Life on Earth. David Attenborough
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.