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The Infinite Variety of Life on Earth

Halfway down the Grand Canyon, you come to 400-million-year-old limestone strata. There are no reptiles to he found here, but there are the bones of strange armored fish. An hour or so later on the way down -- and a hundred million years earlier -- the rocks contain no sign of backboned animals of any kind. There are a few shells and worms that have left behind a tracery of trails in what was the muddy sea floor. Three-quarters of the way down, you are still descending through layers of limestone, but now there is no sign of life whatever. By the late afternoon, you ride at last into the lower gorge where the Colorado River runs green between high rock walls. You are now a vertical mile below the rim and the rocks have been dated to the immense age of 2000 million years. Here you might hope to find evidence for the very beginnings of life. But there are no organic remains of any kind. The dark, fine-grained rocks lie not in horizontal layers like all those above, but are twisted and buckled and riven with veins of pink granite.

Are signs of life absent because these rocks and the limestones directly above are so extremely ancient that all such traces have been crushed from them? Could it be that the first creatures to leave any sign of their existence were as complex as the worms and molluscs found in the 500-million-year-old layers? For many years these questions puzzled geologists. All over the world, rocks of this antiquity were carefully searched for organic remains. One or two odd shapes were found. but most authorities dismissed these as patterns produced by the physical processes of rock formation that had nothing whatever to do with living organisms. Then, during the 1950s, the searchers began to use high-powered microscopes on some particularly enigmatic rocks.

A thousand miles northeast of the Grand Canyon, ancient rocks of about the same age as those beside the Colorado River outcrop on the shores of Lake Superior. Some of them contains seams of a tine-grained flint-like substance called chert. This was well known during the last century because the pioneers used it in their flintlock guns. Here and there, it contains strange white concentric rings a meter or so across. Were these merely eddies in the mud on the bottom of the primaeval seas or could they have been formed by living organisms? No one could be sure and the shapes were given the non-committal name of stromatolite, a word derived from Greek meaning no more than 'stony carpet'. But when researchers cut sections of these rings, ground them down into slices so thin that they were translucent and examined them through the microscope, they found, preserved in the chert, the shapes of simple organisms, each no more than one or two hundredths of a millimeter across. Some resembled filaments of algae; others, while they were unmistakably organic, had no parallels with living organisms; and some looked to be identical with the simplest form of life existing today, bacteria.

It seemed almost impossible to many people that such tiny things as micro-organisms could have been fossilized at all. That relics of them should have survived for such a vast period of time seemed even more difficult to believe. The solution of silica which had saturated the dead organisms and solidified into chert was clearly as fine-grained and durable a preservative as exists. The discovery of the fossils in the Gunflint Chert stimulated further searchers not only in North America but all over the world and other micro-fossils were found in cherts in Africa and Australia. Some of these, astonishingly pre-dated the Gunflint-specimens by a thousand million years.

   
  Questions
   
  1.   How does the author create a sense of vast ages of time in the first paragraph ?
       
  2.   The second paragraph begins with two questions and the rest of the passage provides the answers to them. Summarize these two answers as clearly as you can.
       
  3. (i) Explain fully the meaning of the italic words as they are used in the passage : (a) complex; (b) enigmatic; (c) non-committal; (d) parallels; (e) stimulated
       
    (ii) Write five short sentences, each using one of the words (a)-(e), to illustrate their meaning. Your sentences should not deal with the subject-matter of the passage.
       
  4   Explain in about 150 words what the close study of chert in the 1950s told the scientists.
       
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  Answers
       
  1.   The author describes a journey down the Grand Canyon taking him the equivalent of descending a mile underground. The build-up of that vertical mile was infinitely slow, in fact 2000 million years. He contrasts this with the day's ride which it takes to get from top to bottom. Half a mile equates to 400 million years. The lower half mile equates to the remaining 1600 million years. So we deduce that the weight of the first half mile has greatly compressed the second. Somewhere in the second half, evidence of life seems to run out.
       
  2.   The lowest rocks contain no detectable signs of life. Has the weight on top of them crushed out every trace : well above the bottom layers worms and molluscs are in evidence, but the physical structure of these is complicated. Logically they should have been proceeded by simpler forms. Is it possible that they came into being in their complex form ?
       
  3. (i) (a) complicated, sophisticated
      (b) puzzling, abstruse, hard to understand
      (c) indefinite, failing to convey an exact description
      (d) relationships; in this case, 'obvious signs of connection with ...'
      (e) gave rise to, caused, provided the impetus for
       
       
    (ii) (a) This micro-circuit is too complex for anybody but an expert to understand
      (b) My declaration of love brought an enigmatic smile to her face
      (c) When I asked the bank manager for an overdraft, he was non-committal; 'come back next month', he said ..
      (d) There are few parallels between my violin playing and that of a virtuoso
      (e) I am greatly stimulated in the morning by my first cup of coffee
       
  4. Chert, a fine-grained flint substance found in the rocks beside the Colorado River was also found on the shores of Lake Superior. In the 1950s scientists examined these rocks under high-powered microscopes. Gunflint Chert contained concentric rings a meter across which could have been caused either by swirls in the mud or by living organisms. These were given the Greek name Stromatolite, which merely means 'stony carpet'. However, when specimens were ground so thin that they let the light through they disclosed simple organisms only two hundredths of a millimeter across. Some were like algae or bacteria, others totally unlike living organisms. The preservation of micro-organisms for so long as fossils seemed incredible, but chert was as good a preservative as any. Chert research inspired further searches in North America. Africa and Australia, and some finds were a thousand million years older than Gunflint. ( 147 words )
           
 
 
 
 
 

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