January 3 I look across these snow-covered hills, these old upthrusts that mark my countryside, and think of time, of earths age. Estimates of that age are largely guesswork, but most informed guessers say our particular planet came into being about five billions years ago, give or take a billion. How it came into being is a matter for theorists, but it is now believed to have been whirling around the sun for at least three billion years before any kind of life appeared upon it. How and whence that life came are also matters of theory and speculation. Those who try to pin down its source, I have noticed, sometimes end up in myth or fable, sometimes try to credit omnipotence, and sometime honestly say, We dont know - yet. The first life, according to good evidence now available, appeared on earth about one and a half billion years ago. As far as we know, that life consisted of very small one-celled entities - they may have been neither plant nor animal, but as yet undetermined in form. At first those bits of life lived in the warm waters of the earth, which presumably at that time were not saline as are the oceans of today. From the earliest forms of life evolved, more or less in succession, seaweed, sponges, small worms, shellfish, primitive fish with backbones, and the early amphibians and insects. From the amphibians came the first reptiles, and after them came the giants of their tribe, dinosaurs and their kind. The lizards persisted for about 150 million years. By the end of their age, birds and early mammals had appeared, forerunners of most of the animals we know today. Finally, perhaps a millions years ago, perhaps two million, the earliest men appeared. Not only is the earths age a matter of analytical guesswork, based on the best information available, but so are the approximate dates for the geological and biological eras, periods and epochs. For a long time, for example, it was believed that the earths age could be stated in thousands, not millions or billions, of years. Until recently the geologists and geophysicists based their estimates on sedimentation and erosion rates as revealed in the earths rocks and on the probable rate of salting in the oceans from leaching of the land. Now the atomic physicists have provided a check on those estimates in the radioactive component tests, which indicate how long the elements in the rocks have been radioactively decaying. These tests have already revised a good many geological dates. For example, the last Ice Age was supposed to have ended about 25,000 years ago by geological evidence, but radioactivity tests appear to make the date 11,000 years ago instead. I suppose the age of the mountain behind my house could be determined radioactively, but nobody ever bothered. All I know is that it is a remnant of one of the older mountain upthrusts on this continent. Hal Borlands Book of Days
Posted on: Sat, 03 Jan 2015 01:05:55 +0000
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