Final answer:
Animal cells are generally part of multicellular organisms, with specialized and organized systems of cells within tissues and organs. Unicellular organisms consist of a single cell, while multicellularity involves complexity and cell specialization. The evolution from unicellular to multicellular life has occurred multiple times throughout the history of life on Earth.
Step-by-step explanation:
Animal cells are generally part of multicellular organisms. This means that animals are made up of multiple cells, which have different shapes and perform specialized functions. Each of these cells contributes to the overall structure and function of tissues and organs in the animal body. Unlike unicellular organisms that consist of a single cell performing all the necessary functions for life, multicellular organisms like animals have a complex structure where cells are specialized for tasks such as nerve conduction, muscle contraction, and digestion.
Although most animals and plants are multicellular, the world of living organisms includes a vast array of life forms with different cellular structures. For example, protists can be unicellular; however, some, like kelp, are surprisingly large and multicellular, but do not show cellular specialization. The transition from unicellular to multicellular life has occurred multiple times throughout the evolution of life, with varying degrees of complexity and specialization in multicellular arrangements.
It's noteworthy that every multicellular plant and animal begins life as a single cell, which is a result of the union of an ovum and a sperm. As the organism develops, it becomes a highly organized system of cells, each with its role. True multicellularity, involving specialized cell types, evolved in animals, plants, and some fungi.