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We know one when we see one. A dog is an animal. We are animals. What about a sponge? Sponges are animals, too. That's right—we mean SpongeBob.
All life is divided into six kingdoms:
Animals make up one kingdom. It is our kingdom, unless you happen to be an amoeba (a really smart amoeba). Bacteria and archaea are prokaryotes, generally single-celled organisms without organs or a nucleus separated by membranes. Bacteria, this means you. Prokaryotes are the oldest form of life and were around for millions of years before other kinds of life evolved.
The rest of the kingdoms are eukaryotes. Eukaryotic cells have a nucleus and other organs enclosed by a membrane. Eukaryotes can be single-celled or multi-celled.
Protists are the original kind of eukaryote. If you are that really smart amoeba, this is your group. Protists are single-celled and vary so much that they are hard to categorize. They include algae and the bringers of fun diseases like malaria and sleeping sickness.
Fungi, plants, and animals are multicellular eukaryotes. Plants are the only group of the three that can make their own food, which is called being an autotroph. Animals eat other organisms, which is called being heterotrophic. Why make my own when I can have yours? Another difference is cell walls. Plants have them and animals don't. Fungi are heterotrophs like animals, but have their own kind of cell wall and get their nutrients in an entirely different manner than animals.
So far, we know already know a few things about animals: they are multi-cellular, heterotrophic, and have no cell walls. Animals also generally like to move around. In fact, many of us need to move in order to find all those organisms to eat. We use some unique tissues to do this, like muscles and nerves.
All organisms have to reproduce, but there are many ways to do that. Animals rely on sexual reproduction. Animals have male and female forms and reproduction requires that they work together. It takes two to tango. Usually.
Another characteristic of animals is that their dominant form is diploid. This means that for the majority of an animal's life, it is made up of cells that have two sets of chromosomes. Each diploid cell has a pair of chromosomes, one from each parent. This is not true of plants, for instance, which alternate between diploid and haploid forms and may have either form dominate, depending on the species. Haploid cells have a single set of chromosomes from just one parent. In other words, a haploid organism is a copy of a single adult and a diploid organism is a product of the combination of two parents.
Every animal has almost all or all of these characteristics.
Animals all came from a common ancestor. One organism eventually led to millions of animal species. Here is a current theory on where it all began.