The Loan of Water
Life’s long march to uninhabitable land
We are most comfortable on land. Oceans are like alien worlds that we cross to arrive at another chunk of land. To us therefore land is the real earth that is alive. We have even put a price on every piece of land around us calling it the real estate. However, not only did life not start on the land, but the continents were entirely devoid of life forms for some seven-eighth of the time life has been on our planet. Life’s invasion of continents is a geologically recent phenomenon.
Come to think of it, water being so intimately associated with life, it makes total sense that life originated in the ocean. At one point (roughly 400–500 million years ago), life decided to come out of the oceans and onto the land. As one would expect, life’s mission to conquer land began with the coastal terrains and proceeded inland. We still have vast interiors of continents in the form of deserts and ice sheet covers that do not sustain widespread life. Life’s encroachment of continents is still far from complete. The outbacks of big countries like Canada, Russia and Australia have limited presence of plants and animals, let alone the inhospitable lands such as Antarctica and Greenland.
Still, life has covered a lot of ground. The spread of green colour on the surface is a testimony, although recent human activities have had its undesirable effects. Life’s landward march has been full of struggle, mainly against gravity which tends to remove water from the land surface as soon as possible. Plants and animals are partners in this struggle. Animals can cover ground because of the attribute of motility and hence can take life a little further. Plants are like flagstaffs put on the ground to indicate life’s conquer of that piece of land.
Fundamentally, life requires water to fuel its photosynthesis machinery. Plants consume water (and the omnipresent carbon dioxide from the atmosphere) to produce energy and food that is later converted to more energy. The energy is used up in life’s metabolic activities. In contrast, the energy conversion process of respiration that occurs inside animal cells actually produces water. Such waters are called metabolic water and form substantial portion of body water of desert mammals, insects and birds. Other large animals require a good amount of additional water for maintaining body temperature and for distributing nutrients to every nook and corner of the body. Animals therefore feel the bodily sensation of thirst which prompts them to intake water.
Philosophically speaking, with the aid of the energy supplied by the photosynthesis machinery, one of the critical tasks that animals are meant to expedite is carry water to places where it is normally not available. Animals release water via sweating and excretion. Plants grow with the help of water in such an uncharted territory, put their flags there and produce more food to enable animals to continue the march. This completes the circle of life or rather the spiral of life because life never stops.
The spiral growth isn’t free from problems however. Plants require water to germinate as well as to continue to grow. Animals usually have a shorter life span. For them to continue to provision water for plants they must work over many generations. Generations of animals arrive via reproduction. Now reproduction is a function which requires water in plenty. Catch-22, isn’t it? You consume what you are supposed to provide. Life is continuously working on improving reproductive tools and strategies to mitigate this situation. The objective is to make reproduction less and less dependent on water because there will be less and less of this precious resource as life continues its expedition away from oceans onto higher grounds.
Look at the amphibians, for instance a frog, which were the first animals that started living on land. Having explored the land as adults they return to water when it’s time to reproduce. The male and female release sperms and eggs holding each other tight so as to give the released cells best chance to meet each other and fertilise. The resulting tadpoles stay in water till they are adult enough to come out and do something for the nature. This dependence on water limits an amphibian’s exploration territory. It cannot go too far to be not able to return to water.
At this point let’s remind ourselves of the contribution of aquatic animals in the exploration of land. Many fish species, like Salmon for instance, live in the sea and move upstream through water-filled land channels to lay eggs and end their life. This way they contribute to life on land with the resources brought from the oceans. This lifestyle of theirs not only supports plants on land, but also provides foods to animals like grizzly bears and Bengalis.
Back to the problem of water dependency faced by the first animals on land. Nature found a solution in the form of eggshell. A new group of animals (broad term: amniotes) evolved which could package nutrients rich fluids along with the embryo inside a protective covering. Now the fertilised egg does not have to depend on natural water to grow through the development phases. The egg does not need to be laid in water anymore. This cool idea might have been borrowed from plants that were already producing seeds.
Prior to the invention of seeds, poor plants were too dependent on water which allowed a sperm to swim to a nearby egg for fertilisation. Where water was available in plenty, as in the swampy lands of Carboniferous period when most coal forming plants lived, such a reproduction mechanism worked to perfection. Soon however it was realised that conditions will not always be so favourable as life tries to march further inwards on land. Seeds came about which kept life inside them dormant but intact until a wet ground is found. Then came casings which enclosed many seeds together to be carried away. The use of animals (insects) to do what water did – bring sperm to the egg – began as the world saw visual extravaganza in the form of flowers. Via yet another invention, plants got animals to carry the seeds to a ground where water is likely to be around. We know that invention as fruit. And in case there’s no water available where animals throw seeds after eating the fruits, nature made us humans, who not just watered plants and seeds, but also made large scale provisions of water via dams, bores and irrigation systems. After all, we animals must pay back the plants for the energy and the metabolic water they provide. There is no escape from this loan.
Or is this an investment? An investment by life which is seeking a high return from the higher organisms, even if illegally — by violating the second law of thermodynamics and feeding on negative entropy as Erwin Schrödinger had suggested?