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Recent News
Mar. 25, 2024, 11:54 PM ET (Washington Post)
Top Questions
What is an animal?
Animals are multicellular eukaryotes whose cells are bound together by collagen. Animals dominate human conceptions of life on Earth because of their size, diversity, abundance, and mobility. The presence of muscles and mobility is one of the primary characteristics of the animal kingdom.
What are the two major groups of animals?
The two major groups of animals are vertebrates and invertebrates. Vertebrates have backbones, and invertebrates do not.
When did animals first appear?
Animals first appeared in the Ediacaran Period, about 635 million to 541 million years ago, as soft-bodied forms that left traces of their bodies in shallow-water sediments.
What are the basic functional systems of animals?
Animals' basic functional systems include a musculoskeletal system, for supporting and moving the body; a nervous system, for receiving and processing sensory information and for carrying signals to control muscle and hormone activity; an endocrine system, for secreting hormones to chemically control bodily functions; a digestive system, for receiving and processing food; a circulatory system, for carrying nutrients and oxygen to cells and carrying away their wastes; and a reproductive system, for producing offspring to ensure species survival.
How are animals different from plants and fungi?
Animals differ from plants and fungi in fundamental variations in morphology and physiology, notably in regard to animals’ development of muscles and mobility.
animal, (kingdom Animalia), any of a group of multicellular eukaryotic organisms (i.e., as distinct from bacteria, their deoxyribonucleic acid, or DNA, is contained in a membrane-bound nucleus). They are thought to have evolved independently from the unicellular eukaryotes. Animals differ from members of the two other kingdoms of multicellular eukaryotes, the plants (Plantae) and the fungi (Mycota), in fundamental variations in morphology and physiology. This is largely because animals have developed muscles and hence mobility, a characteristic that has stimulated the further development of tissues and organ systems.
Animals dominate human conceptions of life on Earth not simply by their size, abundance, and sheer diversity but also by their mobility, a trait that humans share. So integral is movement to the conception of animals that sponges, which lack muscle tissues, were long considered to be plants. Only after their small movements were noticed in 1765 did the animal nature of sponges slowly come to be recognized.
In size animals are outdone on land by plants, among whose foliage they may often hide. In contrast, the photosynthetic algae, which feed the open oceans, are usually too small to be seen, but marine animals range to the size of whales. Diversity of form, in contrast to size, only impinges peripherally on human awareness of life and thus is less noticed. Nevertheless, animals represent three-quarters or more of the species on Earth, a diversity that reflects the flexibility in feeding, defense, and reproduction which mobility gives them. Animals follow virtually every known mode of living that has been described for the creatures of Earth.
Animals move in pursuit of food, mates, or refuge from predators, and this movement attracts attention and interest, particularly as it becomes apparent that the behaviour of some creatures is not so very different from human behaviour. Other than out of simple curiosity, humans study animals to learn about themselves, who are a very recent product of the evolution of animals.
Britannica QuizDeadliest Animals QuizThe animal kingdom
Animals evolved from unicellular eukaryotes. The presence of a nuclear membrane in eukaryotes permits separation of the two phases of protein synthesis: transcription (copying) of deoxyribonucleic acid (DNA) in the nucleus and translation (decoding) of the message into protein in the cytoplasm. Compared to the structure of the bacterial cell, this gives greater control over which proteins are produced. Such control permits specialization of cells, each with identical DNA but with the ability to control finely which genes successfully send copies into the cytoplasm. Tissues and organs can thus evolve. The semirigid cell walls found in plants and fungi, which constrain the shape and hence the diversity of possible cell types, are absent in animals. If they were present, nerve and muscle cells, the focal point of animal mobility, would not be possible.
