The coming of sex: how life exploded into a variety of new organisms

Life on Earth consisted of only two kinds of living things until about a billion years ago: the original, simple single-celled bacteria that produced methane and oxygen as waste products, and the newer, more complex multi-cellular organisms that fed off the increasingly abundant supplies of oxygen in the air. For billions of years these tiny organisms were all the life there was. Then something triggered a spectacular and dramatic increase in the pace of evolution. About 1 billion years ago living things developed a radical new form of duplication called sexual reproduction. It revolutionised the way life evolved.

Sexual reproduction helped life shoot forward in complexity, equipping it to survive the challenging conditions on the planet in far fewer generations than it would have taken without it. While it took 2.5 billion years for life to evolve into types of microscopic organism, it took less than half that time for life to transform completely into everything we know today – from fish, amphibians and reptiles to plants, trees, birds, mammals and man.

One of the first men who worked all this out was Gregor Mendel, a monk, born in 1822, who lived in what is now the Czech Republic. He spent most of his life absorbed in the natural world; between 1856 and 1863 he studied more than 28,000 different pea plants. What intrigued him was that when these slightly different plants had seedlings, their differences (or characteristics) would often be carried forward to the next generation. The seedlings had inherited features from their parent plants.

The concept of inheritance was first described by Mendel in 1865. He went on to devise a number of laws that could predict how living characteristics are passed from one generation to another through sexual reproduction, massively increasing the variety of living things.

We are now on the edge of an enormous transformation. The time is just past 21:00 on our 24-hour trek across Earth time. The rest of history will be played out in the final three hours. There's still no life on land, no plants, no trees, no flowers, no insects, no birds or animals, let alone humans. The Earth is very old, but human history is not. Compared to the age of the Earth, everything else we will discover is either young, very young, or just hatching out. Mankind is among the youngest of all.

But the significance of what happened about 550 million years ago is that it is from here on that we begin to get a full, clear picture of what life on Earth was actually like...

Finding fossils: how Victorian detective work changed our view of prehistoric life on earth

Fossils are the impressions of long lost creatures that had hard surfaces, such as bones, shells or teeth which sometimes leave imprints in the rocks. They are wonderful for helping investigators identify what kinds of creatures have lived on the Earth. When the fossil record begins it is rather like a theatre curtain being pulled back to reveal a stage bursting with actors in the middle of a play. Yet it was only relatively recently that we learnt how to read this record.

Charles Doolittle Walcott was born near New York in 1850. As a young boy he found school rather boring. It wasn't that he had no interest in things, rather the opposite. He was so curious that he just wanted to get outside and explore the world for himself – in particular he liked to look for minerals, rocks, birds' eggs and fossils.

By 1909 Walcott had become a well-established fossil collector. One day a freak accident changed the rest of his life. While he was walking high up in the Canadian Rockies his mule slipped and lost a shoe. In the process it turned over a glistening rock of black shale, a type of rock made out of compacted mud and clay. When Walcott stopped to pick up the rock he saw a row of remarkable, flattened, silvery fossils. These were perfectly preserved creatures from the Cambrian Period.

It turned out that the mountainside had collapsed about 505 million years ago, smothering these creatures, killing them instantly and burying them like a time capsule for posterity. Walcott's discovery became one of the richest hoards of fossils ever. It is known as the Burgess Shale (named after Mount Burgess, near to the site where Walcott found the fossils). Walcott returned to the site many times afterwards, and eventually wrote a library shelf of books about his finds. And what a bizarre range of creatures they were.

First, there is the strange-looking Anomalocaris. This was one of the biggest sea-hunters of its day and could grow up to a metre long. It used a pair of grasping arms to capture and hold struggling prey. For a long time, the fossils that make up this extraordinary creature were thought to be three separate living things. The body was identified as a sponge, the grasping arms as shrimps, and the circular mouth as a primitive jellyfish.

Another was the remarkable Hallucigenia. This curious worm-like beast also kept fossil-hunters and scientists scratching their heads. It was thought to have walked on stilt-like legs and to have had a row of soft tentacles on its back which it used for trapping passing food. Thanks to the discovery of similar fossils in other parts of the world (particularly in China), fossil investigators now think they've been looking at it upside down. Instead, it walked on paired tentacle-like legs and used the spines on its back as a form of body armour to protect itself from being eaten.

But nothing in the wildest imagination of science-fiction writers could conceive of such a beast as Opabinia. This swimming gem had five stalky eyes, a fantail for swimming, and a long, grasping arm for feeding. It was smaller than other predators, being only about 4cm long, and there's nothing remotely like it alive today.

One of the most common forms of animal life at that time, and the most common of all the fossils unearthed in the Burgess Shale, were Trilobites. Their fossilised remains look like giant woodlice and have been found all over the world. Trilobites were probably the first creatures ever to be able to see. They had eyes, like those of today's flies, which divide into hundreds of different cells, giving them a kind of mosaic view of the world under the seas.

Natural selection: how a few species evolved into many species

To create a realistic picture of how life on Earth actually evolved, it is important to know when each species lived and died so they can be pieced together in chronological sequence. The genius of one man helped work out how to do this. Charles Darwin (1809–82) worked out that all living things have evolved according to a sequence that is still unfolding today.

Darwin's book, On the Origin of Species (published 1859) explained, for the first time, the theory that all living things originally evolved from a single common ancestor. Darwin worked this out because the fossil record frequently shows new creatures emerging and others disappearing. His conclusion was that all living things are related to each other, but that only those species best suited to the environment of the day survived. His theory gave scientists the first ever way of arranging fossils in different groups and eventually into a rough chronological order.

The upshot of Darwin's theory was the inevitable conclusion that even humans must be descended from simpler forms of life, like apes, and before that from mice, reptiles, fish, and ultimately from those bacteria found at the beginning of life on Earth.

Fossil records of now-extinct species became the key for Darwin's understanding of the evolutionary process. By studying fossils and comparing them with living things today, he could see that each species has adapted itself according to a principle that he called "natural selection". Over successive generations those creatures best equipped to live life on Earth at that time had survived, flourished and become dominant, while those least well equipped had died off, their species eventually becoming extinct.

Many people were outraged at the implications of Darwin's theory when it was first published. Some still are. The suggestion that humans were descended from animals – more specifically apes – threatened the widely held view that mankind was somehow different, superior, to all other living things. Equally as implausible to many was the idea that humans are just another natural species which, like all others, was destined one day to become extinct.

Today, scientists have discovered powerful new ways of dating rocks and fossils which back up Darwin's theories of how creatures have evolved over time. They have been able to construct an accurate picture of how life on Earth has changed since fossils began to appear in rocks about 550 million years ago. They have also made a map of the past called the geologic column which is divided into a number of chronological eras and periods.