1.1Various Organisms, One Biology
Organism, biology, and life science are not particularly old concepts. Although plants, animals, microorganisms, and humans were considered to have their own respective identities before the advent of modern biology, the development of cell theory, laws of heredity, and biochemistry have facilitated the current recognition of several common principles. However, the 1953 discovery of the double helix structure of DNA (Column in Section 5 of Chapter 6 ) enabled us to describe all living things using one biology, a single set of scientific principles. In biology, diversity and unity are generally presented as opposing concepts. Although organisms constantly undergo diversification and extinction, the principles that govern these processes are fundamentally shared. In our life science studies, we begin with an overview of the diversity of organisms (Fig. 1-1).
Humans are the most familiar organisms. They are a mammalian species and are closely related to various other mammals. Animals such as dogs are capable of understanding gestures and human emotions. The mouse is a representative animal model. Examples of the vertebrate species begin with these highly intelligent animals and extend to lizards, frogs, and fish. Moreover, living things include various invertebrates such as insects. In addition to numerous insect species, molluscs such as squid, octopus, and shellfish are familiar invertebrates. Furthermore, invertebrates include animals with a simpler body structure, such as sea anemone, earthworm, and nematodes. Movement, foraging, and eating are common to all the aforementioned animals.
On the other hand, plants include the representative crop rice; and beautiful flowering trees and grasses (angiosperms), which are commonly observed, pine species (gymnosperms); and ferns and mosses that propagate through spores. Characteristics of plants include immobility and photosynthesis. Other examples of organisms that photosynthesize include algae. There are many microscopic unicellular algae, which are invisible to the naked eye, and a majority of these unicellular algae species swim using flagella. Although plants do not move in terms of changing their individual positions, they do perform various types of movement, including the rotational movement of the end of a growing stalk and moving petioles in case of the silk tree.
In the past, it was believed that all organisms were animals or plants. Mushrooms and molds (fungi) are organisms that neither perform photosynthesis nor move. Although they were previously considered as related to plants, molecular phylogenetic analysis has shown them to be completely different. Fungi are often minute creatures. They are neither animals nor plants and are conveniently termed as microorganisms because they are invisible to the naked eye. Fungi are considered microorganisms because of their minute structures, such as hyphae, in addition to their visible portions. Microorganisms contain various species of eukaryotic unicellular organisms, including amoeba and paramecium. Bacteria are smaller microorganisms and include Escherichia coli, which are found in animal digestive tracts, Bacillus subtilis var. natto, which are used to prepare natto (fermented soy beans), and a range of pathogenic bacteria. Although microorganisms exhibiting aerobic respiration and absorbing nutrients from their surroundings may be conspicuous, there are many environmental bacteria that do not perform aerobic respiration (anaerobic) and survive by transforming various substances.
From a larger perspective, one may wonder why such totally different organisms are all considered living. In brief, the defining characteristics of living things are proliferation and metabolism. In addition, we can consider heredity to be a characteristic because proliferation results in several offspring virtually identical to the original organism. Although viruses, which are smaller than bacteria, possess the inheritance characteristic, they are not considered organisms normally because they do not undergo autonomous proliferation or metabolism. The cell as the common basic unit of structure in every organism was established in the 19th century, and the idea that this cell theory is not applicable to some organisms finally ended at the end of the 20th century with the onset of the genomic era. We now introduce some basic characteristics of organisms.