Life on Earth. David Attenborough
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.
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.