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The Age of Plants and Trees

(420 million–65.5 million years ago)

How seaweed eventually evolved into tall trees and forests, and how the ground was covered in a blanket of nutrient-rich soil

FOR MILLIONS OF years heavy rain fell on the earth’s barren land masses, wearing down the ground into a lifeless muddy silt. High levels of carbon dioxide in the atmosphere at this time meant the rain was acidic, increasing rock erosion and weathering. The first plants were just squidgy things that looked like small seaweeds and green mosses. They were descendants of the ancient blue-green algae – the oxygen-producing cyanobacteria – and they clung close to beaches, rivers and streams.

The transformation of these small, soggy clumps of moss into tall, graceful trees that can live thousands of miles from the water’s edge demonstrates some of nature’s most spectacular feats of biological engineering.

Think how hard it would be to design a tall tree that could flourish in the wild. For a start, there is the business of staying upright. Ideally, a forty-metre-high tree should be able to withstand a force-ten hurricane without toppling over. Next, a steady supply of water and nutrients is needed to sustain the whole tree. The bits of the tree that make food – the leaves at the top – must be as near to the sun as possible, which in a thick, dark forest means being tall enough to make sure that all the other trees don’t block out the sunlight. But being tall means being further away from the main source of water, which is stored somewhere in the ground. Finally, if the tree’s family is to flourish in future generations, it must be able to reproduce successfully. That means just dropping seeds willy-nilly on the ground below simply won’t do, because young trees can’t thrive if they have to compete with their parents for sun, food and water. Seeds have to be spread further afield. How is this to be done when trees can’t walk or swim?

Designing a tree is no simple thing, which is probably why the earliest plants – mosses, liverworts and hornworts (the family is called the bryophytes) – stayed exactly where they liked it best: near to the water’s edge, thriving only in inlets and bays in river mouths and beside streams. They completely ducked the idea of being tall; their strategy was to hide from the wind by staying small.

These plants had no proper roots, leaves or internal plumbing system to deliver water and nutrients. But if trees and plants were to colonize the vast tracts of barren land a half-hearted attempt at escaping from the sea like this was no long-term solution. About 420 million years ago, the first signs of a new approach began to emerge in the form of vascular plants. Ultimately, all the world’s trees and forests are derived from them.

The wonders of Rhynie chert

The first vascular plants weren’t anything spectacular to look at. They comprised smallish shoots, only about fifty centimetres high, with thick stems and firm spiny leaves. We know about these plants thanks to a bizarre discovery in a Scottish village called Rhynie, about forty kilometres north-east of Aberdeen. In 1912 local doctor and amateur geologist William Mackie made an extraordinary find when he was exploring a piece of ground: he discovered perfectly preserved species of plants fossilized in the rocks.

About 400 million years ago Rhynie was like a steaming cauldron with boiling-hot pools of bubbling mud. Every so often a giant geyser would spout a huge fountain of scorching water filled with silicon from deep inside the earth. Silicon is one of the elements that form sand and rock. When this silicate water landed on nearby vegetation and plants it didn’t just kill them instantly; when it cooled, it petrified them into perfect fossils of stone.

The fossils of Rhynie are so well preserved that scientists can see exactly what the plants were made of and how they worked. These vascular plants had evolved a chemical called lignin, which toughens the walls of plant cells. Plants that have little or no lignin stay small and floppy – like herbs or garden flowers. Although the stems of plants like these can feel rigid, they are held up only by the force of the water within them. If the water supply dwindles, the herb or flower wilts.

Plants with lignin in them can stay upright even when a drought sets in. With great precision, lignin-toughened cells are stacked and interwoven in carefully constructed layers to make wood – the magical stuff of trees. Lignin also provides tubes through which minerals and water are transported up and around the tree.

The first evidence of lignin came from plants called rhyniophytes (named after their place of discovery in Scotland). These are now extinct, but their descendants are all round us – indeed, everything woody ultimately comes from these early pioneers of the land. Mind you, it took a while for these small plants with toughened stems to become tall, graceful trees. At least forty million years.

By the time we get to the Carboniferous Period (360 million years ago), trees were growing in huge numbers. The earliest, called lycophytes, were simple structures. They had roots and branches which divided into a ‘Y’ shape. But they could also be very big, with some specimens, such as the lepidodendron tree, as wide as two metres and as high as a twelve-storey building.

Life in the world’s first forests

Except for the wind, and maybe a scratching sound inside a hollow log or a faint buzzing in the branches, it was eerily quiet in this prehistoric world. There were few animals, and no birds – it was still far too soon for them. And the landscape looked pretty much the same in all directions – an endless thick, dark greenish-brown, a blur of identical-looking trees. Very few varieties existed at this time. There were also no flowers. The earth would have to wait at least another 150 million years before it could witness a first bloom. Compared to trees, flowers are a modern fad.

The lycophyte trees that dominated ancient forests lacked one ingredient that ultimately led to their graceful decline into extinction some 270 million years ago. They lacked true leaves. They mostly used scales on their trunks and thin green blades on their branches for photosynthesis instead. It was left to a relative of the vascular plants found at Rhynie to come up with the concept of creating little green solar panels attached to the tips of branches. These were the euphyllophytes – literally ‘good leaf plants’. Most trees alive today descend from them. Euphyllophytes quickly grew into several varieties, including ferns and horsetails.

If it weren’t for lycophytes, ferns and horsetails, our modern lives would be very different indeed. These early trees colonized the land in their millions. When they died, most of them sank into swampy marshes, where over millions of years they were compacted, hardened, chemically altered and metamorphosed by heat and pressure, ultimately becoming coal. This source of chemical energy eventually fuelled the Industrial Revolution (see page 260).

While lignin helped trees become strong and leaves trapped the energy of the sun to make food, trees still faced the difficulty of finding a steady and reliable supply of water that somehow had to be channelled all the way to the top of their canopies – water that was often dispersed many metres below the ground. Trees rose to that challenge in two ways. The first relied on cultivating a good crop of friends to help. The second was down to ingenious design.

Tree roots grow downwards to find water. But to help them, they often enlist the support of another group of highly versatile living things. Neither a plant nor an animal, although for a long time they were grouped with plants, fungi form their own separate, almost invisible underground kingdom.

How fungi helped plant life to thrive on land

They came on to land from the sea because their tiny light spores were so easily blown by the wind. They arrived at about the same time as the earliest plants started to grow on the shores. Since then fungi have developed into a huge variety of life forms, ranging from the smallest to the largest living things on earth. Small fungi are just one cell big. Yeast is an example, used in cooking all over the world to make bread. It grows by using a process called fermentation, which converts sugar into alcohol and carbon dioxide. Most fungi live underground. They have elaborate networks of hairs, called hyphae, that gather together in clumps called mycelia. A mushroom or toadstool, which most people think of as a fungus, is simply the fruit of the mycelium, which occasionally pops up above the ground to spread spores so that it can reproduce.

Fungi can have massive mycelia. In fact, the largest living thing on earth today is a fungus. Found recently in the American state of Michigan, this hairy beast stretches underground for over five kilometres, and is estimated to weigh more than ten tonnes. It is also one of the earth’s longest survivors, having lived for well over 1,500 years.

Fungi are the world’s ecological dustmen. They process and digest dead and decaying matter, from the leaves on the ground to the dead skin in between your toes. When human dustmen take away rubbish it is often just burned or thrown into a pit in the ground. Nature’s dustmen, the fungi, not only rot away the dead rubbish of life, but turn it into materials rich in nutrients that fertilize plants and trees to help them grow. Fungi are vital links in the earth’s ongoing cycle of renewal – of life and death.

As so often in nature, different groups of living things team up to mutual benefit. The fungus passes on some of the nutrients and water it gathers to the tree and in return the tree feeds the fungus with sugars produced by its leaves. In this way the tree’s capacity for gathering water and nutrients is dramatically increased, and the fungus gets fed. Sometimes a single fungus lives underground and attaches itself to many trees – so in this way the trees are actually connected together, linked up in a chain as if poised for a medieval dance. This relationship is called mycorrhiza. It has been estimated that 80 per cent of all flowering plants today have some sort of mutually beneficial relationship with underground fungi.

Even with its roots in water, a tree still needs to transport it up to the all-important leaves which manufacture food – and that’s a long way in the case of some trees. For a long time no one could work out how trees did this. Of course, they have no moving parts like heart pumps to do the job for them. It was once thought that external air pressure caused water to flow upwards, as it does when you suck a straw, but air pressure simply isn’t strong enough to pump water up a one-hundred-foot tree.

The answer is down to the ingenious design of tree leaves, which contain millions of tiny perforations, or holes, called stomata. Trees open or close these pores depending on the weather and conditions of the day. When it is hot, water in the leaves evaporates through the stomata, making the sap in the tree trunk more concentrated. This drags underground water up through the trunk and into the leaves at the top of the tree. The name of this process is transpiration.

The trees’ final challenge is to find a way of spreading their offspring, even though they cannot walk or move. The earliest species used the wind to spread spores in much the same way as fungi. The problem is that spores need exactly the right conditions to germinate – usually they must land in wet places, such as marshes or bogs. In dry climates this is a big problem. Then, about 360 million years ago, trees came up with a much better solution. Seeds.

Unlike spores, seeds contain a partly formed tree embryo as well as a substantial food store of sugar, protein and fats. The embryo, with its larder stocked with food, is then encased in a coat (testa), ready for a sometimes epic journey, using one of a number of alternative transport systems.

Seeds dramatically increase a tree’s chances of successful reproduction. They are tougher than spores; they can survive droughts; they take their own food rations with them and some can even float. The first seed-bearing trees were the cycads, which have been traced back to about 270 million years ago – about 40 million years before the first dinosaurs appeared. About 130 species of cycads are still living today, although many are under threat of extinction owing to the destruction of their habitats. These trees also mastered the art of sexual reproduction, which usually requires the genes of two different trees to combine to make a seed. But that leaves a final, apparently insuperable challenge: how could two different parents mix their genes when they are both, literally, rooted to the ground?

How rising levels of oxygen transformed life on land

The solution came from other forms of life that had emerged on to the land. A few small worm-like creatures probably emerged from the sea at about the same time as the earliest plants and mosses, about 420 million years ago. What tempted them ashore had something to do with the rising quantities of oxygen in the air. Oxygen levels had steadily increased over millions of years until they levelled off at the beginning of the Cambrian Period, about 530 million years ago. Then there was a blip between four and two hundred million years ago.

This blip was caused by the luscious green forests now covering the land. Plants and trees dramatically increased the amount of oxygen in the atmosphere. The effect was a bit like lining the shores with sweets. Life in the seas just couldn’t resist the temptation to come ashore for a taste. When the first sea creatures came crawling out they found that the adjustment wasn’t too hard, with all that extra oxygen in the air to give them a boost. Today about 21 per cent of the air we breathe is oxygen. But 350 million years ago, with the arrival of the carboniferous forests, oxygen levels shot up to perhaps as much as 35 per cent.

An age of giants

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 twenty-five pounds.

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 quarry in East Kirkton where the limestone in the wall had come 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 thirty centimetres long, with a vicious-looking barbed tail and a protective outer skeleton. This huge creature, an eurypterid, probably grew more than two metres 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 lungs for breathing air. These were adapted from its gills and protected by pockets of hard outer skin. The giant scorpion also had pairs of legs, so it could walk on land.

The Beast of Bolsover

Some of the world’s first insects also belong to this period. Dragonflies were the 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 to add a little extra distance to a big 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 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 rivals.

Smaller insects eventually developed an ingenious design to protect themselves. They evolved sophisticated folding wings, just like those we see in houseflies today. Folding wings allowed the 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 ever 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, or all-round, 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, England, where a giant 300-million-year-old dragonfly fossil was discovered by two coalminers. With a twenty-centimetre 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.

Worms emerge and insects evolve

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 fertilizing the trees’ genes to produce a new crop of seeds.

A blanket of nutrient-rich soil

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 fertilize 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 mean 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?

Flower power

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 via 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.

The flower is a powerful technology used by many plants and trees to reproduce and spread. Evolution must have been at its magical best when the first flowers evolved, because the designs it came up with to aid fertilization and spread seeds are among the most spectacular of all.

Plants used the 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 is maybe no accident that flying insects such as bees, moths and butterflies first appeared alongside the earth’s first 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 co-operation – 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 juice

Fruit is the female part of a flower, the ovary, which once it has been fertilized 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 to bury seeds in a ready-made meal. A passing animal might help itself to the fruit, 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 like 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 (145–65.5 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 up 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 and 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 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. Nipas 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, many pieces are missing in a puzzle that was, and is still, Darwin’s ‘abominable mystery’.