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|The science of life|
Animals (from the Latin for "having breath") are the most diverse group of life on Earth, consisting of currently around two million known species belonging to the kingdom Metazoa (literally "following animals", see below). Most of these species are insects, namely beetles. Animals are all motile at some point during their lives, have many cells except in the zygotic stage, and their cells do not have walls. However, the definition of "animal" has changed over the centuries.
 History and definition
In all likelihood, humans have recognised the basic concept of "animal" as long as we have had the capacity to do so, long before we knew anything about cells. That woolly mammoth moves around, eats food, and when killed can be considered meat. This made "animals" 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 you. 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|
The first real attempt to classify life was Aristotle in 350 BC, who put it into two groups; "plants" and "animals". Aristotle's methods were based on superficial characteristics, like habitat, means of movement, and whether or not they have red blood.
Aristotle's natural history was mostly forgotten in the Dark Ages, mainly retained by the Church for things like what can or can't be eaten during Lent, until in the Enlightenment, the need for a better system of classification re-emerged. Carl Linnaeus 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 this has since been abandoned). Linnaeus initially divided his animal kingdom into six classes:
- Quadrupedia (now mammals excluding the whales and dolphins, initially classified as fish)
- Amphibia (including reptiles)
- Pisces (remaining vertebrates [sensu stricto])
- Insecta (= Arthropoda [sensu praesens])
- Vermes (the rest of the modern animal kingdom not in the above groups, plus amoebae)
The tenth edition of the Systema, published in 1758, also saw the true establishment of binomial nomenclature to name species now used today. As the decades passed, the taxonomic levels of family and phylum were added, Vermes was split up into various groups, and many other such changes occurred. As of c. 1980, "animals" were divided 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 Myxozoa 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 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 adhaerans; these simple animals had many cells and an embryology including a gastrula, 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.
In the past 20 or so years, 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
As stated in the introduction, the modern Kingdom Metazoa includes permanently (exc. 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 like Linnaeus classified them). Typically, the "animals" are listed as being heterotrophic (i.e, needing to eat other organisms to survive), although lineages like the beard worms, emerald sea slugs, 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.
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 ago. 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
- ↑ Read the book in English online
- ↑ "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".
- ↑ Latin, "in the present sense".
- ↑ See Wikipedia.