Animals are a major group of multicellular,
eukaryotic organisms of the
kingdom Animalia or
Metazoa. Their
body plan eventually becomes fixed as they
develop, although some undergo a process of
metamorphosis later on in their life. Most animals are
motile, meaning they can move spontaneously and independently. All animals are also
heterotrophs, meaning they must ingest other organisms or their products for
sustenance.
Most known animal
phyla appeared in the fossil record as marine species during the
Cambrian explosion, about 542 million years ago.
Etymology
The word "animal" comes from the
Latin word
animalis, meaning "having
breath".
[1] In everyday colloquial usage, the word often refers to non-
human members of kingdom Animalia. Sometimes, only closer relatives of humans such as
mammals and other
vertebrates are meant in colloquial use.
[2] The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects and humans.
[3]
Characteristics
Structure
With a few exceptions, most notably the
sponges (Phylum
Porifera) and
Placozoa, animals have
bodies differentiated into separate
tissues. These include
muscles, which are able to contract and control locomotion, and
nerve tissues, which send and process signals. Typically, there is also an internal
digestive chamber, with one or two openings.
[10] Animals with this sort of organization are called metazoans, or
eumetazoans when the former is used for animals in general.
[11]
All animals have
eukaryotic cells, surrounded by a characteristic
extracellular matrix composed of
collagen and elastic
glycoproteins.
[12] This may be calcified to form structures like
shells,
bones, and
spicules.
[13] During development, it forms a relatively flexible framework
[14] upon which cells can move about and be reorganized, making complex structures possible. In contrast, other
multicellular organisms, like
plants and fungi, have cells held in place by cell walls, and so develop by progressive growth.
[10] Also, unique to animal cells are the following intercellular junctions:
tight junctions,
gap junctions, and
desmosomes.
[15]
Reproduction and development
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 — 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.
[26]
Food and energy sourcing
Most animals indirectly use the energy of
sunlight by eating plants or plant-eating animals. Most plants use light to convert
inorganic molecules in their environment into
carbohydrates,
fats,
proteins and other biomolecules, characteristically containing
reduced carbon in the form of carbon-hydrogen bonds. Starting with
carbon dioxide (CO
2) and
water (H
2O), photosynthesis converts the energy of sunlight into chemical energy in the form of simple sugars (e.g.,
glucose), with the release of molecular
oxygen. These sugars are then used as the building blocks for plant growth, including the production of other biomolecules.
[10] When an animal eats plants (or eats other animals which have eaten plants), the reduced carbon compounds in the food become a source of energy and building materials for the animal.
[33] They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion.
[34] [35]
Origin and fossil record
Animals are generally considered to have
evolved from a
flagellated eukaryote.
[39] Their closest known living relatives are the
choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges.
[40] Molecular studies place animals in a supergroup called the
opisthokonts, which also include the choanoflagellates,
fungi and a few small parasitic
protists.
[41] The name comes from the posterior location of the
flagellum in motile cells, such as most animal spermatozoa, whereas other
eukaryotes tend to have anterior flagella.
[42]
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.
[45]
Aside from them, most known animal phyla make a more or less simultaneous appearance during the
Cambrian period, about 542 million years ago.
[46] It is still disputed whether this event, called the
Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible.
Some paleontologists suggest that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.
[47] Trace fossils such as tracks and burrows found in the
Tonian era indicate the presence of
triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex as
earthworms.
[48] During the beginning of the Tonian period around 1 billion years ago, there was a decrease in
Stromatolite diversity, which may indicate the appearance of grazing animals, since stromatolite diversity increased when grazing animals went extinct at the
End Permian and
End Ordovician extinction events, and decreased shortly after the grazer populations recovered. However the discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist
Gromia sphaerica casts doubt on their interpretation as evidence of early animal evolution.
[49][50]
Groups of animals
The relative number of species contributed to the total by each phylum of animals.
Porifera, Radiata and basal Bilateria
The sponges (
Porifera) were long thought to have diverged from other animals early.
[52] They lack the complex organization found in most other phyla.
[53] Their cells are differentiated, but in most cases not organized into distinct tissues.
[54] Sponges typically feed by drawing in water through pores.
[55] Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum.
[56] However, a phylogenomic study in 2008 of 150 genes in 29 animals across 21 phyla revealed that it is the
Ctenophora or comb jellies which are the basal lineage of animals, at least among those 21 phyla. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.
[57]
Among the other phyla, the Ctenophora and the
Cnidaria, which includes
sea anemones,
corals, and
jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.
[58] Both have distinct tissues, but they are not organized into
organs.
[59] There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called
diploblastic.
[60] The tiny
placozoans are similar, but they do not have a permanent digestive chamber.
The remaining animals form a
monophyletic group called the
Bilateria. For the most part, they are
bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is
triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a
coelom or pseudocoelom. There are exceptions to each of these characteristics, however — for instance adult
echinodermsare radially symmetric, and certain
parasitic worms have extremely simplified body structures.
Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to two major lineages: the
deuterostomes and the
protostomes, the latter of which includes the
Ecdysozoa,
Platyzoa, and
Lophotrochozoa. In addition, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the
Acoelomorpha,
Rhombozoa, and
Orthonectida. The
Myxozoa, single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Medusozoa as well.
Deuterostomes
Deuterostomes differ from the other Bilateria, called
protostomes, in several ways. In both cases there is a complete digestive tract. However, in protostomes, the first opening of the gut to appear in embryological development (the
archenteron) develops into the mouth, with the anus forming secondarily. In deuterostomes the anus forms first, with the mouth developing secondarily.
[61] In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes, it forms through
invagination of the endoderm, called enterocoelic pouching.
[62] Deuterostome embryos undergo radial
cleavage during cell division, while protostomes undergo spiral cleavage.
[63]
In addition to these, the deuterostomes also include the
Hemichordata, or acorn worms.
[68] Although they are not especially prominent today, the important fossil
graptolites may belong to this group.
[69]
The
Chaetognatha or arrow worms may also be deuterostomes, but more recent studies suggest protostome affinities.
Ecdysozoa
The
Ecdysozoa are protostomes, named after the common trait of growth by moulting or
ecdysis.
[70] The largest animal phylum belongs here, the
Arthropoda, including
insects,
spiders,
crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the
Onychophora and
Tardigrada, are close relatives of the arthropods and share these traits.
The ecdysozoans also include the
Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water.
[71] A number are important parasites.
[72] Smaller phyla related to them are the
Nematomorpha or horsehair worms, and the
Kinorhyncha,
Priapulida, and
Loricifera. These groups have a reduced coelom, called a pseudocoelom.
The remaining two groups of protostomes are sometimes grouped together as the
Spiralia, since in both embryos develop with
spiral cleavage.
Platyzoa
Lophotrochozoa
The
Lophotrochozoa include two of the most successful animal phyla, the
Mollusca and
Annelida.
[77][78] The former, which is the second-largest animal phylum by number of described species, includes animals such as
snails,
clams, and
squids, and the latter comprises the segmented worms, such as
earthworms and
leeches. These two groups have long been considered close relatives because of the common presence of
trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented.
[79] Now, this is generally considered
convergent evolution, owing to many morphological and genetic differences between the two phyla.
[80]
The Lophotrochozoa also include the
Nemertea or ribbon worms, the
Sipuncula, and several phyla that have a ring of ciliated tentacles around the mouth, called a
lophophore.
[81] These were traditionally grouped together as the lophophorates.
[82] but it now appears that the lophophorate group may be
paraphyletic,
[83] with some closer to the nemerteans and some to the molluscs and annelids.
[84][85] They include the
Brachiopoda or lamp shells, which are prominent in the fossil record, the
Entoprocta, the
Phoronida, and possibly the
Bryozoa or moss animals.
[86]
Model organisms
Because of the great diversity found in animals, it is more economical for scientists to study a small number of chosen species so that connections can be drawn from their work and conclusions extrapolated about how animals function in general. Because they are easy to keep and breed, the fruit fly
Drosophila melanogaster and the nematode
Caenorhabditis elegans have long been the most intensively studied metazoan
model organisms, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their
genomes, but as many
genes,
introns, and
linkages lost, these ecdysozoans can teach us little about the origins of animals in general. The extent of this type of evolution within the superphylum will be revealed by the crustacean, annelid, and molluscan
genome projects currently in progress. Analysis of the
starlet sea anemone genome has emphasised the importance of sponges, placozoans, and
choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.
[87]
An analysis of the
homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals was more complex than previously assumed.
[88]
Other model organisms belonging to the animal kingdom include the house mouse (
Mus musculus) and zebrafish (
Danio rerio).
History of classification
Aristotle divided the living world between animals and
plants, and this was followed by
Carolus Linnaeus (Carl von Linné), in the first hierarchical classification.
[89] 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.
See also
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