Life on Earth. David Attenborough
club mosses and horsetails were still, for the most part, swamp-dwellers, and there they now stood in dense ranks, thirty metres tall, some with woody trunks two metres in diameter. The compacted remains of their stems and leaves today form coal. The great thicknesses of the seams are impressive evidence of the abundance and persistence of the early forests. Other species of both these groups also spread farther inland and there mingled with ferns. These had developed true leaves, large spreading structures with which to collect as much light as possible. They grew tall with curving trunks, like the tree ferns that still thrive in tropical rainforests.
Wood horsetails (Equistetum sylvaticum) Columbia River, Gorge National Scenic Area, Oregon, USA.
The height of these first forests must have caused considerable problems for their animal inhabitants. Once, there had been a superabundance of leaves and spores close to the ground. Now the soaring trunks had raised this source of food high in the sky, creating a dense canopy that cut out much of the light. The floor of these forests was, at best, only sparsely vegetated and great areas may have been entirely without any living leaves. Some of the multi-legged vegetarians found their fodder by clambering up the trunks.
There may have been another factor that induced these creatures to leave the ground. About this time, animals of a completely new kind joined the invertebrates on the land. They had backbones and four legs and wet skins. They were the first amphibians and they too were carnivorous. A description of their origins and fate will have to wait until we have followed the development of the invertebrates to its climax, but their presence at this stage must be mentioned if the scene in these first jungles is not to be misrepresented.
Virtually all of the new-style invertebrate families still survive. Among the most numerous are the bristletails and springtails. Although they are little known and infrequently seen, they are enormously abundant. There is hardly a spadeful of soil or leaf litter anywhere in the world that does not contain some of them. Indeed, the springtails, or collembola, are probably the most abundant arthropods on the planet. Most are only a few millimetres long. Of those new families, only one is commonly noticed – the silverfish that glides smoothly across cellar floors or is occasionally discovered making a meal of the dried glue in the bindings of books. Its body is clearly segmented but it has very many fewer divisions than the millipede. It has a well-defined head with compound eyes and antennae; a thorax bearing three pairs of legs, the result of fusing together three segments; and a segmented abdomen which, while it no longer carries limbs on each segment, retains little stumps as signs that it once possessed them. Three thin filaments trail from its rear end. It breathes like the millipedes by means of tracheae, and it reproduces in a manner reminiscent of those early land invertebrates, the scorpions. The male silverfish deposits a bundle of sperm on the ground and then, one way or another, he entices the female to walk over it. When that happens, she is stimulated to take it up into her own sexual pouch.
There are several thousand different species of bristletails and collembola. They vary considerably in their anatomy and, as is often the case when considering the simpler members of a big group, it is sometimes difficult to decide whether a particular characteristic represents a truly primitive survival or one that has become secondarily reduced to suit a particular way of life. The silverfish, for example, has compound eyes but other members of the group are blind. All lack wings. Some even lack tracheae and breathe through their chitinous skeleton which is particularly thin and permeable. Is this because they never had them or because they have lost them?
Marine springtail/bristletail (Petrobius maritimus) adult resting on stones, Lough Muree, County Clare, Ireland.
Many such debatable questions raised by the anatomy of these creatures still wait universally agreed answers. However, they all have six legs and tripartite bodies and these characteristics clearly link them to that great and varied group of land invertebrates, the insects. They appeared many millions of years after the earlier groups were well established. Geneticists have now shown that collembolla, as well as the insects, including the silverfish, are all closely related to one particular group of water-living crustaceans, the remipedia (the name means ‘oar-foot’), which today are found only in the pools and streams of caves.
The primitive insects must have found some of their food by climbing the trunks of the early tree ferns and horsetails. The ascent was doubtless relatively easy. The climb down, involving long detours over the upward-pointing leaf-bases, may have been very much more laborious and time-consuming. Whether or not the prevalence of such obstacles had anything to do with the next developments, we cannot be sure. It is certain, however, that some of these primitive insects did develop a much swifter and less laborious method of getting down. They flew.
We have no direct evidence of how they achieved flight, but the living silverfish provides a clue. On the back of its thorax it has two flap-like sideways extensions of the chitinous shell that look as though they might be the rudiments of wings. The early wings may not have served initially for flight. Insects, like all animals, are greatly affected by body temperature. The warmer they are, the quicker the energy-producing chemical reactions of their body can proceed and the more active they can be. If their blood were to be circulated through thin flaps extending laterally from the back, they could certainly warm themselves very effectively and quickly in the sunshine. If, furthermore, these flaps had muscles at their base, they could be tilted to face squarely to the sun’s rays. Insect wings do originate as flaps on the back and they do, initially, have blood flowing in their veins, so such a theory seems very plausible.
However this may be, insects with wings appeared some 350 million years ago. The earliest so far discovered are dragonflies. There were several species, most about the size of those living today. But for the dragonflies as for millipedes and other groups that have pioneered a new environment, the absence of competition allowed some early forms to develop to an enormous size, and dragonflies eventually appeared with a wingspan of 70 centimetres, the largest insects ever to exist. When the air became more thickly populated, such extravagant forms disappeared.
Living dragonflies have two pairs of wings which have simple joints to them: they can only move up and down and cannot be folded back. Even so, they are highly accomplished flyers, shooting over the surface of a pond in a blur of gauzy wings at up to 30 kph. At such speeds, they need accurate sense organs if they are to avoid damaging collisions. A tuft of hair on the front of the body helps them to check that their motion through the air is straight, but their primary navigational guidance comes from huge mosaic eyes on either side of the head, which provide superbly accurate and detailed vision.
Because of this dependence on sight, most dragonflies do not fly at night, although there are some that migrate vast distances over the oceans, flying from India to Africa and stopping off at the islands of the Maldives along the way. All are daytime hunters, flying with their six legs crooked in front of them to form a tiny basket in which they catch smaller insects. That fact alone makes it clear that they must have been preceded into the air by other herbivorous forms which, judging from the primitive nature of their anatomy, were probably related to cockroaches, grasshoppers, locusts and crickets.
The presence of these large populations of insects, whirring and buzzing through the air of the ancient forests, was eventually to play an extremely important part in a revolution that was taking place among the plants.
The early trees, like their predecessors, the mosses and liverworts, existed in two alternating forms, a sexual generation and an asexual one. Their greater height posed no problem for spore dispersal: if anything it was a help, since up in the treetops spores were more easily caught by the wind and carried away. The distribution of sex cells, however, was a different matter. Hitherto, it had been achieved by the male cells swimming through water, a process which demanded that the sexual generation be small and live close to the ground. That of ferns, club mosses and horsetails still is. The spores of these plants develop into a thin filmy plant called the thallus which looks not unlike a liverwort and releases its sex cells from its underside where there is permanent moisture. After its eggs have been fertilised, they grow into tall plants like the previous spore-producing