Lava cactus (Brachycereus nesioticus) growing in lava field, coast of Fernandina, Galapagos Islands.
The continents were not stationary. They drifted slowly over the earth’s surface, driven by the convection currents moving deep in the earth’s mantle. When they collided, the sedimentary deposits around them were squeezed and rucked up to form new mountain ranges. As the geological cycles repeated themselves for some three thousand million years, and the volcanoes exploded and spent themselves, the land remained barren. In the sea, however, life burgeoned.
Some marine algae no doubt managed to live on the edges of the seas, rimming the beaches and boulders with green, but they could not have spread far beyond the splash zone, for they would have dried out and died. Then between 450 and 500 million years ago, some forms developed a waxy covering, a cuticle, which warded off desiccation. Even this, however, did not totally emancipate them from water. They could not leave it because their reproductive processes depended on it.
Algae reproduce themselves in two ways – by straightforward asexual division and by the sexual method, which is of great importance in the the evolutionary process. Sex cells will only develop further if they meet each other and fuse in pairs. To make these journeys and achieve these meetings, they need water.
This problem still besets the most primitive land plants living today – both the flat, moist-skinned ones known as liverworts, and the filamentous ones covered with green scales, the mosses. They use these two methods of reproduction, sexual and asexual, in alternate generations. The familiar green moss is the generation which produces the sex cells. Each large egg remains attached to the top of the stem, while the smaller microscopic sperms are released into water and wriggle their way up to fertilise it. The egg then germinates while still attached to the parent plant and produces the next asexual generation – a thin stem with, at its tip, a hollow capsule. In this, great numbers of grain-like spores are produced. When the atmosphere becomes dry, the capsule wall expands until it suddenly snaps apart, throwing the spores into the air to be distributed by the wind. Those that land on a suitably moist site then develop into new plants.
Moss filaments have no rigidity. Some kinds achieve a modest height by packing closely together in cushions and so giving one another support, but their soft, permeable, water-filled cells do not provide enough strength to enable individual stems to stand upright. Plants like these are very likely to have been among the earliest forms to colonise the moist margins of the land, but so far no fossil relics of undoubted mosses have been discovered from this early period.
The first land plants we have indentified, dating from over 400 million years ago, are simple leafless branching strands which occur as filaments of carbon in the rocks of central Wales and in some cherts in Scotland. Like mosses, they had no roots, but when their stems are carefully prepared and examined under the microscope, they are seen to contain structures that no moss possesses – long, thick-walled cells that must have conducted water up the stem. These structures gave them strength and enabled them to stand several centimetres tall. That may not sound very imposing, but it represented a major advance in life’s colonisation of the land.
Apple moss (Bartrimia pomiformis) with spore capsules, Inverness-shire, Scotland, UK.
Endive Pellia liverwort (Pellia endiviifolia) in centre growing through common liverwort (Marchantia polymorpha), the latter bearing cups containing gemmae (used in asexual reproduction). Lathkill Dale, Peak District National Park, Derbyshire, UK.
Such plants, together with primitive mosses and liverworts, formed green tangled carpets, miniature forests that spread inland from the edges of estuaries and rivers, and into these crept the first animal colonists from the sea. They were segmented creatures, ancestors of today’s millipedes, well suited by their chitinous armour to movement on land. At first they doubtless kept close to the edge of the water, but wherever there was moss there was both moisture and vegetable debris and spores to eat. With the land to themselves, these pioneering creatures flourished. Their name millipede, ‘thousand legs’, is something of an overstatement. No species alive today has many more than two hundred legs, and some have as few as eight. Nevertheless, some of them grew to magnificent dimensions. One of them was two metres long and must have had a devastating effect on the plants as it browsed its way through the wet green bogs. It was, after all, as long as a cow.
The external skeleton inherited from their water-living forebears needed few modifications for life on land, but the millipedes did have to acquire a different method of breathing. The feathery gill attached to a stalk alongside the leg that had served their aquatic relatives, the crustaceans, would not work in air. In its place, the millipedes developed a system of breathing tubes, the tracheae. Each tube begins at an opening on the flank of the shell and then branches internally into a fine network that leads ultimately to all the organs and tissues of the body, the tips even entering individual specialised cells called tracheoles that deliver gaseous oxygen to the surrounding tissues and also absorb waste.
Reproduction out of water, however, posed problems for the millipedes. Their marine ancestors had relied, like the algae, on water to enable their sperm to reach their eggs. On land the solution was an obvious one – male and female, being well able to move about, must meet and transfer the sperm directly from one to the other. This is exactly what millipedes do. Both sexes house their reproductive cells in glands close to the base of the second pair of legs. When the male meets the female in the mating season, the two intertwine. The male reaches forward with his seventh leg, collects a bundle of sperm from his sex gland and then clambers alongside the female until the bundle is beside her sexual pouch and she is able to take it in. The process looks rather laborious but at least it is not dangerous. Millipedes are entirely vegetarian. Fiercer invertebrates, which came to the moss jungles to prey on this grazing population of millipedes, could not indulge in such trusting relationships.
Three groups of these predatory creatures still survive today – centipedes, scorpions and spiders. Like their prey, they are members of the segmented group of animals, though the degree to which they have retained divisions in their bodies varies considerably. The centipedes are as clearly and extensively segmented as their close relatives the millipedes. The scorpions show divisions only in their long tails; and most spiders have completely lost all signs of segmentation, except for a few Southeast Asian species which retain clearly recognisable relics of their segmented past.
The scorpions that live today have not only fearsome-looking claws but a large venom gland drooping from the end of a long thin tail with a sharp curving sting. Their copulations cannot be the somewhat hit-and-miss gropings practised by the millipedes. Approaching such an aggressive and powerful creature is a dangerous enterprise even if the move is made by another individual of the same species and its intentions are purely sexual. There is a real risk of it being regarded not as a mate but a meal. So scorpion mating demands, for the first time among the animals that have appeared so far in this history, the ritualised safeguards and placations of courtship.
The male scorpion approaches the female with great wariness. Suddenly he grabs her pincers with his. Thus linked, with her weapons neutralised, the pair begin to dance. Backwards and forwards they move with their tails held upright, sometimes even intertwined. After some time, their shuffling steps have cleared the dancing ground of much of its debris. The male then extrudes a packet of sperm from the genital opening beneath his thorax and deposits it on the ground. Still grasping the female by the claws, he jerks and heaves her forward until her sexual opening, also on her underside, is brought directly above the sperm packet. She takes it up, the partners disengage and then go their separate ways. The eggs eventually hatch inside the mother’s pouch, the young crawl out and clamber up on to her back. There they stay for about a fortnight until they have completed their
