Final answer:
Animals are eukaryotic, multicellular organisms without cell walls that are typically motile and reproduce sexually. They are heterotrophic, requiring ingestion of food for energy, and have specialized tissues. Their evolution from unicellular ancestors represents a significant event in the history of life on Earth.
Step-by-step explanation:
Characteristics of Animals, Animals are eukaryotic, multicellular organisms that are distinguished by their lack of cell walls, heterotrophic nutrition, and the ability to move or be motile at some stage in their lifecycle. They typically have specialized tissues, including nervous, muscular, connective, and epithelial that perform various functions.
Unlike plants and fungi, animals ingest food sources directly, being either carnivores, herbivores, omnivores, or parasites. The evolution of eukaryotes led to the development of complex multicellular structures, which can be traced back to approximately 1.5 billion years ago.
These eukaryotic cells have nuclei and organelles enclosed within membranes, separating them from the simpler prokaryotic cells. Animals, as multicellular eukaryotes, exhibit a range of sizes and complexities, extending from minute yeasts to large organisms such as giant sequoias and dinosaurs.
Most animals are capable of sexual reproduction, leading to offspring that go through developmental stages, following a predetermined body plan – a notable distinction from plants. The ability to move, complex cell structures, and sexual reproduction are fundamental traits that unify the diverse animal kingdom.