Cosmos by Carl Sagan Read Free Book Online Page A

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. 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 ultraviolent 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. The products ofthis 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. (We will return to this subject later.)
    This was the earliest ancestor of deoxyribonucleic acid, DNA, 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. 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. 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. 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 organisms 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. 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 books of their DNA code, producing new varieties ready for