Extra oxygen explains why Stan Wood, a sharp-eyed commercial fossil-hunter from Scotland, did so well out of a dilapidated old limestone farm wall that he spotted next to a school football field in 1984.
He thought the wall might contain some interesting fossils, so he bought it from some developers who were about to knock it down – for £25.
The fossils Wood found inside were so important that he ended up selling them for more than £50,000. He spent some of the money buying the disused old quarry in East Kirkton where the limestone in the wall came from. After bringing in some heavy digging machinery, he made some even more amazing discoveries. He found the fossil of a giant, air-breathing scorpion at least 30cm long, with a vicious-looking barbed tail and a giant protective outer skeleton. This huge creature, a Eurypterid, probably grew more than 2m long, bigger than most humans.
The higher concentrations of oxygen in the air meant many creatures could grow much bigger than their living descendants can today because energy-rich oxygen could diffuse further into an organism's breathing system. Stan Wood's scorpion, estimated at about 335 million years old, shows how this ancient beast had mastered two of the essential challenges for creatures that came ashore. It used primitive types of lungs for breathing air. These were adapted from its gills, and were protected by pockets of hard outer skin. The giant scorpion also had pairs of legs, so it could walk on land.
Some of the world's first insects also belong to this period. Dragonflies were most spectacular of all. How they learned to fly is still a mystery. But it probably had something to do with the arrival of plants and trees. Wouldn't it make sense for an insect to just jump or glide from one tree to another, rather than climb all the way down and then up again? Something like this is what led the dragonfly to develop its wings. They grew out of the same kind of pockets of hard outer skin as those found in the giant scorpion.
Perhaps to begin with they used these small flaps just for jumping, maybe adding a little extra distance to make a big first leap. Gradually the flaps grew larger, until such acrobatics as gliding, diving and finally flapping became possible. Of course, flapping is an extremely energetic thing to do. Happily, in the oxygen-rich atmosphere of those days these first flyers were immersed in just the right stuff for trying something new and tiring. The extra oxygen also made the air thicker, so it was easier for the dragonflies to get lift-off.
Extra oxygen also helped them grow big. These colourful prehistoric flies were as large as today's seagulls. They leaped, jumped and flew from tree to tree, totally unchallenged. They had complete command of the skies, feeding off other insects as and when they liked. They had no rival.
Smaller insects eventually developed an ingenious design to protect themselves. They evolved defences in the shape of sophisticated folding wings, just like those we see in house flies today. Folding wings allowed smaller insects to crawl into narrow spaces where the larger, fixed-winged predators, the dragonflies, could not go. Flying ( neopterous) insects are by far the largest group alive today which means that the folding wing probably counts as one of nature's most successful inventions.
Another important requirement for land-based creatures is the ability to see. Dragonflies developed highly sophisticated compound eyes with 30,000 facets, each one a tiny eye, neatly arranged to give nearly 360-degree vision.
Dragonfly fossils have been found in many parts of the world, but the most spectacular came from the small mining town of Bolsover, in Derbyshire, where a giant 300-million-year-old dragonfly fossil was discovered by two coalminers. With a 20cm wingspan, this is the oldest known dragonfly fossil, and far bigger than any dragonfly alive today. For a few days dragonfly fever gripped Britain, the newspapers had a field day and the legend of the "Beast of Bolsover" was born.
Life goes on: the role of worms and beetles
The first ever land animal was probably a relation of the velvet worm. It wriggled out of the sea, feeding off the earliest plants and mosses that were clinging to the shore. Descendants of this creature, the common ancestor of the arthropod family, went on to develop legs, becoming the first millipedes and centipedes. Once on the land, these early arthropods gradually evolved into a wide variety of insects, combining the first few segments of their worm-like bodies to form a head, and adapting at least one pair of legs into feelers. Over time, other segments merged to form the thorax (upper body) and abdomen (lower body and tail).
One of the most significant insects to emerge was the beetle. Today there are probably more species of beetle than of any other living creature. Over 350,000 different types have been discovered so far, which is about 40 per cent of all known insect species, but experts believe there may be between five and eight million types in all.
Beetles bring us back to the final engineering challenge faced by the world's first sexually active trees, the Cycads. These were the insects that came to their assistance. As they rummaged in the undergrowth and up into the leaves of the trees, they transferred yellow pollen powder from the male parts of one Cycad tree on to the female parts of another, so fertilising the trees' genes to produce a new crop of seeds.
Soil power: how life covered the earth in a nutrient-rich blanket
Beetles, other insects, worms and fungi are jointly responsible for attending to the land's most precious sustaining life force of all: the soil. Like constant gardeners, they recycle organic matter – fallen leaves and rotting trees – into nutrients that fertilise the soil for tomorrow's plants and trees. Without living things, there would be no soil. The Earth would be nothing more than dust and rock, like the surface of the Moon, Mars or Venus. Some of the rock might weather and dissolve in the rain to be washed back into the sea in the form of mud and silt, but the crumbly black-brown stuff that makes vegetable gardens grow would never have formed were it not for life on Earth. Over the course of millions of years all the soil on Earth is renewed and regenerated. This is called the soil cycle.
There is nothing now left of the soil from the Carboniferous Period. The oldest soil today is just a few million years old. Wind, water, ice and the movement of the tectonic plates means that soil, like rock and salt, is always being churned up or washed away. Soil, which is made up of weathered rock, minerals and organic matter appeared first when plants and trees started to grow on the land in large numbers during the Carboniferous Period. Plants established themselves in cracks between rocks that had been pummelled by centuries of rain and weather. Their developing roots broke down the rock further.
Since plants were a rich source of food, they attracted fungi, worms and other tiny arthropods such as mites that live off organic matter. For the last 400 million years these creatures have been digging up the earth and turning it over, exposing it to the air and rain with their burrowing, allowing the weather and the elements to break up the soil so that it's always ready for new life to take seed.
With plentiful supplies of food in the form of plants and trees, more oxygen than ever, a cooling climate and a landscape ideal for providing shelter (either in the branches of trees or in soil which could be burrowed into), the scene was now set for life's next major episode. What would the descendants of those backboned, four-finned creatures such as the lungfish make of this rich, earthy paradise now?
The great floral mystery: when and where did the world's flowers first appear?
Charles Darwin wrote in 1879 in a letter to a friend, the botanist Joseph Hooker, that he could not understand the sudden appearance of flowering plants in the fossil record. Where on Earth did they spring from? "The rapid development of all the higher plants in recent geological times is an abominable mystery... I should like to see this whole problem solved."
To this day, no one has really come up with a decent explanation. Unlike some of those wilder theories about life's ingredients arriving on Earth on a meteorite from outer space, there is no question of the same being true for flowers. Yet about 130 million years ago the world's first flower fossils suddenly start to show up.
Some experts think flowers arose as long as 250 million years ago, but fossils this old have never been found. Others think that several evolutionary phases occurred in quick succession, accounting for flowers' sudden appearance in the rocks. The fact remains that flowers appeared for sure only about 130 million years ago and as yet there is no clear evidence that they lived much before then.
Flowering plants and trees made a massive impact on life on Earth. Without them, life today would be very different indeed. More than 75 per cent of all the food humans eat (directly or indirectly) comes from flowering trees and plants. No longer was the Earth dominated by endless streaks of browns, greens and blues. For the first time there were blooms of red, yellow, orange, purple and pink.
A flower is a powerful technology used by many plants and trees to reproduce so that they can spread all over the world. Evolution must have been at its magical best when the first flowers evolved, because the designs it came up with to aid fertilisation and spread seeds are among the most spectacular of all.
They did it by using that tried and tested strategy that the older trees knew best: they put all their effort into making friends. Flower power helped plants and trees recruit armies of other creatures to help them spread to all corners of the Earth. It maybe no accident that flying insects such as bees, moths and butterflies first appeared alongside the Earth's first spring of flowers.
Flowers and pollinating insects such as bees evolved together – a process called co-evolution. Flowers needed bees as much as bees needed flowers. Each developed ways of helping the other survive better because they both stood to make gains from mutual cooperation – one providing food, the other a means of transport. With the help of a pollinator such as a beetle or bee, genes from male and female flowers could mix to produce new seeds with their own unique genetic code. Flowers developed a huge range of incentives to get animals on land and in the air to carry their pollen and seeds to other places.
Fruit is the female part of a flower, the ovary, which, once it has been fertilised, changes its shape and form to help disperse seeds. Sometimes these seeds travel on the wind, sometimes by water, or sometimes by sticking to an animal's fur. So the downy white parachute of a dandelion seed is a fruit. And so is the acorn from an oak tree, or a prickly burr. The most ingenious transportation method of all is by burying seeds in a ready-made meal. A passing animal might help itself to the morsel, digest it for a day or two, and then obligingly spread the indigestible seeds as it moves along by scattering them in its dung, giving them an additional growth shot in the shape of a godparent's gift of manure. Seeds are built to be tough. They can survive the most upset, unpleasant of stomachs.
Not all fruits rely on being eaten. Some trees, like the coconut palm, developed other strategies, such as floating their large seeds over thousands of miles of water from one coast to another. Or there's the curious sandbox tree, whose fruit explodes like a firework, scattering its seeds up to a hundred metres away. Cotton fruits produce fibres that stick to animal skins as a way of spreading their seeds. Nuts are edible seeds designed to be carried off by animals which hoard them for the winter. Almost always they leave some uneaten, allowing these seeds to grow into new plants in a new place.
Before fruit there was the wind – nature's most traditional method of spreading pollen and dispersing seeds and spores. The wind is still a favourite among many flowers, especially grasses (e.g. wheat and barley). Dandelions have tiny parachutes to catch the breeze, and the helicopter-like wings of the sycamore seed work well too. Flowers that use the wind for pollination as well as seed dispersal have no need to attract animals or insects, so they don't bother with big, showy flowers. They save energy by keeping their petals small and discreet.
An important new group of plants evolved during the Cretaceous Period (145 million-65.5 million years ago) called Monocots. Unlike most other plants (called Dicots), they came up with the ingenious trick of growing back to front. Instead of new growth being added to the tips of the leaves – a Conifer's greenest, youngest shoots are always at the ends of each branch – a Monocot's leaves grow upwards from a central, often submerged, bud. The new design was an instant success, because it meant that if a plant's leaves got nibbled by a passing dinosaur it didn't lose its most recent growth, because this was safely tucked away at the bottom. Grasses are Monocots that use this design to recover quickly after being grazed by animals. In fact, many grasses like to be grazed. It strengthens their stems, but doesn't damage their potential for new growth, since the growth bud (called the apical bud) is always kept beneath the ground, out of harm's way.
So successful was this design that grasslands have come to cover as much of the Earth's surface as all the other plant and tree species combined. What's more, some time during the Cretaceous Period a completely new type of tree had evolved. Unlike the ancient Cycads and Conifers, Monocot palm trees grow from a bud at the top of a thick, scaled trunk. There are more than 2,600 types of palm trees alive today.
The most ancient palm tree fossils – from the nipa palm – date from around 112 million years ago. These are rather special, because their trunks and roots are sunk in marshy swamps or riverbanks. Their way of spreading themselves takes some beating: these are trees that swim. They tie themselves together by the roots and, using the force of the tides and water, break loose into floating islands that can carry cargo in the form of small groups of animals, which use them as rafts to float from one place to another.
Although today an enormous amount of research has gone into studying the fossil record and the genetic ancestry of modern plants and trees, there are many pieces that are still missing in a puzzle that was, and remains, Darwin's "abominable mystery".