two new, identical molecules." "When this first happened, primitive cells formed new cells and life on earth had appeared." "But sometimes there is a mistake, a mutation." "These caused variations in the first cells, and natural selection sorted them out." "Those that were as well or better suited to their environment survived." "The rest died." "And so, over tens of millions of years, a variety of bacteria-like organisms developed, thrived and invaded new environments on earth." "Evolution had truly begun." "We can get a glimpse of what those first stirrings of life were like in the hot volcanic springs of such places as Yellowstone Park in Wyoming." "And in these springs, staining them a whole variety of colours, there flourish microorganisms." "Microorganisms that look to be almost identical to some of the earliest fossils we know." "Tufts of bacteria grow where the water is hottest." "In cooler areas, other bacteria deposit silica in strange-coloured crusts." "These bacteria represent the next big step." "For they are probably very like the first forms to manufacture food inside their own cell walls with the help of energy from the sun:" "light." "One of the raw materials they needed was hydrogen." "At first, they got it as sulphuretted hydrogen, which occurs in volcanic gases." "There's some around here." "This place smells a bit of rotten eggs." "And there are such bacteria flourishing in the hot water of these springs." "But then that link with volcanoes was broken." "Some forms of bacteria arose which got their hydrogen from a much more widespread and easily available source: water." "That was a crucial stage in the history of life." "Because if you take hydrogen from water, you are left, as a by-product, with oxygen." "These new blue-green bacteria, or cyanophytes, still exist." "As slime on wet rocks or in ponds covered with silver bubbles." "It was they that first contributed oxygen in large quantities to the atmosphere." "Under the microscope, you can see that they're very simple structures." "Some form chains, others are isolated beads." "On a larger scale, they form mats with bacteria in the cooler springs of Yellowstone." "Some of these blue-greens deposit lime as part of the chemistry of their body processes." "And in one place in the world, here in a bay on the coast of Western Australia, they grow large and huge to form these great pillars." "What makes this place so special is that the mouth of the bay is almost blocked by a bar of sand covered with sea grass." "This restricts the flow of the tide in and out, with the result that these waters are extremely salty." "So virtually none of the creatures which eat blue-greens can survive here." "So these blue-greens, these very primitive organisms, can grow uncropped, just as they did when they were the most advanced form of life 2,000 million years ago, at the beginning of life on earth." "And here is an explanation for those extraordinary shapes that we saw on the shores of Lake Superior." "This is as close as we may get to a scene of the world when life was beginning to stir." "Now, life had reached the point of no return." "The oxygen accumulated and formed a layer of ozone in the atmosphere, screening off ultraviolet rays, the very source of energy that had helped create the first life." "So it could never begin in the same way again." "Outwardly, things changed little for hundreds of millions of years." "But eventually, the stage was set for a new and dramatic step." "To find evidence of that development, you need go no further than your local pond." "Most microscopic organisms here are just single cells." "Yet each is much more complex than any bacteria." "Some, like this amoeba, seem to have animal characteristics." "And some appear to be simple plants." "Yet others seem to be half-animal and half-plant." "In terms of complexity, they are as different from a bacterium as man is from a jellyfish." "To see why, we have to look inside one with an electron microscope." "The DNA, unlike that in a bacterium, is enclosed in its own compartment." "Other parts of the cell resemble and act like blue-greens." "These look more like bacteria and are a source of energy." "This cell is driven by a tail that resembles yet another type of bacterium." "So it appears that this tiny creature is composed of a committee of smaller ones." "And many now believe that it was by some form of collaboration between primitive cells that such organisms came into existence." "But it took a long time for life to reach this stage, probably not until some 1,200 million years ago, say early September in our "life on earth" year." "These plant cells belong to this new, advanced type." "Many kinds of them still abound in fresh water and the sea, and they form the basic food of other simple organisms." "Some of them have delicate skeletons of silica." "This is another kind, with chambered shell of chalk, and so small that several would fit on the head of a pin." "The animals, in essence, are like an amoeba, to which they're closely related." "They catch their food with sticky threads." "When something tangles with them, it's drawn inside and digested in a special compartment." "The cells can reproduce by splitting into two, as bacteria do." "But some cells have more complicated methods of reproduction." "These have temporarily joined so they can exchange genes." "Later, they will part and then divide in the normal way." "In other cases, cells shuffle their genes and then divide to produce a very special kind of cell with only half the number of genes of the parent." "These special cells are eggs." "Meanwhile, other members of the same species are also producing sex cells with half-rations of shuffled genes." "This time, they're quite different in form." "They have tails." "They are sperm cells." "They're chemically attracted to the egg, and the first one to find it down there penetrates the wall." "After getting inside, it swims towards the nucleus and unites with it, so the full complement of genes is restored." "But now it's in a new combination, different from either parent." "When this mechanism developed, the extent and frequency of variation greatly increased." "As a result, the pace of evolution accelerated." "One of the most successful groups of single-celled creatures in this microscopic world are the ciliates." "They're covered in beating hairs, the cilia, which drive them through the water." "The cilia also create currents which waft particles of food into their gullets." "These particular ciliates are stalked and remain anchored to one spot." "But others are large and mobile and actively hunt for their food." "These ciliates are among the larger single-celled creatures, just visible to the naked eye." "Above this size, the chemical processes inside become difficult and inefficient." "But size can be achieved in a different way, by grouping cells together in an organised colony." "This volvox, almost the size of a pinhead, is composed of hundreds of cells, each with a tail, but all beating in a co-ordinated way." "Inside, daughter colonies are formed and the tiny, delicate globe ruptures to release them." "Eventually, this co-ordination between cells was taken a stage further." "Sponges appeared." "There are about 5,000 species of sponges in existence today, and in all of them, the colonial bonds between their constituent cells are remarkably loose." "Individual cells may crawl around over the surface like amoebae." "If a sponge is forced through a sieve so that it's broken down into separate cells, they will, if left alone, reorganise themselves to form a new sponge." "What is more, each kind of cell will take up its proper place." "Some are specialised to form the walls." "Others are pump cells that line the walls of the channels with which the sponge is riddled." "By beating their tiny threads, they create currents, drawing in water through the pores on the sides then pumping it out at the top after the food has been strained off." "The structure is supported by yet other cells which make tiny needles, and these build to form a skeleton." "In the so-called glass sponges, they're made of silica." "Modern science is only some 200 or 300 years old, and yet already it's provided us with some profound insights into the workings of our world." "But there's still a great deal we don't know." "Take this sponge skeleton, for example." "How on earth did the microscopic sponge cells, one of the most primitive organisms we know, collaborate to build out of a million splinters of silica this complex and beautiful structure which is sometimes called Venus's flower basket?" "Some religious people will maintain that it is the work of God, and that is all that need be said." "Some scientists say it is only a matter of time before we will provide a more detailed explanation than that." "Either way, it remains an awesome and beautiful object." "But sponges, in an evolutionary sense, are a dead end." "They have no true mouth, no gut, no muscles, no nervous system." "But this has." "It's a jelly-like creature with just two layers of cells." "The inner one lines a cavity which has a single opening." "Its design may be simple, but it is a fully coordinated, multi-celled animal." "It's one of several kinds of comb jellies, which swarm in the oceans but which are so transparent, they are hardly ever noticed." "To appreciate the full beauty of comb jellies, you need special lighting." "They swim with rows of cilia arranged like combs, and their beating produces interference colours, like a rainbow." "This pulsating bell is a close relation of the comb jelly." "Technically, it's called a medusa, after the unfortunate lady in the Greek myth who had snakes on her head for hair." "Its tentacles have stings for capturing prey." "Once caught, it's transferred to the mouth at the centre." "Comb jellies and medusae both have muscle fibres and a simple nervous system." "But most medusae have a surprise." "They begin their lives in a completely different form, like this." "These may look like plants, but they're animals." "Each structure began when a tiny free-swimming creature developed from the fertilised egg of a medusa and settled on the sea bed or some weed." "From it sprang a tiny branching twig bearing flower-like individuals called polyps." "These filter feed with the aid of beating cilia and grow, putting out more branches with polyps on them." "Each polyp is basically equivalent to a swimming medusa." "In some species, medusae can bud directly off the branch and swim away." "In others, they are born from special vessels." "All these medusae, not much bigger than a pinhead, have been produced by a process that involves no sex." "Eventually, they develop sexual cells which will be released into the sea to produce larvae to begin new colonies of polyps again." "This alternation of generations between sexual and non-sexual methods of reproduction has given these creatures and their relatives great scope for variety." "Larger medusae carry quantities of jelly in their umbrellas to make them more robust in rough seas." "These are the true jellyfish, and many lead the same type of double life, having a stationary polyp phase as well as a swimming one." "There's a surprising variety of types of jellyfish." "Some are able to feed on quite large prey." "This one has ruffles in which there are many pores for netting microscopic food." "This shallow-water species uses pulsating movements to create currents of water that bring it food." "It's an obvious deduction that such simple things as jellyfish appeared very early in the development of life." "But for a long time, there was no actual proof that they did." "After all, proof could only come from fossils, and who could suppose that an insubstantial jellyfish could be fossilised, let alone survive in rocks from the earliest period?" "And then, about 30 years ago, in these sandstones in the Flinders Ranges in southern Australia, which are probably about 650 million years old, people found things like this." "At first, many scientists refused to believe that these faint impressions were the remains of jellyfish." "But by now, enough specimens have been discovered to make quite sure that that indeed is what they are." "What's more, almost a dozen different species have now been discovered." "Such fossils as these reveal that at a very early period, jellyfish existed in many different forms, just as they do today." "This, though a close relative of the jellyfish, is, strictly speaking, not one creature but a colony of polyps, one that has gone to sea and assumed much the same structure as a true jellyfish." "Another colony that is built on the same principle is the Portuguese man-of-war." "It has no swimming bell but a bag filled with gas that supports the whole colony." "To avoid drying out, the colony can dip the sail into the water from time to time." "Long tentacles trail behind for lengths of up to 50 metres." "The colony begins with just one founding member which buds off two lines of other individuals." "They in turn bud off others, some specialised for feeding, some for reproduction, and some to catch prey." "As with all jellyfish and their relatives, the tentacles have special stinging cells." "Each contains a coiled, barbed tube which discharges on contact with its prey." "And from the end of each comes a drop of paralysing poison." "Animals like the Portuguese man-of-war are highly complicated, and you might think they're recent developments in the world of jellyfish." "In fact, one of the fossils from the Flinders Range suggests that such colonies existed 650 million years ago." "The impression in this rock is thought to be from a gas bag of such a colony of polyps, which was blown inshore and cast up on the sandy beaches that today form the sandstones of the Flinders Ranges." "And that's not all." "Alongside those jellyfish, in the same rocks, there are the remains of other closely-related creatures." "These beautiful impressions are of animals in which the equivalent of the medusa remained very small and attached to one another to form a colony." "And we can be pretty sure that this is what that was because similar creatures are alive today and living only about 40 miles away from here in the sea." "These are sea pens." "On either side of the stem are polyps which are specialised for feeding and reproduction." "This living one bears a remarkable resemblance to the fossil." "They were given the name sea pen when people wrote with quills, and apt it must have seemed, for the skeleton is flexible and horny." "They belong to a group called the soft corals." "This is another kind, a soft, flabby organism rather ghoulishly known as dead man's fingers." "Soft corals of one kind or another can grow in depths of up to 6,000 metres, but stony corals, the ones which produced limestone skeletons and form reefs, can live no deeper than 40 metres." "The coral polyps live only on the surface of these structures, each in its tiny limestone cell and connected to its neighbours by thin strands, so that the whole skin is a living network." "As new ones sprout from the connecting branches, they secrete cells which grow over the early ones and stifle them." "The coral tissues contain plants." "Tiny single-celled green algae." "Like all plants, they release oxygen, which helps the coral polyps to respire." "They also assimilate carbon dioxide, taking it from the water." "And that helps the corals to form their gigantic skeletons of lime." "Each species branches and buds in a different way." "And so the colony produces its own individual shape." "The reef may look like some fantastic multicoloured jungle of plants and flowers, but when you touch one, it has the hard, incongruous scratch of stone." "The coral organisms are tiny and simple, yet they grow on such a scale, and their stony skeletons are so durable, that they may well have been the first signs of life that could be detected from outer space." "Certainly, this Great Barrier Reef can be seen from the moon." "So it may well be that if a passing astronaut came this way several hundred million years ago, he might have noticed in the deep blue seas of the earth a few mysterious, beautiful shapes in turquoise, and guessed that life on earth had really started."