A New History of Life Read Free

Is either alive, and at what point does each pass from chemical reaction to chemical reactions powering life? Yet if the essential characteristic of a living cell is homeostasis—the ability to maintain a steady and more or less constant chemical balance in a changing environment—it follows that metabolism had to come first. Eating before breeding seems to be the accepted view at the present time, but as in so much dealing with the origin of life, disquieting questions remain.
ENERGY AND THE DEFINITION OF LIFE
    The role of energy in maintaining life can now be added to our definition of life. We have already defined life as metabolizing, replicating, and evolving. But let us not consider life from an energy flow and order-disorder continuum. Just having energy is clearly not sufficient as a basis of life; there must be an interaction with the energy, and that interaction at a very basic level is needed to maintain a state of nonequilibrium order. Without energy, life goes to nonlife, so life must be something whose very definition is coupled with energy acquisition and energy dumping. Life maintains itself by having states that allow it to become progressively more orderly through the input of energy flow. Our kind of life does this by maintaining a relatively small numberof combinations of carbon, oxygen, nitrogen, and hydrogen (and some other elements in smaller volumes). Eventually, a degree of complexity and integration is reached, and maintained, that we call life. The inflow of energy must be sufficient to overcome the tendency of the chemistry within the body that we call life to revert back to its equilibrium condition—nonlife.
    One of the universally accepted definitions of life is that it metabolizes. For Earth life, the primary sources of energy are from the heat of the Earth or from the sun, itself the energy arising from the sun’s thermonuclear fusion reactions. By far the most common way that life taps into solar energy is through photosynthesis. In this process, sunlight provides the energy to convert carbon dioxide and water into complex carbon compounds with many chemical bonds that store energy. By breaking these bonds, energy is released.
    Life on Earth uses a variety of biochemical reactions, and they all involve the transfer of electrons. But this system works only if there is what might be called an electrochemical gradient: the steeper the gradient, the more energy that can be realized. This means that some types of metabolism yield far more energy than others, just as some kinds of environments have more energy to harvest than others. The organic (carbon-containing) compounds containing the greatest amount of stored energy are fats and lipids—long chain carbons that have much energy tied up in their chemical bonds.
    Metabolism is the sum of all the chemical reactions occurring within an organism. A virus is very small; typical viruses are from 50 to 100 nm in diameter, where nm stands for nanometer, or 10 -9 meter. They come in two general types: one group is enclosed in a shell of protein, the second by both a protein shell and an additional membrane-like envelope. Within this covering is the most important part of the virus, its genome, made up of a nucleic acid component. In some there is DNA, in others only RNA. The number of genes also varies widely, with some having as few as three genes and others (such as smallpox) having more than 250 individual genes. In fact, there is a huge variety of viruses, and if they were considered alive, they would be classified across a great taxonomic spread. But common wisdom treats them asnonliving. The viruses that contain only RNA show that RNA by itself, in the absence of DNA, is capable of storing information, and serving as a de facto DNA molecule. 9 This finding is strong evidence that there may have been an “RNA world” 10 before DNA and life as we know it originated. And there is an even more striking implication of the presence of RNA