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
