4.1 Life and Water¶
Wherever the shore line ran there was life, and that life went on in and by and with water as its home, its medium, and its fundamental necessity.
The first jelly-like beginnings of life must have perished whenever they got out of the water, as jelly-fish dry up and perish on our beaches today. Drying up was the fatal thing for life in those days, against which at first it had no protection. But in a world of rain-pools and shallow seas and tides, any variation that enabled a living thing to hold out and keep its moisture during hours of low tide or drought met with every encouragement in the circumstances of the time. There must have been a constant risk of stranding. And, on the other hand, life had to keep rather near the shore and beaches in the shallows because it had need of air (dissolved of course in the water) and light.
No creature can breathe, no creature can digest its food, without water. We talk of breathing air, but what all living things really do is to breathe oxygen dissolved in water. The air we ourselves breathe must first be dissolved in our lungs; and all our food must be liquefied before it can be assimilated. Water-living creatures which are always under water, wave the freely exposed gills by which they breathe in that water, and extract the air dissolved in it. But a creature that is to be exposed for any time out of the water must have its body and its breathing apparatus protected from drying up. Before the seaweeds could creep up out of the Early Paleozoic seas into the intertidal line of the beach, they had to develop a tougher outer skin to hold their moisture.
Before the ancestor of the sea scorpion could survive being left by the tide it had to develop its casing and armour. The trilobites probably developed their tough covering and rolled up into balls, far less as a protection against each other and any other enemies they may have possessed, than as a precaution against drying. And when presently, as we ascend the Paleozoic rocks, the fish appear, first of all the back-boned or vertebrated animals, it is evident that a number of them are already adapted by the protection of their gills with gill covers and by a sort of primitive lung swimming- bladder, to face the same risk of temporary stranding.
Now the weeds and plants that were adapting themselves to intertidal conditions were also bringing themselves into a region of brighter light, and light is very necessary and precious to all plants. Any development of structure that would stiffen them and hold them up to the light, so that instead of crumping and flopping when the waters receded, they would stand up outspread, was a great advantage. And so we find them developing fibre and support, and the beginning of woody fibre in them. The early plants reproduced by soft spores, or half-animal “gametes”, that were released in water, were distributed by water and could only germinate under water. The early plants were tied, and most lowly plants today are tied, by the conditions of their life cycle, to water. But here again there was a great advantage to be got by the development of some protection of the spores from drought that would enable reproduction to occur without submergence. So soon as a species could do that, it could live and reproduce and spread above the, high-water mark, bathed in light and out of reach of the beating and distress of the waves. The main classificatory divisions of the larger plants mark stages in the release of plant life, from the necessity of submergence by the development of woody support and of a method of reproduction that is more and more defiant of drying up. The lower plants are still the prisoner attendants of water. The lower mosses must live in damp, and even the development of the spore of the ferns demands at certain stages extreme wetness. The highest plants have carried freedom from water so far that they can live and reproduce if only there is some moisture in the soil below them. They have solved their problem of living out of water altogether.
The essentials of that problem were worked out through the vast aeons of the Proterozoic Age and the early Paleozoic Age by nature’s method of experiment and trial. Then slowly, but in great abundance, a variety of new plants began to swarm away from the sea and over the lower lands, still keeping to swamp and lagoon and water-course as they spread.