Life on Earth
The Burgess Shales also contain superbly preserved examples of trilobites like those in the Moroccan limestones. Their body armour was constructed partly of calcium carbonate and strengthened by a horny substance called chitin, a material that forms the external skeletons of insects. But chitin, unlike skin, does not expand, so any animal with such an external chitinous skeleton has to shed it regularly if it is to grow – as indeed insects do today. Many of the trilobite fossils we find are in fact these empty suits of armour. Sometimes they are concentrated in great drifts, having been sorted by sea currents, as shells sometimes are when they are swept up on beaches today. The underwater avalanches in the Burgess Shales Basin, however, swept down not just discarded armour but living trilobites and buried them. Mud particles filtered into the animals’ bodies and preserved the finest details of their anatomy. So in them we can still see the paired jointed legs that are attached to each body segment, the feathery gill associated with each leg, two feelers at the front of the head, and the gut running the entire length of the body. Even the muscle fibres along the back, which enabled the animal to roll itself up into a ball, are still recognisable in some exceptional specimens.
Trilobites, as far as we know, were the first creatures on earth to develop high-definition eyes. They are mosaics, a cluster of separate components, each with its own lens of crystalline calcite orientated in the precise position in which it transmits light most efficiently, much like the eyes of today’s insects. One eye may contain 15,000 elements, and would have given its owner an almost hemispherical field of view. Late in the dynasty, some species developed an even more sophisticated kind of eye and one that has never been paralleled by any other animal. Here the components are fewer but larger. Their lenses are much thicker and it is thought that these species lived where there was little light and needed thick lenses to collect and concentrate what light there was. However, the optical properties of a simple calcite lens in contact with water are such that it transmits light in a diffused way and cannot bring it to a sharply focused point. To do this, a two-part lens is needed which has a waved surface at the junction between its two elements. And this is exactly what these trilobites evolved. The lower element of the double lens was formed by chitin and the surface between the two conforms to the mathematical principle that human scientists did not discover until 300 years ago when they tried to correct the spherical aberration of lenses in their newly invented telescopes.
As the trilobites spread through the seas of the world, they diversified into a great number of species. Many seem to have lived on the seafloor, chomping their way through mud. Some colonised the deep seas where there was little light and lost their eyes altogether. Others, to judge from the shape of their limbs, may well have paddled about, legs uppermost, scanning the seafloor below with their large eyes.
In due course, as creatures of many kinds and varying ancestries came to live on the bottom of the seas, the trilobites lost their supremacy. Two hundred and fifty million years ago, their dynasty came to an end. One relation alone survives, the horseshoe crab. It’s a misleading name for it is not a crab and only half its shell bears any resemblance to a horseshoe. Measuring 30 centimetres or so across, it is many times bigger than most known trilobites and its armour no longer shows any signs of segmentation. Its front section is a huge domed shield, on the front of which are two bean-shaped compound eyes. A roughly rectangular plate, hinged to the back of the shield, carries a sharp spike of a tail. But beneath its shell, the animal’s segmentation is clear. It has several pairs of jointed legs with pincers on the end, and behind these there are plate-like gills, large and flat like the leaves of a book.
Tower-eyed trilobite (Erbenochile erbeni) from the Timrahrhart Formation, Morocco.
Horseshoe crab (Limulus polyphemus) group spawning at high tide at sunset, Cape May, New Jersey.
Horseshoe crabs are seldom seen, for they live at considerable depths. Some inhabit Southeast Asian waters, others are found in the seas along the North Atlantic coast of America. Every spring, they migrate towards the coast. Then on three successive nights, when the moon is full and the tides are high, hundreds of thousands emerge from the sea. The females, their huge shells glinting in the moonlight, move towards the shore, dragging smaller males behind them. Sometimes four or five males, in their anxiety to reach a female, cling to one another and form a chain. As she reaches the edge of the water, the female half buries herself in the sand. There she sheds her eggs and the males release sperm. For kilometre after kilometre along the dark beaches, the living tide of horseshoe crabs is so thick that they form a continuous strip, like a causeway of giant cobbles. The breakers sometimes overturn them and they lie in the sand, with their legs waving, their stiff tails slowly swivelling, in an effort to lever themselves right side up. Many fail and are abandoned by the receding tide to die as thousands more swim in the shallows, pressing forward to take their turn.
This scene must have been enacted every spring for several hundred million years. When it began, the land was without life of any kind, and on such beaches the eggs were safe from sea-dwelling marauders. Perhaps this is why the horseshoe crabs developed the habit. Today beaches are not quite so safe, for hordes of gulls and small wading birds congregate to share the prodigious feast. But many of the fertilised eggs remain buried deep among the sand grains where they will stay for a month until, once more, high water reaches this part of the beach, stirring the sand and releasing the larvae to swim freely in the sea.
Although the trilobites were so successful, they were by no means the only armoured creatures to develop from the segmented worms. So did a group that must have been among the most alarming of all marine monsters – the sea scorpions, called scientifically the Eurypterids. Some grew to a length of two metres and were the largest arthropods ever known to have existed. However, in spite of their appearance and huge claws, many of them were filter feeders. Presumably, their fearsome claws were used in fights between one another rather than in subduing prey. Like the trilobites, they disappeared at the end of the Permian period.
One group related to the trilobites did however survive and today is extremely successful. They differed in one seemingly trivial but nonetheless diagnostic characteristic. They have not one but two pairs of antennae on their heads. They lived alongside the trilobites, comparatively unobtrusively for hundreds of millions of years, and then, when the trilobite dynasty came to an end, it was they who took over. They are the crustaceans. Today there are about 35,000 species of crustacean – seven times as many as there are of birds. Most prowl among the rocks and reefs – crabs, shrimps, prawns and lobsters. Some – the barnacles – have taken up a static life. Others – the krill which forms the food of whales – swim in vast shoals.
Robber crab (Birgus latro) climbing coconut tree, Aldabra Seychelles.
An external skeleton is highly versatile; it serves the tiny water flea as well as it does the giant Japanese spider crab that measures over three metres from claw to claw. Each crustacean species modifies the shape of its many paired legs for particular purposes. Those at the front may become pincers or claws; those in the middle, paddles, walking legs or tweezers. Some have feathery branches, gills through which oxygen is absorbed from the water. Others develop attachments so that they can carry eggs.
The limbs, which are tubular and jointed, are operated by internal muscles. These extend from the end of one section, along its length, to a prong from the next section which projects across the joint. When the muscle contracts between these two attachment points, the limb hinges. Such joints can only move in one plane, but crustaceans deal with that limitation by grouping two or three on a limb, sometimes close together, each working in a different plane so that the free end of the limb can move in a complete circle.
The external shell, however, gives the crustaceans the same problem as it gave the trilobites. It will not expand, and since it completely encloses their bodies, the only way they can grow is to shed it periodically. As the time for the moult approaches, the animal absorbs much of the calcium carbonate from its shell into its blood. It secretes a new, soft and wrinkled skin beneath the shell. The outgrown armour splits at the back and the animal pulls itself out, leaving the shell more or less complete, like a translucent ghost of its former self. Now, because the animal’s skin is soft, it must hide, but it grows fast and swells its body by absorbing water and stretching out the wrinkles of its new carapace. Gradually this hardens so the animal can again venture into a hostile world.
The hermit crab partly avoids this complicated and hazardous process by having a shell-less hind part and protecting it with a discarded mollusc shell, slipping into a new one in a minute or so whenever it has the need.
The external skeleton has one incidental quality which has had momentous results. Mechanically, it works almost as well on land as it does in water, so that, providing a creature can find a way of breathing, there is little to prevent it walking straight out of the sea and up the beach. Many crustaceans, indeed, have done so – sand shrimps and beach hoppers stay quite close to the sea; and pill bugs and penny sows have colonised moist ground throughout the land.
The most spectacular of all these land-living crustaceans is the robber crab. It is found on islands in the Indian Ocean and the western parts of the Pacific. At the back of its main carapace, at the junction with the first segment of its abdomen, there is an opening to an air chamber lined with moist puckered skin through which the animal absorbs oxygen. This monster is so big it can embrace the trunk of a palm tree between its outstretched legs. It climbs with ease, and once in the palm’s crest, cuts down with its gigantic pincers the young coconuts on which it feeds. It has to return to the sea to lay its eggs, but otherwise it is entirely at home on land.
Other descendants of the marine invertebrates have also left the water. Among the molluscs there are the snails and the shell-less slugs, but these emerged from water relatively recently in the group’s history. The first to make the move to land were probably descendants of the segmented worms, the millipedes. Their droppings have been found fossilised in the rocks of Shropshire. They were followed by pioneers which recent DNA studies show to have been crustaceans. And some of these made such a success of life in their new surroundings that they eventually gave rise to the most numerous and diverse group of all land animals – the insects.
THREE
The First Forests
There are few more barren places on earth than the plains surrounding a volcano in the aftermath of its eruption. Black tides of lava lie spilt over its flanks like slag from a furnace. Their momentum has gone but they still creak, and boulders still tumble as the flow settles. Steam hisses between the blocks of lava, caking the mouths of the vents with yellow sulphur. Pools of liquid mud, grey, yellow or blue, boiled by the subsiding heat from far below, bubble creamily. Otherwise all is still. No bush grows to give shelter from the scouring wind; no speck of green relieves the black surface of the empty ash plains.
This desolate landscape has been that of much of the earth for the greater part of its history. The first volcanoes to appear on the surface of the cooling planet erupted on a far greater scale than any that we know today, building entire mountain ranges of lava and ash. Over the millennia, the wind and rain destroyed them. Their rocks weathered and turned to clay and mud. Streams transported the debris, particle by particle, and strewed it over the seafloor beyond the margins of the land. As the deposits accumulated, they compacted into shales and sandstone.
Lava cactus (Brachycereus nesioticus) growing in lava field, coast of Fernandina, Galapagos Islands.
The continents were not stationary. They drifted slowly over the earth’s surface, driven by the convection currents moving deep in the earth’s mantle. When they collided, the sedimentary deposits around them were squeezed and rucked up to form new mountain ranges. As the geological cycles repeated themselves for some three thousand million years, and the volcanoes exploded and spent themselves, the land remained barren. In the sea, however, life burgeoned.
Some marine algae no doubt managed to live on the edges of the seas, rimming the beaches and boulders with green, but they could not have spread far beyond the splash zone, for they would have dried out and died. Then between 450 and 500 million years ago, some forms developed a waxy covering, a cuticle, which warded off desiccation. Even this, however, did not totally emancipate them from water. They could not leave it because their reproductive processes depended on it.
Algae reproduce themselves in two ways – by straightforward asexual division and by the sexual method, which is of great importance in the the evolutionary process. Sex cells will only develop further if they meet each other and fuse in pairs. To make these journeys and achieve these meetings, they need water.
This problem still besets the most primitive land plants living today – both the flat, moist-skinned ones known as liverworts, and the filamentous ones covered with green scales, the mosses. They use these two methods of reproduction, sexual and asexual, in alternate generations. The familiar green moss is the generation which produces the sex cells. Each large egg remains attached to the top of the stem, while the smaller microscopic sperms are released into water and wriggle their way up to fertilise it. The egg then germinates while still attached to the parent plant and produces the next asexual generation – a thin stem with, at its tip, a hollow capsule. In this, great numbers of grain-like spores are produced. When the atmosphere becomes dry, the capsule wall expands until it suddenly snaps apart, throwing the spores into the air to be distributed by the wind. Those that land on a suitably moist site then develop into new plants.
Moss filaments have no rigidity. Some kinds achieve a modest height by packing closely together in cushions and so giving one another support, but their soft, permeable, water-filled cells do not provide enough strength to enable individual stems to stand upright. Plants like these are very likely to have been among the earliest forms to colonise the moist margins of the land, but so far no fossil relics of undoubted mosses have been discovered from this early period.
The first land plants we have indentified, dating from over 400 million years ago, are simple leafless branching strands which occur as filaments of carbon in the rocks of central Wales and in some cherts in Scotland. Like mosses, they had no roots, but when their stems are carefully prepared and examined under the microscope, they are seen to contain structures that no moss possesses – long, thick-walled cells that must have conducted water up the stem. These structures gave them strength and enabled them to stand several centimetres tall. That may not sound very imposing, but it represented a major advance in life’s colonisation of the land.
Apple moss (Bartrimia pomiformis) with spore capsules, Inverness-shire, Scotland, UK.
Endive Pellia liverwort (Pellia endiviifolia) in centre growing through common liverwort (Marchantia polymorpha), the latter bearing cups containing gemmae (used in asexual reproduction). Lathkill Dale, Peak District National Park, Derbyshire, UK.
Such plants, together with primitive mosses and liverworts, formed green tangled carpets, miniature forests that spread inland from the edges of estuaries and rivers, and into these crept the first animal colonists from the sea. They were segmented creatures, ancestors of today’s millipedes, well suited by their chitinous armour to movement on land. At first they doubtless kept close to the edge of the water, but wherever there was moss there was both moisture and vegetable debris and spores to eat. With the land to themselves, these pioneering creatures flourished. Their name millipede, ‘thousand legs’, is something of an overstatement. No species alive today has many more than two hundred legs, and some have as few as eight. Nevertheless, some of them grew to magnificent dimensions. One of them was two metres long and must have had a devastating effect on the plants as it browsed its way through the wet green bogs. It was, after all, as long as a cow.
The external skeleton inherited from their water-living forebears needed few modifications for life on land, but the millipedes did have to acquire a different method of breathing. The feathery gill attached to a stalk alongside the leg that had served their aquatic relatives, the crustaceans, would not work in air. In its place, the millipedes developed a system of breathing tubes, the tracheae. Each tube begins at an opening on the flank of the shell and then branches internally into a fine network that leads ultimately to all the organs and tissues of the body, the tips even entering individual specialised cells called tracheoles that deliver gaseous oxygen to the surrounding tissues and also absorb waste.
Reproduction out of water, however, posed problems for the millipedes. Their marine ancestors had relied, like the algae, on water to enable their sperm to reach their eggs. On land the solution was an obvious one – male and female, being well able to move about, must meet and transfer the sperm directly from one to the other. This is exactly what millipedes do. Both sexes house their reproductive cells in glands close to the base of the second pair of legs. When the male meets the female in the mating season, the two intertwine. The male reaches forward with his seventh leg, collects a bundle of sperm from his sex gland and then clambers alongside the female until the bundle is beside her sexual pouch and she is able to take it in. The process looks rather laborious but at least it is not dangerous. Millipedes are entirely vegetarian. Fiercer invertebrates, which came to the moss jungles to prey on this grazing population of millipedes, could not indulge in such trusting relationships.
Three groups of these predatory creatures still survive today – centipedes, scorpions and spiders. Like their prey, they are members of the segmented group of animals, though the degree to which they have retained divisions in their bodies varies considerably. The centipedes are as clearly and extensively segmented as their close relatives the millipedes. The scorpions show divisions only in their long tails; and most spiders have completely lost all signs of segmentation, except for a few Southeast Asian species which retain clearly recognisable relics of their segmented past.
The scorpions that live today have not only fearsome-looking claws but a large venom gland drooping from the end of a long thin tail with a sharp curving sting. Their copulations cannot be the somewhat hit-and-miss gropings practised by the millipedes. Approaching such an aggressive and powerful creature is a dangerous enterprise even if the move is made by another individual of the same species and its intentions are purely sexual. There is a real risk of it being regarded not as a mate but a meal. So scorpion mating demands, for the first time among the animals that have appeared so far in this history, the ritualised safeguards and placations of courtship.
The male scorpion approaches the female with great wariness. Suddenly he grabs her pincers with his. Thus linked, with her weapons neutralised, the pair begin to dance. Backwards and forwards they move with their tails held upright, sometimes even intertwined. After some time, their shuffling steps have cleared the dancing ground of much of its debris. The male then extrudes a packet of sperm from the genital opening beneath his thorax and deposits it on the ground. Still grasping the female by the claws, he jerks and heaves her forward until her sexual opening, also on her underside, is brought directly above the sperm packet. She takes it up, the partners disengage and then go their separate ways. The eggs eventually hatch inside the mother’s pouch, the young crawl out and clamber up on to her back. There they stay for about a fortnight until they have completed their first moult and can fend for themselves.
Mediterranean/European scorpion (Buthus occitanus) stinging a spider (Amourobius sp.).
Spiders, too, must be extremely cautious in their courtship. Matters are made even more hazardous for the male because he is nearly always smaller than the female. And he prepares for his encounter with his mate long before he meets her. He spins a tiny triangle of silk a few millimetres in length and deposits a drop of sperm on to it from the gland that lies underneath his body. He then sucks it into the hollow first joint of his pedipalp, a special limb at the front of his body. Now he is ready.
The courtships of spiders are beguilingly various and ingenious. Jumping spiders and wolf spiders hunt primarily by sight and have excellent eyes. The courting male, consequently, relies on visual signals to make the female aware of his presence and his purpose. His pedipalps are brightly coloured and patterned, and as soon as he sights a female, he begins to signal with them in a kind of manic semaphore. Nocturnal spiders, on the other hand, depend largely on an extremely delicate sense of touch to find their prey. When they meet one another, they gingerly caress each other’s long legs, and only after a great deal of hesitation do they come to closer quarters. Web-making spiders are sensitive to the vibrations on their silken threads that tell them when a victim has blundered into the web. So when the male of such a species approaches a female hanging, large and menacing, on her web, or lurking hidden beside it, he signals to her by twanging the threads at one side in a special and meaningful way which he trusts the female will recognise. Other species put their faith in bribery. The male catches an insect and carefully parcels it up in silk. Holding this in front of him, he cautiously approaches the female and presents it to her. While she is occupied in examining the gift, he quickly scuttles over her and ties her to the ground with bonds of silk. Only then does he risk an embrace.
All these techniques lead to the same conclusion. The male, having survived every danger, places his pedipalp close to the female’s genital opening, squirts out the sperm and then hastily retreats. It has to be recorded that in spite of all his precautions he sometimes fails to make his getaway in time and the female eats him after all. But in terms of the transmission of his genes, the male’s disaster is of limited consequence: he lost his life after, not before, he had completed his purpose.