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Animals (from the Latin animale for "having breath"[1]) are a diverse group of life on Earth, consisting currently of around two million known species belonging to the biological kingdom Animalia, also known as Metazoa (literally "following animals", see below). Most of these species are insects; many of those are beetles. Animals are all motile at some point during their lives, have many cells (except in the zygotic stage), get their nutrients from other sources and have cells without walls. However, the definition of "animal" has changed over the centuries.

History and definition[edit]

In all likelihood, humans have recognised the basic concept of "animal" as long as we have had the capacity to do so - by circular definition - long before we knew anything about cells. Woolly mammoths move around, eat food, and when killed can be considered meat. This made "animals" (such as edible locusts) distinct from "bugs". This simple, intuitive method would have worked well enough for our hunter-gatherer ancestors, because it was important to recognise what you can eat and what can eat youWikipedia. When humans begun to take a look at the seas, they saw that things like "shellfish" didn't necessarily move, but were still a form of "meat". "Animals" could be considered as having a "flesh and blood", as reflected in a biblical statement, Genesis 9:2-4:

...the dread of you [ Noah and his family] shall be upon every beast of the earth, and upon every fowl of the air, upon all that moveth upon the earth, and upon all the fishes of the sea; into your hand are they delivered. Every moving thing that liveth shall be meat for you....But flesh with the life thereof, which is the blood thereof, shall ye not eat.
—Genesis 9:2-4, KJV

An early attempt to classify life came in 350 BCE from Aristotle, who put living things into two groups; "plants" and "animals"[2]. Aristotle used methods based on superficial characteristics, like habitat, means of movement, and whether or not subjects have red blood.

Aristotle's natural history was mostly forgotten in the Dark Ages, mainly retained by the Church to specify what good Christians can or can't eat during Lent.

Some circles have tended to characterise animals using the word "creature" (implying a sense of things created) - which has the advantage of grouping humans with other animals; whereas the term "beast" emphasises the "sub-human" characteristics of large vertebrates.

In the Enlightenment, due to mounting importance of observation-based and evidence-based knowledge, the need for a better system of classification re-emerged. John RayWikipedia first coined the term "species" biologically and classified plants in Historia Plantarum (published in 1686), using a system based on similarities and differences on close observation rather than predefined "kinds" or "types". Ray's work would later influence Carl LinnaeusWikipedia, who published the first edition of his Systema Naturae in 1735, which established "Animalia" and "Vegetablia" as the two "kingdoms" of life (Linnaeus also included a "mineral kingdom" to classify the inorganic natural world, but science has since abandoned it). Linnaeus initially divided his animal kingdom into six classes:

  • Quadrupedia (all mammals excluding the whales and dolphins, initially classified as fish)
  • Aves
  • Amphibia (including reptiles)
  • Pisces (remaining vertebrates [sensu stricto][3])
  • Insecta (= Arthropoda [sensu praesens][4])
  • Vermes (the rest of the modern animal kingdom not in the above groups, plus amoebae)

The tenth edition of the Linnean Systema, published in 1758, also saw the true establishment of binomial nomenclature still used today to name species. As the decades passed, biologists added the taxonomic levels of family and phylum, split up Vermes into various groups, and made many other such changes occurred. As of c. 1980, zoology divided "animals" into the following subkingdoms:

  • Protozoa ("first animals"), undifferentiated eukaryotes that do not have rigid cell walls or produce filamentous threads. Members have no embryology, so the MyxozoaWikipedia were included in their ranks.
  • Mesozoa ("middle animals", believed to be midway between protozoa and "true animals"), simple many-celled vermiform parasites of cephalopods and others[5] Members have a very simple embryology.
  • Parazoa ("near/beside animals", believed to be the closest relatives of "true" animals), including the sponges (phylum Porifera) and the Trichoplax adhaeransWikipedia; these simple animals had many cells and an embryology including a gastrulaWikipedia, but no true differentiated tissues.
  • Metazoa ("following animals", because they followed the parazoa in chronological order) included all modern animals except sponges and trichoplaxes (see Parazoa, above). Now considered equal to the modern Eumetazoa.

Since about 1990, evolutionary relationships have become an integral part of taxonomy. The field of phylogenetics has taken over, which led to the Protozoa and Metazoa being dissolved and split into various groups. The modern kingdom Metazoa includes 1980's Metazoa, Mesozoa, Myxozoa, and Parazoa. The word "protozoan" is still sometimes used as an informal group, or evolutionary "grade".

Modern definition[edit]

As stated in the introduction, the modern Kingdom Metazoa includes permanently (excluding the zygote) many-celled organisms that undergo development (including at least one motile stage) in their life history. All the Metazoa and only the Metazoa produce a structural protein, collagen (this criterion prevents creatures like the Volvox from being animals as Linnaeus classified them). Typically, the "animals" are listed as being heterotrophic (i.e, needing to organic material to survive), although lineages like the beard wormsWikipedia, emerald sea slugsWikipedia, and possibly the Ediacaran biota (see below) house bacteria that produce their nutrients from within so these lineages do not need to eat other organisms.

At law[edit]

Animals are chattel property. Wild animals freely roaming are considered the common property of all, and those who manage to capture or kill one acquire it as their property.[6] Given animals' status as items of chattel property, the human owners of animals are entitled to kill them, for food, sport, or any reasons; these rights, however, are subject to the police power of the state, which may restrict them in order to guard public health, welfare, or morality.[7]


There are two leading opinions regarding the origins of animals: the explanation supported by hundreds of thousands of peer-reviewed papers by thousands of scientists over 150+ years from all walks of life, gathered and refined through the most rigourous methods possible and the bullshit explanation.


The kingdom Metazoa's oldest fossil record appears in the Ediacaran period, 630 million to 542 million years ago. As indicated by genetic and fossil records, the first animals were a bit like sponges, jellyfish, and sea pens. The very first animals with a distinct head and tail appeared 600 million years agoWikipedia. Over a period of time lasting up to 80 million years, these bilaterians diversified into various body plans and gained hard parts like armour, shells, and bone. This important evolutionary process has been named the Cambrian explosion because initially, before soft-bodied fossils were discovered, it appeared several major groups of animals appeared to appear from nothing. Appearances are deceiving, however. Animals have since diversified since then.


Some people believe with no empirical evidence to support their views that some old psychopath poofed every living thing into existence over a period of six days. Animals are said to have been created over the fifth and sixth days of this week. Humans are also believed to be distinct from animals because humans have the ability to be "born again" and accept God into their hearts. These claims are with absolutely no supporting evidence, and have massive refuting evidence, although they somehow gain quite a following.

See also[edit]


  1. See Online Etymology Dictionary discussion. The Latin root-word anima can mean "breath" or "soul" - hence some scope for theological confusion over whether animals can have souls and whether human animals alone have divinely-imparted souls/breath. Compare concepts like inspirationWikipedia - literally "breathing in" - or "spirit".
  2. Read the book in English online
  3. "Vertebrata" is considered distinct from "Craniata", as supported by some (not all) genetic and morphological analyses. (The other hypothesis is that lampreys and hagfish are closely related, forming one group, Cyclostomata. This is supported by other, mostly genetic, analyses. You can argue either way with fair levels of support.) The hagfish are included in the latter but not the former. When Linnaeus added the hagfish, he included them in "Vermes".
  4. Latin, "in the present sense".
  5. See Wikipedia.Wikipedia
  6. Bostock-Ferari Amusement Co. v. Brocksmith, 34 Ind. App. 566, 73 N.E. 281 (1905)
  7. Youngstown v. Traylor, 123 Ohio St. 3d 132, 914 N.E.2d 1026 (2009)