January 17, 2010
| By one billion years ago, plants, working cooperatively, had made a stunning change in the environment of the Earth. Green plants generate molecular oxygen. Since the oceans were by now filled with simple green plants, oxygen was becoming a major constituent of the Earth’s atmosphere, altering it irreversibly from its original hydrogenrich character and ending the epoch of Earth history when the stuff of life was made by nonbiological processes. But oxygen tends to make organic molecules fall to pieces.Despite our fondness for it, it is fundamentally a poison for unprotected organic matter. |
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January 17, 2010
We are, each of us, a multitude.Sex seems to have been invented around two billion years ago. Before then, new varieties of organisms could arise only from the accumulation of random mutations – the selection of changes, letter by letter, in the genetic instructions. Evolution must have been agonizingly slow. With the invention of sex, two organisms could exchange whole
paragraphs, pages and topics of their code, producing new varieties ready for the sieve of selection. Organisms are selected to engage in sex – the ones that find it uninteresting quickly become extinct. And this is true not only of the microbes of two billion years ago. We humans also have a palpable devotion to exchanging segments of today. |
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January 17, 2010
| The cells in a drop of blood contain a different sort of molecular factory, the mitochondrion, which combines food with oxygen to extract useful energy.These factories exist in plant and animal cells today but may once themselves have been free-living cells.By three billion years ago, a number of one-celled plants had joined together,perhaps because a mutation prevented a single cell from separating after splitting in two.The first multicellular or ganisms had evolved. Every cell of your body is a kind of commune, with once free-living parts all banded together for the common good. And you are made of a hundred trillion cells. |
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January 17, 2010
| There were as yet no predators. Some molecules reproduced themselves inefficiently, competed for building blocks and left crude copies of themselves. With reproduction, mutation and the selective elimination of the least efficient varieties, evolution was well under way, even at the molecular level. As time went on, they got better at reproducing. Molecules with specialized functions eventually joined together, making a kind of molecular collective the first cell. Plant cells today have tiny molecular factories, called chloroplasts, which are in charge of photosynthesis – the conversion of sunlight, water and carbon dioxide into carbohydrates and oxygen. |
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January 17, 2010
Every lifeform on Earth has a different set of instructions, written out in essentially the same
language. The reason organisms are different is the differences in their nucleic acid instructions. A mutation is a change in a nucleotide, copied in the next generation, which breeds true. Since mutations are random nucleotide changes, most of them are harmful or lethal, coding into existence nonfunctional enzymes. It is a long wait before a mutation makes an organism work better. And yet it is that improbable event, a small beneficial mutation in a nucleotide a ten-millionth of a centimeter across, that makes evolution go.Four billion years ago, the Earth was a molecular Garden of Eden. |
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January 17, 2010
The products of this early chemistry were dissolved in the oceans, forming a kind of organic
soup of gradually increasing complexity, until one day, quite by accident, a molecule arose
that was able to make crude copies of itself, using as building blocks other molecules in the
soup.This was the earliest ancestor of deoxyribonucleic acid , the master molecule of life on Earth. It is shaped like a ladder twisted into a helix, the rungs available in four different molecular parts, which constitute the four letters of the genetic code. These rungs,called nucleotides, spell out the hereditary instructions for making a given organism. |
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January 17, 2010
| The Earth condensed out of interstellar gas and dust some 4.6 billion years ago. We know from the fossil record that the origin of life happened soon after, perhaps around 4.0 billion years ago, in the ponds and oceans of the primitive Earth. The first living things were not anything so complex as a one-celled organism, already a highly sophisticated form of life. The first stirrings were much more humble. In those early days, lightning and ultraviolet light from the Sun were breaking apart the simple hydrogen-rich molecules of the primitive atmosphere, the fragments spontaneously recombining into more and more complex molecules. |
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January 17, 2010
The secrets of evolution are death and time – the deaths of enormous numbers of lifeforms that were imperfectly adapted to the environment; and time for a long succession of small mutations that were by accident adaptive, time for the slow accumulation of patterns of favorable mutations. Part of the resistance to Darwin and Wallace derives from our difficulty in imagining the passage of the millennia, much less the aeons. What does seventy million years mean to beings who live only one-millionth as long? We are like butterflies who flutter for a day and think it is forever.
What happened here on Earth may be more or less typical of the evolution of life on many worlds; but in such details as the chemistry of proteins or the neurology of brains, the story of life on Earth may be unique in all the Milky Way Galaxy. |
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January 17, 2010
| I glumly nodded assent.Muller switched on the overhead light and smiled benignly. It was an old story.There was a kind of moth that had adapted to Drosphila genetics laboratories. It was nothing like a fruit fly and wanted nothing to do with fruit flies. What it wanted was the fruit flies’ molasses. In the brief time that the laboratory technician took to unstopper and stopper the milk bottle – for example, to add fruit flies – the mother moth made a divebombing pass, dropping her eggs on the run into the tasty molasses. I had not discovered a macro-mutation. I had merely stumbled upon another lovely adaptation in nature, itself the product of micromutation and natural selection. |
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January 17, 2010
| It was my unhappy task to explain it to him.With heavy heart I knocked on his office door. Come in,’ came the muffled cry. I entered to discover the room darkened except for a single small lamp illuminating the stage of the microscope at which he was working. In these gloomy surroundings I stumbled through my explanation. I had found a very different kind of fly. I was sure it had emerged from one of the pupae in the molasses. I didn’t mean to disturb Muller but Does it lookmore like Lepidoptera than Diptera?’ he asked, his face illuminated from below. I didn’t know what this meant, so he had to explain: Does it have big wings? Does it have feathery antennae? |
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