John Barrow
The Artful Universe

The world is full of plants and animals that have grown sensitive to the cycle of night and day, the seasonal cycle of the Sun's heat, and the monthly pull of the tides. Ocean tides raised by the waxing and waning of the Moon influenced the evolution of crustaceans and amphibians. The development of intertidal regions in which conditions alternated between submersion and drying may have encouraged the spread of life from sea to land. Changing conditions stimulate the evolution of a breed of complexity that leads ultimately to life because it creates conditions in which variation makes a difference to the prospects for survival.

There are clear imprints of an annual period in life-cycles of animals. Evolutionary adaptation will favour the survival of innate 'clocks' that time the birth of offspring to coincide with times when the chances of survival are highest, especially in the temperate regions where the seasons change abruptly. An impressive example is provided by the spawning of the grunion fish in southern Californian waters. They spawn at the highest reach of the spring tide, when the Moon is dark or full, leaving their spawn after burrowing half of their bodies into the sand. As successive tides are lower, the eggs remain out of reach of marine predators. They hatch two weeks later, when the tide has turned, just in time to be helped into the sea by the next advancing high tide. A lack of respect for this tidal cycle would be penalized by predators, and organisms with innate timing-triggers in step with tidal variations will prosper at the expense of those that lack them. Because tidal forces are manifestations of the same monthly cycle of lunar variations that alter the fraction of the Moon's face that can be seen by reflected sunlight at night, it is possible to synchronize with tidal cycles by various means: by sensing the forces directly, by sensing moonlight variations, or by behavioural variations in the intertidal region.

Animals sense the changing of the seasons by a response to the length of the hours of daylight. There are remarkable examples of the accuracy of this sensing, which optimizes female fertility to coincide with the spring equinox. A critical daylight length seems to trigger mating activity. Experiments show there may be just two phases: light-loving and dark-loving. In the first phase, when light falls on the body it enhances growth and activity; in the second Phase these things are inhibited. On long days, more light stimulates stronger biochemical responses. Yet the situation is not always so simple. Creatures can have their internal clocks reset by exposing them to artificial environments. Much argument has occurred among biologists about the respective roles of Eternal, genetically regulated clocks and of external influences in explainingbiological cycles. It appears that living things have baseline rhythms, inherited through adaptations to the environment, which can be shifted by changes in the environment and entrained into new cycles.

The day and the year are the simplest of our time divisions. The length of the day is determined by the period taken by the Earth to spin round once upon its axis. The day would last much longer if the Earth rotated more slowly, and diurnal variations would not exist at all if the Earth possessed no rotation. In that case, living things would be divided into three distinct populations: one for the dark side, one for the light side, and a third for the twilight zone in between. The day could not be dramatically shorter because there is a limit to how fast a body can spin before it starts to part company with things on its surface and disintegrate. The length of the day is in fact very slowly lengthening, by about two-thousandths of a second every century, because of the pull of the Moon. Over the vast periods of time required for significant geological or biological change this small change becomes quite significant. The day would have been eleven hours shorter two thousand million years ago when the oldest known fossilized bacteria were alive. Direct evidence of this change imprinting itself upon living things has been found in some coral reefs in the Bahamas. Daily and annual growth bands (rather like tree rings) are laid down in the coral, and by counting how many daily bands are in each annual band one can determine how many daily cycles there were in a year. Contemporary coral growths display about 365 bands for each year, roughly as expected, while 350 million-year-old corals, near by, display about 400 daily rings in each annual band, indicating that the day was then only about 21. hours long. This is almost exactly the value that we would expect at that time in the past, given the rate at which the Moon's pull is changing. If we extrapolate back to the formation of the Earth, then the young Earth might have had days lasting only about six hours. Thus, if the Moon did not exist, our day would probably be only a quarter of its present length. This would have consequences for the Earth's magnetic field as well. With a day of only six hours, the more rapid rotation of charged particles within the Earth would produce a terrestrial field about three times stronger than at present. Magnetic sensing would be a more cost-effective adaptation for living things on such a world. But the most far-reaching environmental effects of a shorter day would follow from the far stronger winds that would whip across the planet's rotating surface. The extent of erosion by wind and waves would be very great. There would be selective pressure towards smaller trees, and for plants to grow smaller, stronger leaves that were less susceptible to removal. This might well alter the course of the evolution of the Earth's atmosphere by delaying the early conversion of its carbon dioxide atmosphere into oxygen by the action of photosynthesis.

The year is determined by the time that it takes for the Earth to complete one orbit of the Sun. This period of time is by no means haphazard. The temperatures and energy output from stable stars are fixed by the unchanging strengths of the forces of Nature. Biological activity can occur on a planet only it its surface temperature is not extreme. Too hot, and molecules fry; too cold, and they freeze; but, in between, there is a range in which they can multiply, and grow in complexity. The narrow range within which water is liquid may well be the optimal one for the spontaneous evolution of life. Water offers a wonderful environment for the evolution of complex chemistry because it enhances both the mobility and the build-up of large concentrations of molecules.

These constraints of temperature ensure that living beings must find themselves on planets that are neither too close, nor too far, from the star they orbit. They will lie in a 'habitable zone' around a central star of the typical, middle-aged sort, that is typified by the Sun. Those orbits will need to be quite close to circular if these planets are to stay in the habitable zone throughout their orbital journeys. If they move in wildly eccentric oval orbits, like those of the comets that periodically pass our way, they will then alternately experience conditions of extreme cold and intense heat, rendering the evolution of complexity and life most unlikely. The law of gravitation fixes the time that a planet will take to complete its orbit if its distance from the parent star is known. Planets that are habitable thus have the length of their 'year' determined very closely by unalterable constants of Nature. These considerations show us that planet-based life will find itself in a periodic environment. Moreover, the cycles of change introduced by its rotation, and by its motion round its parent star, will be not dissimilar to those that characterize our own situation, because all are strongly linked to the conditions necessary for the maintenance of any constant habitable environment. Adaptations to periodic change will be ones that all intelligent life should share.

One can speculate about which aspects of the world would have left the deepest imprint upon our common view of the world in primitive antiquity. There is the clear division between the Earth and sky, separated by the horizon; the pull of the Earth's gravity orients 'up' and 'down', wherever we go. These experiences are invariable; but others, like the cycles of darkness and light, are periodic. The Sun dominates the daytime hours—the source of heat and light. At night, its role is taken by the Moon and the stars, which straddle the sky in the fuzzy band that we call the Milky Way. All conscious beings on habitable planets orbiting stable stars will be under similar influences. Sun-gods and moon-gods are the most widespread objects of worship in human history; their veneration may well extend far beyond the bounds of our solar system.

  The World was all before them, where to choose
Their place of rest, and Providence their guide:
They, hand in hand, with wand'ring steps and slow,

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Through Eden took their solitary way.