Cosmos - Carl Sagan [146]
For a whale to live there are many things it must know how to do. This knowledge is stored in its genes and in its brains. The genetic information includes how to convert plankton into blubber; or how to hold your breath on a dive one kilometer below the surface. The information in the brains, the learned information, includes such things as who your mother is, or the meaning of the song you are hearing just now. The whale, like all the other animals on the Earth, has a gene library and a brain library.
The genetic material of the whale, like the genetic material of human beings, is made of nucleic acids, those extraordinary molecules capable of reproducing themselves from the chemical building blocks that surround them, and of turning hereditary information into action. For example, one whale enzyme, identical to one you have in every cell of your body, is called hexokinase, the first of more than two dozen enzyme-mediated steps required to convert a molecule of sugar obtained from the plankton in the whale’s diet into a little energy—perhaps a contribution to a single low-frequency note in the music of the whale.
The information stored in the DNA double helix of a whale or a human or any other beast or vegetable on Earth is written in a language of four letters—the four different kinds of nucleotides, the molecular components that make up DNA. How many bits of information are contained in the hereditary material of various life forms? How many yes/no answers to the various biological questions are written in the language of life? A virus needs about 10,000 bits—roughly equivalent to the amount of information on this page. But the viral information is simple, exceedingly compact, extraordinarily efficient. Reading it requires very close attention. These are the instructions it needs to infect some other organism and to reproduce itself—the only things that viruses are any good at. A bacterium uses roughly a million bits of information—which is about 100 printed pages. Bacteria have a lot more to do than viruses. Unlike the viruses, they are not thoroughgoing parasites. Bacteria have to make a living. And a free-swimming one-celled amoeba is much more sophisticated; with about four hundred million bits in its DNA, it would require some eighty 500-page volumes to make another amoeba.
A whale or a human being needs something like five billion bits. The 5 × 109 bits of information in our encyclopaedia of life—in the nucleus of each of our cells—if written out in, say, English, would fill a thousand volumes. Every one of your hundred trillion cells contains a complete library of instructions on how to make every part of you. Every cell in your body arises by successive cell divisions from a single cell, a fertilized egg generated by your parents. Every time that cell divided, in the many embryological steps that went into making you, the original set of genetic instructions was duplicated with great fidelity. So your liver cells have some unemployed knowledge about how to make your bone cells, and vice versa. The genetic library contains everything your body knows how to do on its own. The ancient information is written in exhaustive, careful redundant detail—how to laugh, how to sneeze, how to walk, how to recognize patterns, how to reproduce, how to digest an apple.
Eating an apple is an immensely complicated process. In fact, if I had to synthesize my own enzymes, if I consciously had to remember and direct all the chemical steps required to get energy out of food, I would probably starve. But even bacteria do anaerobic glycolysis, which is why apples rot: lunchtime for the microbes. They and we and all creatures in between possess many similar genetic instructions. Our separate gene libraries have many pages in common, another reminder of our common evolutionary heritage. Our technology can duplicate only a tiny fraction of the intricate biochemistry that our bodies effortlessly perform: we have only just begun to study these processes. Evolution, however, has had billions of years of practice. DNA