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The cellular structure – essentially a small container that represents the smallest possible complete unit of a replicating, evolving life form, is a characteristic of life on Earth. However, is this compartmentalization universal? One feature of the cell is the accumulation of molecules at sufficiently high concentrations to carry out the diversity of reactions associated with life. Natural environments where water is available, such as lakes or the oceans, tend to dilute molecules. One could imagine a hypothetical scenario where cell contents became very concentrated. Consider a small pond in which lots of organic molecules and other ions and inorganic substances accumulate and that by chance they result in a primitive form of metabolism with reactions producing new compounds that are cycled in the pond. This “cell,” however, cannot go anywhere, and it cannot reproduce, since it is isolated in a small depression in the ground. Thus, we might suspect that for evolutionary biology to occur, for replicating, evolving life forms to be distributed across the surface of a planet, they must, in some way, be in containers or cells that can be dispersed to different environments and then selected for survival – to evolve. You might like to continue this discussion about whether cellularity is a fundamental and necessary characteristic of any form of replicating, evolving life particularly after you have read the sections on viruses and prions.

Prokaryotes were discovered in the seventeenth century by Dutch fabric maker, Antonie van Leeuwenhoek (1632–1723). Van Leeuwenhoek was keen to improve the quality of the cloth that he was selling, which enticed him to develop small microscopes to observe the fibers in his fabrics (Figure 5.1). As he was an inquisitive man, he used his new devices to examine samples of pond water. What he found were tiny creatures, which at the time he called “animalcules,” or little animals.

Figure 5.1 Early microbiology. (a) Antonie van Leeuwenhoek, discoverer of microbes, and (b) one of his first microscopes. The microscope (about 10 cm long) is held up to the eye, and objects are observed through the tiny glass lens.

Source: Reproduced with permission of Jeroen Rouwkema.

The creatures he observed had different shapes and he published numerous papers through The Royal Society in which he described these organisms and their appearances. Remarkably, he managed, even with his primitive microscopes, to observe some of the major shapes (morphologies) of microorganisms, including coccoids (spheres), rods, spiral organisms, and microbes with a bent cell shape. He even observed bacterial movement or motility, which you can see in Figure 5.2.

Figure 5.2 Leeuwenhoek's diagrams in the 1670s showing the first drawing of prokaryotes and their diverse morphologies. In the part labeled “fig: B (C–D)” you can see a depiction of bacterial motility.

Leeuwenhoek even showed that he could kill bacteria. In a letter of 1684, he wrote: “I took a very little wine-Vinegar and mixt it with the water in which the scurf [tooth plaque he had scrapped from his teeth] was dissolved, whereupon the Animals dyed presently.”

For the moment, we leave historical accounts. However, I have no compunction here in recommending the now rather classic book The Microbe Hunters, written in 1926 by Paul de Kruif, an amusing and popular account of the early history of microbiology, which remains relevant today.