Animals

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Biography

Animals are a major group of multicellular, eukaryotic organism s of the Kingdom (biology)|kingdom Animalia or Metazoa . Their body plan eventually becomes fixed as they Developmental biology|develop , although some undergo a process of metamorphosis later on in their life. Most animals are Motility|motile , meaning they can move spontaneously and independently. All animals are also heterotroph s, meaning they must ingest other organisms or their products for sustenance .

Most known animal phylum|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 ". In everyday colloquial usage, the word often refers to non- human members of kingdom Animalia. Sometimes, only closer relatives of humans such as mammal s and other vertebrate s are meant in colloquial use. The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects and humans.

Characteristics

Animals have several characteristics that set them apart from other living things. Animals are eukaryote|eukaryotic and mostly multicellular , which separates them from bacteria and most protist s. They are heterotroph ic, generally digesting food in an internal chamber, which separates them from plant s and alga e. They are also distinguished from plants, algae, and fungus|fungi by lacking rigid cell wall s. All animals are motility|motile , if only at certain life stages. In most animals, embryo s pass through a Blastula|blastula stage , which is a characteristic exclusive to animals.

Structure

With a few exceptions, most notably the sponge s (Phylum Porifera ) and Placozoa , animals have body|bodies differentiated into separate biological tissue|tissues . These include muscle s, which are able to contract and control locomotion, and nervous system|nerve tissues , which send and process signals. Typically, there is also an internal digestion|digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoan s when the former is used for animals in general.

All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoprotein s. This may be calcified to form structures like animal shell|shells , bone s, and spicule s. During development, it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organism s, like plants and fungi, have cells held in place by cell walls, and so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: tight junction s, gap junction s, and desmosome s.

Reproduction and development

Nearly all animals undergo some form of sexual reproduction . They have a few specialized Gamete|reproductive cells , which undergo meiosis to produce smaller, motile spermatozoon|spermatozoa or larger, non-motile ovum|ova . These fuse to form zygote s, which develop into new individuals.

Many animals are also capable of asexual reproduction . This may take place through parthenogenesis , where fertile eggs are produced without mating, budding, or fragmentation (reproduction)|fragmentation .

A zygote initially develops into a hollow sphere, called a blastula , which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invagination|invaginates to form a gastrula with a digestive chamber, and two separate germ layer s& nbsp;— an external ectoderm and an internal endoderm . In most cases, a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs.

Food and energy sourcing

All animals are heterotroph s, meaning that they feed directly or indirectly on other living things. They are often further subdivided into groups such as carnivore s, herbivore s, omnivore s, and parasite s.

Predation is a biological interaction where a predator (a heterotroph that is hunting) feeds on its prey (the organism that is attacked).Begon, M., Townsend, C., Harper, J. (1996). Ecology: Individuals, populations and communities (Third edition). Blackwell Science, London. ISBN 0-86542-845-X, ISBN 0-632-03801-2, ISBN 0-632-04393-8. Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of the prey.. Britannica.com. Retrieved on 2011-11-23. The other main category of consumption is detritivory , the consumption of dead organic matter . It can at times be difficult to separate the two feeding behaviour s, for example, where parasite|parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. Selective pressures imposed on one another has led to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations .

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 carbohydrate s, fat s, protein s and other biomolecules, characteristically containing Reduction (chemistry)|reduced carbon in the form of carbon-hydrogen bonds. Starting with carbon dioxide (CO₂) and water (H₂O), 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. 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. 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.

Animals living close to hydrothermal vent s and cold seep s on the Seabed|ocean floor are not dependent on the energy of sunlight. Instead chemosynthesis|chemosynthetic archaea and bacteria form the base of the food chain .

Origin and fossil record

Animals are generally considered to have Evolution|evolved from a flagellate d eukaryote. Their closest known living relatives are the choanoflagellate s, collared flagellates that have a morphology similar to the choanocytes of certain sponges. Molecular studies place animals in a supergroup called the opisthokont s, which also include the choanoflagellates, fungus|fungi and a few small parasitic protist s. 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.

The first fossils that might represent animals appear in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia. These fossils are interpreted as being early sponges. They were found in 665-million-year-old rock.

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 biota|Ediacaran or Vendian biota . 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.

Aside from them, most known animal phyla make a more or less simultaneous appearance during the Cambrian period, about 542 million years ago. 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. Trace fossil s such as tracks and burrows found in the Tonian era indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5& nbsp;mm wide) and complex as earthworms . 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 Permian–Triassic extinction event|End Permian and Ordovician–Silurian extinction event|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.

Groups of animals

Porifera, Radiata and basal Bilateria

Phylogenetic analysis suggests that the Porifera and Ctenophora diverged before a clade that gave rise to the Bilateria , Cnidaria and Placozoa .Ryan JF, Pang K; NISC Comparative Sequencing Program, Mullikin JC, Martindale MQ, Baxevanis AD (2010) The homeodomain complement of the ctenophore Mnemiopsis leidyi suggests that Ctenophora and Porifera diverged prior to the ParaHoxozoa. Evodevo 1(1):9

The sponges ( Porifera ) were long thought to have diverged from other animals early. They lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges typically feed by drawing in water through pores. Archaeocyatha , which have fused skeletons, may represent sponges or a separate phylum. 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.

Among the other phyla, the Ctenophora and the Cnidaria , which includes sea anemone s, coral s, and jellyfish , are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. Both have distinct tissues, but they are not organized into organ (anatomy)|organs . 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 . The tiny placozoan s 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 Symmetry (biology)|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& nbsp;— for instance adult echinoderm s are radially symmetric, and certain parasitic worm s 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 deuterostome s and the protostome s, 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

Deuterostome s differ from the other Bilateria, called protostome s, 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. 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. Deuterostome embryos undergo radial Cleavage (embryo)|cleavage during cell division, while protostomes undergo spiral cleavage.

All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the Echinodermata and Chordate|Chordata . The former are radially symmetric and exclusively marine, such as starfish , sea urchin s, and Holothuroidea|sea cucumbers . The latter are dominated by the vertebrate s, animals with backbones. These include fish , amphibian s, reptile s, bird s, and mammal s.

In addition to these, the deuterostomes also include the Hemichordata , or acorn worms. Although they are not especially prominent today, the important fossil graptolite s may belong to this group.

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 . The largest animal phylum belongs here, the Arthropoda , including insect s, spider s, crab s, 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. A number are important parasites. 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

The Platyzoa include the phylum Platyhelminthes , the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors. A number of parasites are included in this group, such as the Trematoda|flukes and tapeworm s. Flatworms are Body cavity#No coelom (Acoelomate)|acoelomates , lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha .

The other platyzoan phyla are mostly microscopic and Body cavity#Pseudocoelomates|pseudocoelomate . The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida , Micrognathozoa , and possibly the Cycliophora . These groups share the presence of complex jaws, from which they are called the Gnathifera (phylum)|Gnathifera .

Lophotrochozoa

The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida . The former, which is the second-largest animal phylum by number of described species, includes animals such as snail s, clam s, and squid s, and the latter comprises the segmented worms, such as earthworm s and leech es. 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. Now, this is generally considered convergent evolution , owing to many morphological and genetic differences between the two phyla.

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 . These were traditionally grouped together as the lophophorates. but it now appears that the lophophorate group may be paraphyletic , with some closer to the nemerteans and some to the molluscs and annelids. 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.

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 organism s, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their genome s, but as many gene s, intron s, and genetic linkage|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 project s currently in progress. Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellate s, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.

An analysis of the Homoscleromorpha|homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals was more complex than previously assumed.

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 plant s, and this was followed by Carolus Linnaeus (Carl von Linné), in the first hierarchical classification. 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 in the 10th edition of Systema Naturae|Vermes , Insecta in the 10th edition of Systema Naturae|Insect a, Pisces in the 10th edition of Systema Naturae|Pisces , Amphibia in the 10th edition of Systema Naturae|Amphibia , Aves in the 10th edition of Systema Naturae|Aves , and Mammalia in the 10th edition of Systema Naturae|Mammalia . Since then the last four have all been subsumed into a single phylum, the chordate|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.

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