Increased technological ability to examine the char­acteristic properties of organisms has permitted the establishment of more detailed and accurate classi­fication systems for living things.

Interestingly, none of these classification systems considers the viruses. The viruses, perhaps as they should be, are treated as nonliving entities. An exam­ination of the genetic molecules (RNA or DNA) of viruses indicates that they probably evolved from their respective host cells. They probably did not evolve in a hereditary lineage from one virus to the next. Hence, it is appropriate to classify viruses in re­lation to their host cells, for example, as a tobacco mosaic virus or a human immunodeficiency virus.

FIG. 2-4 According to endosymbiotic theory the chloroplasts and mitochondria of con­temporary eukaryotic cells evolved from prokaryotic cells living within ancestral eukary­otic cells that lacked these organelles, providing photosynthetic and respiratory capabili­ties, respectively.

30 CHAPTER 2 DIVERSITY OF THE MICROBIAL WORLD

HIGHLIGHT

Evolution of Microorganisms

We do not know exactly how the first living organism developed. However, it is possible to show in laboratory experiments that some chemicals that could have accu­mulated in the primitive atmosphere of Earth sponta­neously collect into spheres when wet with water (see Figure). These spheres, called micelles, resemble the cells that are the fundamental organizational units of all liv-

ing systems. A micelle is separated from the surround­ing environment by a chemical boundary layer. Its structure allows restricted exchange of materials with the surroundings while permitting the maintenance of a high degree of internal organization. It is likely that about 3.6 billion years ago the chemicals that accumu­lated within a micelle, including nucleic acid molecules

A micelle that forms spontaneously when certain chemicals interact with water resembles the structure of a living cell.

SURVEY OF MICROORGANISMS

ACELLULAR NONLIVING MICROORGANISMS

Viruses, viroids, and prions are acellular (noncellular), nonliving microorganisms that can replicate within the confines of a living cell of a compatible host organism (Table 2-1). It is this capacity for mul­tiplying (replicating one's own structure) within a host cell that gives them their "lifelike character" and distinguishes them from other nonliving chemical combinations of molecules. A host cell is a compati­ble cell of a living organism within which a specific virus, viroid, or prion is capable of being replicated.

Viruses, viroids, and prions are obligate intracellular parasites that do not have an independent capacity to carry out life functions. When these acellular mi­croorganisms are replicated within the cells of host organisms, they use the living cells' metabolic func­tions. Often in doing so they disrupt normal cellular functions, producing diseases in those organisms. Each virus, for example, produces characteristic symptoms when it replicates within a host organism, as typified by the characteristic mosaic pattern on the leaves of a tobacco plant when tobacco mosaic virus

that can transmit hereditary information, permitted es­sential life functions to occur. These functions were the ability to process materials and energy and to repro­duce. Thus the first living microorganism could have evolved on Earth. Paleobiologists believe they have found fossilized imprints of microbial communities that existed 3.5 to 3.8 billion years ago in western Australia and Greenland.

Of the chemicals that may have accumulated in a mi­celle, RNA (ribonucleic acid) appears to have hid a key role. RNA, like the hereditary molecule of all contempo­rary living organisms, DNA (deoxyribonucleic acid), can encode genetic information. Also some RNA mole­cules can act as catalysts for chemical reactions, a role most often played by protein catalysts (enzymes) in liv­ing cells. In 1993, scientists at the Scripps Research Insti­tute, made an artificial RNA molecule. When they mixed it with proteins it began to reproduce. As long as they continued to supply proteins, the RNA molecule churned out copies of itself.

In a similar manner the first microorganism probably used RNA as catalyst and guiding template and energy from the organic compounds that accumulated sponta­neously in the primordial atmosphere or on the Earth's surface. Generally speaking, organic compounds are substances that contain carbon and hydrogen. With these compounds a living cell could have carried out chemical reactions. (Collectively these reactions are called metabolism.) From these reactions the living cell could obtain energy and transform substances into the materials needed to survive and reproduce. There

would have been no molecular oxygen (0₂) in the prim­itive atmosphere. Therefore this first microorganism would have been an anaerobe, that is, an organism that grows and reproduces without using molecular oxygen.

Reproduction of this first microbial cell produced other living cells. Each of the new cells also reproduced, forming new cells with hereditary information that could be passed on to their progeny. In this continuous chain of descendents, errors or changes in the replica­tion of DNA occasionally occurred so that some of the new cells received somewhat differing hereditary char­acteristics. Thus new microorganisms evolved. Some of the microorganisms that evolved could synthesize com­plex organic compounds from the carbon dioxide in the atmosphere. Some of these microorganisms were photo-synthetic and were able to use light energy to make or­ganic compounds. The metabolism of these microorgan­isms gradually changed conditions in the environment so that other organisms could evolve.

The first photosynthetic microorganisms were able to grow only in the absence of oxygen. Later—probably 2 billion years ago, based on geologic evidence—micro­organisms evolved that could produce molecular oxy­gen from water. This oxygen-producing photosynthesis made possible other ways for cells to obtain energy, in­cluding aerobic respiration (a life-supporting process that uses oxygen). Over the next 0.6 billion years, the pace of evolution apparently quickened. While many microorganisms became extinct, an astonishing number of new organisms developed, including many types of plants, animals, and microorganisms.