Life on Earth
A typical jellyfish is a saucer fringed with stinging tentacles. This form is called a medusa after the unfortunate woman in a Greek myth who was loved by the god of the sea and as a result had her hair changed by a jealous goddess into snakes. Jellyfish are constructed from two layers of cells. The jelly which separates them gives the organism a degree of rigidity needed to withstand the buffeting of the sea. They are quite complex creatures. Their cells, unlike those of the sponge, are incapable of independent survival. Some are modified to transmit electric impulses and are linked into a network which amounts to a primitive nervous system; others are able to contract in length and so can be considered as simple muscles. There are also stinging cells with coiled threads inside them, the unique possessions of the jellyfish tribe. When food or an enemy comes near, the cell discharges the thread, which is armed with spines like a miniature harpoon and often loaded with poison. It is these cells in the tentacles that will sting you if you unluckily brush against a jellyfish when swimming.
Jellyfish reproduce by releasing eggs and sperm into the sea. The fertilised egg does not develop into another jellyfish directly but becomes a free-swimming creature quite different from its parents. It eventually settles down on the bottom of the sea and grows into a tiny flower-like organism called a polyp. In some species, this sprouts, through branching twigs, into other polyps. They filter-feed with the aid of tiny beating cilia. Eventually, the polyps bud in a different way and produce miniature medusae which detach themselves and wriggle away to take up the swimming life once more.
Portugese man o’war (Physalia physalis) split level showing float and tentacles, Indo-pacific.
This alternation of form between generations has allowed all kinds of variations within the group. The true jellyfish spend most of their time as free-floating medusae with only the minimum period fixed to the rocks. Others, like the sea anemones, do the reverse. For all their adult lives they are solitary polyps, glued to the rock, their tentacles waving in the water ready to trap prey that may touch them. Yet a third kind are colonies of polyps but ones that have, confusingly, given up their attachment to the sea bottom and sail free like medusae. The Portuguese man o’war is one of these. Chains of polyps dangle from a float filled with gas. Each chain has a specialised function. One kind produces reproductive cells; another absorbs sustenance from captured prey; another, heavily armed with particularly virulent stinging cells, trails behind the colony for up to fifty metres, paralysing any fish that blunder into it.
It seems an obvious assumption that these relatively simple organisms appeared very early in the history of animal life, but for a long time there was no proof that they actually did so. Hard evidence could only come from the rocks. Even if microorganisms can be preserved in chert, it is difficult to believe that a creature as large but as fragile and insubstantial as a jellyfish could retain its shape long enough to be fossilised. But in the 1940s some geologists noticed very odd shapes in the ancient Ediacara Sandstones of the Flinders Ranges in southern Australia. These rocks, now thought to be about 650 million years old, were believed to be completely unfossiliferous. Judging from the size of the sand grains of which they are composed and the ripple marks on the surface of their bedding planes, they had once formed a sandy beach. Very occasionally, flower-like impressions were detected on them, some the size of a buttercup, some as big as a rose. Could these be the marks left by jellyfish stranded on the beach, baked in the sun and then covered by a wash of fine sand by the next tide? Eventually enough of these shapes were collected and studied for it to be undeniable that this is just what they must be.
Since then, other assemblages of living organisms of this extreme age have been discovered in many parts of the world – the Charnwood Forest in the heart of England, Namib Desert in southwest Africa, on the flanks of the Ural Mountains and the shores of the White Sea in Russia. But the most impressive and richest of all these discoveries have been made on the Avalon peninsula in Newfoundland. There the rocks, which are around 565 million years old, are exposed in dramatic cliffs. The strata have been tilted and folded, as one might expect in deposits of such extreme age, but not so severely that they have destroyed or even seriously distorted the fossils they contain. These are so abundant that in places it is impossible to walk over the exposed surface of a layer without treading on examples that any museum in the world would regard as one of its greatest treasures. They have been preserved in extraordinary perfection, seemingly by falls of volcanic ash from nearby volcanoes which buried them almost instantaneously, so creating what have been called death masks. There is a rich variety of shapes that are still being catalogued – spindles, fronds, discs, mats, plumes and combs, by far the richest record of any of the communities that flourished in the seas of the world during this extremely ancient period. Many seem to be unrelated to anything alive today and may perhaps be regarded as evolution’s failed experiments. One or two, however, bear at least a superficial resemblance to living marine creatures called sea pens that are still common today.
The name sea pen was given them when people wrote with quills, and very apt it must have seemed, for not only are they shaped like feathers but their skeleton is flexible and horny. They grow sticking up vertically on sandy seafloors, some only a few centimetres long, some half as tall as a man. At night they are particularly spectacular for they glow with a bright purple luminescence, and if you touch them, ghostly waves of light pulsate along their slowly writhing arms.
Sea pens are also called soft corals. Stony corals, their relatives, often grow alongside them and they too are colonial creatures. Their history is not as ancient as that of the sea pens, but once they had appeared, they flourished in immense numbers. An organism that produces a skeleton of stone and lives in an environment where deposits of ooze and sand are being laid down is an ideal subject for fossilisation. Huge thicknesses of limestone in many parts of the world consist almost entirely of coral remains and they provide a detailed chronicle of the development of the group.
The coral polyps secrete their skeletons from their bases. Each is connected with its neighbours by strands that extend laterally. As the colony develops, new polyps form, often on these connecting sections, and their skeletons grow over and stifle earlier polyps. So the limestone the colony builds is riddled with tiny cells where polyps once lived. The living ones form only a thin layer on the surface. Each species of coral has its own pattern of budding and so erects its own characteristic monument.
Corals are very demanding in their environmental requirements. Water that is muddy or fresh will kill them. Most will not grow at depths beyond the reach of sunlight for they are dependent upon single-celled algae that grow within their bodies. The algae photosynthesise food for themselves and in the process absorb carbon dioxide from the water. This assists the corals in the building of their skeletons, and releases oxygen which helps the corals respire.
The first time you dive on a coral reef is an experience never to be forgotten. The sensation of moving freely in three dimensions in the clear sunlit water that corals favour is, in itself, a bewitching and other-worldly one. But there is nothing on land that can prepare you for the profusion of shapes and colours of the corals themselves. There are domes, branches and fans, antlers delicately tipped with blue, clusters of thin pipes that are blood red. Some seem flower-like, yet when you touch them they have the incongruous scratch of stone. Often different coral species grow beside one another, mingled with sea pens arching above and beds of anemones that wave long tentacles in the current. Sometimes you swim over great meadows that consist entirely of one kind of coral; sometimes in deeper water, you discover a coral tower hung with fans and sponges that extends beyond your sight into depths of darkest blue.
Purple sea pen (Virgularia gustaviana) on sandy sea bed. Rinca, Indonesia.
But if you swim only during the day, you will hardly ever see the organisms that have created this astounding scene. At night, with a torch in your hand, you will find the coral transformed. The sharp outlines of the colonies are now misted with opalescence. Millions of tiny polyps have emerged from their limestone cells to stretch out their minuscule arms and grope for food.
Coral polyps are each only a few millimetres across, but, working together in colonies, they have produced the greatest animal constructions the world had seen long before humans appeared. The Great Barrier Reef, running parallel to the eastern coast of Australia for over 1,600 kilometres can be seen from the moon. So if, some 500 million years ago, astronauts from some other planet passed near the earth, they could easily have noticed in its blue seas a few new and mysterious turquoise shapes; and from them they might have guessed that complex life on earth had really started.
Table corals (Acropora spp.) on remote reef. Komodo National Park, Indonesia.
TWO
Building Bodies
The Great Barrier Reef swarms with life. The tides surging through the coral heads charge the water with oxygen and the tropical sun warms it and fills it with light. All the main kinds of sea animals seem to flourish here. Phosphorescent purple eyes peer out from beneath shells; black sea urchins swivel their spines as they slowly perambulate on needle tip; starfish of an intense blue spangle the sand; and patterned rosettes unfurl from holes in the smooth surface of coral. Dive down through the pellucid water and turn a boulder. A flat ribbon, striped yellow and scarlet, dances gracefully away and an emerald green brittle star careers over the sand to find a new hiding place.
The variety at first seems bewildering, but leaving aside primitive creatures like jellyfish and corals which we have already described, and the much more advanced backboned fish, nearly all can be allocated to one of three main types: shelled animals, like clams, cowries and sea snails; radially symmetrical creatures, like starfish and sea urchins; and elongated animals with segmented bodies varying from wriggling bristle worms to shrimps and lobsters.
The principles on which these three kinds of bodies are built are so fundamentally different that it is difficult to believe that they can be related to one another except right at the very roots of the evolutionary tree. The fossil record bears this out. All three groups, being sea-dwellers, have left behind abundant remains, and the details of their separate dynastic fortunes can be traced through the rocks for hundreds of millions of years. The walls of the Grand Canyon show that animals without backbones, invertebrates, came into existence long before the vertebrates such as fish. But just below the layer of gently folded limestones that contain the earliest of the invertebrate fossils, the strata change radically. Here the rocks are highly contorted. They had once formed mountains. These were eroded and eventually covered with the sea that deposited the limestone now lying above them. The episode occupied many millions of years and during all that time there were no deposits. As a consequence, this junction in the rocks represents a huge gap in the record. To trace the invertebrate lines back to their origins, we must find another site where rocks were not only deposited continuously throughout this critical period, but have survived in a relatively undistorted condition.
Such places are few, but one lies in the Atlas Mountains of Morocco. The bare hills behind Agadir in the west are built of blue limestones so hard that they ring under the fossil hunter’s hammer. The beds of rock are slightly tilted but otherwise undistorted by earth movements. On the crest of the passes, the rocks yield fossils. They are not very many, but if you look hard enough you can collect quite a range of species. All fossils found anywhere in the world in rocks of this age can be placed in one or other of those three main groups we identified on the reef. There are tiny shells, the size of your little fingernail, called brachiopods; radially symmetrical organisms looking like stalked flowers called crinoids; and trilobites, segmented creatures that resemble woodlice.
The limestones at the top of the Moroccan succession are about 560 million years old. Beneath them lie more layers extending downwards for thousands of metres, seemingly unchanged in character. In them, surely, must be evidence about the origins of those three great invertebrate groups.
But it is not so. As you clamber down the mountainside over the strata, the fossils suddenly disappear. The limestone seems to be exactly the same as that at the head of the pass, so the seas in which it was laid down must surely have been very similar to those that produced fossiliferous rocks. There are no signs of a revolutionary change in physical conditions. It is simply that at one time the ooze covering the seafloor contained shells of animals – and before that it did not.
This abrupt beginning to the fossil record is not just a Moroccan phenomenon, though you can see it here more vividly than in most places. It occurs in almost all the rocks of this age throughout the world. The microfossils from the cherts of Lake Superior and South Africa show that life had started long, long before. In the theoretical year of life, shelled animals do not appear until early November. So the bulk of life’s history is undocumented in the rocks. Only at this late date, about 600 million years ago, did several separate groups of organisms begin to leave records of any abundance by secreting shells. Why this sudden change should have come about, we do not know. Perhaps before this time the seas were not at the right temperature or did not have the chemical composition to favour the deposition of the calcium carbonate from which most marine shells and skeletons are constructed. Whatever the reason, we have to look elsewhere for evidence of the origins of the invertebrates.
A living crinoid: the great west indian sea lily (Cenocrinus asterius), 180–250 metres depth, Caribbean.
Flatworm (Maiazoon orsaki) Raja Ampat, Irian Jaya, Indonesia, Pacific Ocean.
We can find some living clues back on the reef. Fluttering over the coral heads, hiding in the crevices or clinging to the underside of rocks, are flat leaf-shaped worms. Like jellyfish, they have only one opening to their gut through which they both take in food and eject waste. They have no gills and breathe directly through their skin. Their underside is covered with cilia which by beating enable them to glide slowly over surfaces. Their front end has a mouth below and a few light-sensitive spots above so that the animal can be said to have the beginnings of a head. These flatworms are the simplest creatures to show signs of such a thing.
Eye-spots, to be of any use, must be linked to muscles so that the animal can react to what it senses. In flatworms all that exists is a simple network of nerve fibres. There are a few thickenings in some of them, but these can hardly be described as brains. Yet the flatworms can learn the kind of things that would help even this simplest of animals to survive, such as avoiding a particularly dangerous place or remembering where food can be found.
Today we know of some 3,000 species of flatworm in the world. Most are tiny and water-living. You can find freshwater ones in most streams simply by dropping a piece of raw meat or liver into the water. If the underwater vegetation is thick, flatworms are likely to glide out in some numbers and settle on the bait. In humid tropical forests, the ground is usually moist enough for some species to live on land, and many are likely to appear, undulating on the mucus that they secrete from their undersides. One of these species grows to a length of about 60 centimetres. Other flatworms have taken to the parasitic life and live unseen within the bodies of other animals – including us.
Liver flukes still retain the typical flatworm form. Tapeworms are also members of the group, though they look very different, for after burying their heads in the walls of their host’s gut, they bud off egg-bearing sections from their tail end. These segments remain attached while they mature, eventually forming a chain that may be as much as 10 metres long. The whole creature, as a result, looks as though it is divided into segments, but in fact these separate living packets of eggs are quite different from the permanent internal compartments of a truly segmented creature like an earthworm.
Flatworms are very simple creatures. Members of one free-swimming group lack a gut altogether and look very like the tiny free-swimming coral organisms before they settle down to a sedentary life. So there is little difficulty in believing those researchers who conclude from a study of the detailed structure of both adult and larva that the flatworms are descended from simpler organisms like corals and jellyfish.
During the period when these first marine invertebrates were evolving, between 600 and 1,000 million years ago, erosion of the continents was producing great expanses of mud and sand on the seabed around the continental margins. This environment must have contained abundant food in the form of organic detritus falling from the waters above as the single-celled organisms that floated in the surface waters died and drifted downwards. It also offered concealment and protection for any creature that lived within it. The flatworm shape, however, is not suited to burrowing. A tubular form is much more effective, and eventually worms with such a shape appeared. Some became active burrowers, tunnelling through the mud in search of food particles. Others lived half buried with their front end above the sediment. Cilia around their mouths created a current of water and from it they filtered their food.
Some of these creatures lived in a protective tube. In time, the shape of the top of this was modified into a collar with slits in it. This improved the flow of water over the tentacles. Further modification and mineralisation eventually produced a two-part protective shell around the front end. These were the first brachiopods, including Lingulella, an example of a species that has existed virtually unchanged for hundreds of millions of years.
The front end of a brachiopod is really quite complicated. Within the shell, it has a mouth surrounded by a group of tentacles. They are covered with beating cilia which create a current in the water. Any food particles in it are caught by the tentacles and then passed by them down to the mouth. While doing this, the tentacles perform another and important function, for the water brings with it dissolved oxygen which the animal needs in order to respire. The tentacles absorb it and so, in effect, they become gills. The shell enclosing the tentacles not only gives protection to these soft delicate structures, but concentrates the water into a steady stream so that it flows more effectively over them.
The brachiopods elaborated this design considerably over the next million years or so. One group developed a hole at the hinge end of one of the valves through which the worm-like stalk emerged to fasten the animal into the mud. This gave the shell the look of an upside-down Aladdin oil lamp, with the stalk as the wick, and so the group as a whole gained the name of lamp shell. The tentacles within the shell eventually became so enlarged that they had to be supported by delicate spirals of limestone.
There are other shelled worms to be found alongside the brachiopods in these ancient rocks. In one kind the elaborated worm did not attach itself to the seafloor but continued to crawl about and secreted a small conical tent of shell under which it could huddle when in danger. This was the ancestor of the most successful group of all these shelled worms, the molluscs, and it too has a living representative, a tiny organism called Neopilina, which was dredged up in 1952 from the depths of the Pacific. Today there are about 80,000 different species of molluscs with about as many again known from their fossils. You can find some of them in your garden; they are the snails and the slugs.
Brachiopods (Glottidia albida).
The lower part of the molluscan body is called the foot. Its owner moves itself about by protruding the foot from the shell and rippling its undersurface. Many species carry a small disc of shell on the side of it which, when the foot is retracted into the shell, forms a close-fitting lid to the entrance. The upper surface of the body is formed by a thin sheet that cloaks the internal organs and is appropriately called the mantle. In a cavity between the mantle and the central part of the body, most species have gills which are continually bathed by a current of oxygen-bearing water, sucked in at one end of the cavity and expelled at the other.
The shell is secreted by the upper surface of the mantle. One whole group of molluscs has single shells. The limpet, like Neopilina, produces shell at an equal rate right round the circumference of the mantle and so builds a simple pyramid. In other species, the front of the mantle secretes faster than the rear and creates a shell in a flat spiral, like a watch spring. In yet others, maximum production comes from one side so that the shell develops a twist and becomes a turret. The cowrie concentrates its secretion along the sides of the mantle, forming a shell like a loosely clenched fist. From the slit along the bottom, it protrudes not only its foot but two sections of its mantle which in life may extend over each flank of the shell and meet at the top. These lay down the marvellously patterned and polished surface characteristic of cowries.
Blue limpet (Patella coerulea), showing underside.
The single-shelled molluscs feed not with tentacles within the shell like the brachiopods but with a radula, a ribbon-shaped tongue, covered with rasping teeth. Some use it to scrape algae from the rocks. Whelks have developed a radula on a stalk which they can extend beyond the shell and use to bore into the shells of other molluscs. Through the holes they have drilled, they poke the tip of the radula and suck out the flesh of their victim. Cone shells also have a stalked radula but have modified it into a kind of gun. They slyly extend it towards their prey – a worm or even a fish – and then discharge a tiny glassy harpoon from the end. While the tethered victim struggles, they inject a venom so virulent that it kills a fish instantly and can even be lethal to human beings. They then haul the prey back to the shell and slowly engulf it.
A heavy shell must be something of a handicap when actively hunting, and some carnivorous molluscs have taken to a faster if riskier life by doing without it altogether and reverting to the lifestyle of their flatworm-like ancestors. These are the sea slugs (nudibranchs) and they are among the most beautiful and highly coloured of all invertebrates in the sea. Their long soft bodies are covered on the upper side with waving tentacles of the most delicate colours, banded, striped and patterned in many shades. Though they lack a shell, they are not entirely defenceless, for some have acquired secondhand weapons. These species float near the surface of the water on their feathery extended tentacles and hunt jellyfish. As the sea slug slowly eats its way into its drifting helpless prey, the stinging cells of the victim are taken into its gut, complete and unsprung. Eventually these migrate within the sea slug’s tissues and are concentrated in the tentacles on its back. There they give just the same protection to their new owners as they did to the jellyfish that developed them.