Zoology as a Part of Biology

Animals form a distinct branch on the evolutionary tree of life. It is a large and old branch that originated in the Precambrian seas over 600 million years ago. Animals form part of an even larger limb known as eukaryotes, organisms whose cells contain membrane-enclosed nuclei. This larger limb includes the plants and fungi. Perhaps the most distinctive characteristic of the animals as a group is their means of nutrition, which consists of eating other organisms. This basic way of life has led to the evolution of many diverse systems for locomotion and for capturing and processing a wide array of food items.

Animals can be distinguished also by the absence of properties that have evolved in other eukaryotes. Plants, for example, have evolved the ability to use light energy to produce organic compounds (photosynthesis), and they have evolved rigid cell walls that surround their cell membranes; photosynthesis and cell walls are absent from animals. Fungi have evolved the ability to acquire nutrition by absorption of small organic molecules from their environment, and they have a body plan consisting of tubular filaments called hyphae; structures of this kind are absent from the animal kingdom.

Some organisms combine the properties of animals and plants. For example, Euglena (Figure 1-10) is a motile, single-celled organism that resembles plants in being photosynthetic, but it resembles animals in its ability to eat food particles. Euglena is part of a separate eukaryotic lineage that diverged from those of plants and animals early in the evolutionary history of eukaryotes. Euglena and other unicellular eukaryotes are sometimes grouped into the kingdom Protista, although this kingdom is an arbitrary grouping of unrelated lineages that violates taxonomic principles (see Classification and Phylogeny of Animals).

The fundamental structural and developmental features evolved by the animal kingdom are presented in detail in Principles of Development and Architectural Pattern of an Animal.