A New History of Life Read Free Page A

viruses.
    Viruses are parasites. They are technically termed obligatory intracellular parasites, as they are unable to reproduce without a host cell. In most cases, viruses infiltrate cells of living organisms and hijack the protein-forming organelles and start making more of themselves,turning the invaded cells into virus-producing factories. Viruses have a huge effect on the biology of their hosts.

    Our revised version of the tree of life, which includes viruses and now-extinct RNA life. This requires a new taxonomic category, one that is higher than domains (which are above kingdoms). RNA life is currently not definable on the accepted tree of life. (From Peter Ward, Life As We Do Not Know It , 2006)

    The greatest argument against including viruses as alive is the fact that they are unable to replicate on their own—and thus seemingly fail this major test of whether or not an object is living. But it must be remembered that viruses are obligate parasites, and parasites tend to undergo substantial morphological and genetic changes in adapting to their hosts.
    We can also ask if other parasites are alive. Parasitism, which is essentially a highly evolved form of predation, is generally the result of a long evolutionary history. Parasites are not primitive creatures. But like our viruses, they have stages that do not seem fully alive . Cryptosporidium and Giardia , both parasites on humans and other mammals, have resting stages that are as dead as any virus outside of its host. Without the hosts, these two organisms (and thousands of other species as well) will not live, perhaps cannot be classified as living. Yet when in their hosts, they show all the hallmarks of life as we know it: they metabolize, they reproduce, and they undergo Darwinian selection. But if we accept that viruses are alive, and this is increasingly accepted, we must radically reassess the tree of life as it is currently accepted.
    In studying life on Earth, two questions can be posed: What is the simplest assemblage of atoms that is alive? And what is the simplest life form on Earth, and what does it need to stay alive? To answer these questions we must look at what current Earth life needs to attain and maintain the state of life described above. To do this we must briefly digress into chemistry of the materials that all Earth life uses to attain and then maintain life.
THE NONLIVING BUILDING BLOCKS OF EARTH LIFE
    Of all the molecules making up Earth life, perhaps none is more important than water, and water in a single phase: it has to be liquid water, and not ice or water vapor (a gas). Earth life is composed of molecules bathed in liquid, and while the number of molecules thatcan be found in life is staggeringly large, in fact there are only four main kinds of molecules used by Earth life: lipids, carbohydrates, nucleic acids, and proteins. All of these are either bathed in liquid, in this case water with salt in it, or serve as an outer wall to contain the other molecules and water.
    Lipids—what we call fat—are key ingredients in the cell membranes of Earth life. They are water resistant due to an abundance of hydrogen atoms, but they contain few oxygen and nitrogen atoms. Lipids are the major components of the cell boundary or wall that separates the outside environment from the fluid-filled interior of what we call life. These membranes, although delicate, provide control of substances in and out of cell.
    Carbohydrates are the second major class of structures that Earth life is made of, and they are what we informally call sugars. By linking together a number of them, we can form a polysaccharide, which means “many sugars.” Sugars, be they linked or single, are important building blocks in that they can be combined with themselves or with other organic and inorganic molecules to form larger molecules.
    Sugars are also important in forming the next category of building block, nucleic acids. This group contains the stored genetic