Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus (Carl von Linne), in the first hierarchical classification.[91] Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic protozoa were originally considered animals because they move, but are now treated separately. In Linnaeus's original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out. The above lists represent our current understanding of the group, though there is some variation from source to source.

Nearly all animals undergo some form of sexual reproduction.[16] They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova.[17] These fuse to form zygotes, which develop into new individuals.[18] Many animals are also capable of asexual reproduction.[19] This may take place through parthenogenesis, where fertile eggs are produced without mating, budding, or fragmentation.[20] A zygote initially develops into a hollow sphere, called a blastula,[21] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.[22] In most other groups, the blastula undergoes more complicated rearrangement.[23] It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers Ч an external ectoderm and an internal endoderm.[24] In most cases, a mesoderm also develops between them.[25] These germ layers then differentiate to form tissues and organs. The first fossils that might represent animals appear in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia.[43] These fossils are interpreted as being early sponges. They were found in 665-million-year-old rock.[43] The next oldest possible animal fossils are found towards the end of the Precambrian, around 610 million years ago, and are known as the Ediacaran or Vendian biota.[44] These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all