Page one- the tides

From Isaac Asimov's The Tragedy of the Moon (1972)

To begin with, man might conceivably not exist at all if Earth had had no Moon. The dry land might have remained untenanted.

Life began in the sea some three billion years ago or more, and for at least 80 per cent of its entire history on this planet, it remained in the sea. Life is adapted for the surface layers of the ocean primarily, and only by the power of versatile adjustment over many generations has it succeeded in colonizing the surface's borderlands: downward into the abyss, outward into the fresh-water rivers and lakes, and outward/upward into the land and air. Of the borderlands, dry land must in its way have been most exotic; as impossible to sea life as the surface of the Moon is to us. If we imagine a primitive sea creature intelligent enough to have speculated about land life, we can be sure he would be appalled by the prospect. On land, an organism would be subjected to the full and eternal pull of gravity, to the existence of wild oscillations of temperature both daily and yearly, to the crushing need to get and retain water in an essentially water- free environment , to the need to get oxygen out of dry, and desiccating, air rather than out of mild water solution.

Such a sea creature might imagine itself emerging from the sea in a water-filled land suit with mechanical grapples to support him against gravity, insulation against temperature change, and so on.

The sea life of half a billion years ago had, however, no technology to help it defeat the land. It could only adapt itself over hundreds or thousands of generations to the point where it could live on land unprotected.

But what force drove it to do so, in the absence of a deliberate decision to do so?

The tides ...

Life spread outward into the rims of the ocean, where the sea water rose up against the continental slopes and then fell back twice each day. And thousands of ,species of seaweed and worms and crustaceans and mollusks and fish rose and fell with those tides. Son e were exposed on shore as the sea retreated, and of those a very few survived, because they happened, for some reason, to be the best able to withstand the nightmare of land existence until the healing, life-giving water returned.

Species adapted to the temporary endurance of dry land developed, and the continuing pressure of competitor saw to it that there was survival value to be gained in developing the capacity to withstand dry-land conditions for longer and ever-longer periods.

Eventually species developed that could remain on land indefinitely. About 425 million years ago, plant life began cautiously to green the edges of the continent. Snails, spiders, insects developed to take advantage of a new food supply. Some 400 million years ago, certain fish were crawling on new-made limbs over the soggy mud flats.

(Actually we are descended from fresh-water creatures who probably came to endure land as a result of the periodic drying of ponds, but they could have completed the colonization only because the tides had already populated the continents and produced an ecology to become part of.)

And of course the tides are the product of the Moon. The Sun, to be sure, also produces tides, nearly half the size of those produced by the Moon today, but that smaller to-and-fro wash of salt water would represent a smaller drive towards land and might have led to the colonization of the continents much later in time, if at all.

Indeed, hundreds of millions of years ago, when land life was evolving, the Moon was surely closer to Earth, and the tides were considerably more ample. It is even possible that the Moon was captured late in the existence of life and that it was the long period of giant tides that followed which produced the necessary push for the colonization of the land.