Jump to: navigation, search
Parent taxa:
(check the following menu and phylogeny - the taxon in bold refers to the topic on this page)

Eukarya - The Eukaryotes
Taxonomy phylogeny and hypothetical timeline (not to scale)
* Domain: EUKARYA
* Subdomain: "Protista"
* Subdomain: Metabionta
** Kingdom: Plantae
** Kingdom: Fungi
** Kingdom: Metazoa

ref. Whittaker & Margulis, 1978; Mayr 1990
Arch.Paleopr Mesopr  Neoproterozoic Phanerozoic

`--o Eukarya
   |--Metamonada ------------------------------
   `--+--Discicristata ------------------------
      `--+--Rhizaria --------------------------
         `--o Metabionta
            |--+--o Chromalveolata:
            |  |  |--Alveolata ----------------
            |  |  `--Chromista ----------------
            |  `--o Archaeplastida 
            |     |--Rhodophyta ---------------
            |     `--Chlorobionta  ------------
            `----------+--Amoebozoa  ----------
                       `--o Opisthokonta 
                             |--Fungi ---------
                             `--Metazoa -------

(See also Alternative Eukarya phylogeny)

Metazoa - Multicelluar Animals

<-- Graphic goes here-->



The Metazoa or Animalia are a major group or kingdom of multi­cellular eukaryotic organisms. In general they are capable of locomotion, responsive to their environment, and feed by consuming other organisms. Their body plan becomes fixed as they develop, usually early on in their development as embryos, although some undergo a process of metamorphosis later on. They are found in all habitable environments, and range in size from microscopic to the blue whale in the sea and the largest sauropod dinosaurs on land.

They are thought to have evolved from a unicellular choanoflagellate-like common ancestor, sometime during the Ediacaran period.

Early on, Metazoa developed body layers, organs, a nervous system (nerve net) and finally a head, advanced sensory organs, and a brain, leading to the development of intelligence. For this reason, philosophers - as metazoans - tend to chauvinistically present the group as part of a straight line of evolution. In fact, the Metazoa or Animal Kingdom are only one of a number of groups of advanced multicellular Eukaryotes. However, they are certainly the most numerous of these in terms of numbers of species, although this is thanks mostly the extraordinarily prolific quality of terrestrial arthropods.

(The following menu and phylogeny refers to subtopics of this page)

Taxonomy Phylogeny
   `==Porifera (paraphyletic?)
      `==Radiata (paraphyletic?)
            `==Acoelomorpha (paraphyletic?)
               |  |--Ecdysozoa
               |  `--Lophotrochozoa


the following three sections are from Wikipedia:

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and usually multicellular (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking cell walls.


With a few exceptions, most notably the sponges (Phylum Porifera), animals have bodies differen­tiated into separate tissues. These include muscles, which are able to contract and control locomotion, and a nervous system, which sends and processes signals. There is also typically an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans 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 glycoproteins. This may be calcified to form structures like shells, bones, and spicules. 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 organisms 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 junctions, gap junctions, and desmosomes.

Reproduction and development

Nearly all animals undergo some form of sexual reproduction. Adults are diploid or polyploid. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These fuse to form zygotes, 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, or in some cases through 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 invaginates to form a gastrula with a digestive chamber, and two separate germ layers - 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.

Most animals grow by indirectly using the energy of sunlight. Plants use this energy to convert sunlight into simple sugars using a process known as photosynthesis. Starting with the molecules Carbon Dioxide (CO2) and Water (H2O), photosynthesis converts the energy of sunlight into chemical energy stored in the bonds of Glucose (C6H12O6) and releases Oxygen (O2). These sugars are then used as the building blocks which allow the plant to grow. When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by 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. This process is known as glycolysis.

Animals who live close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the energy of sunlight. Instead, chemosynthetic archaea and eubacteria form the base of the food chain.

Linnaean Hierarchy Local Cladogram

Domain: Eukarya
Kingdom: Metazoa


           | ? Vendozoa (relationship uncertain)
           |-Mesomycetozoa (=Choanozoa)
           `-+-Choanomonada (=Choanoflagellida)
                 | `?-Archaeocyatha (Porifera?)
                   | '-Placozoa
                   |   '-Trichoplax adhaerens
                         | ? Vendozoa
                             | |-Ecdysozoa
                             | `-Lophotrochozoa

Stratigraphic Range: Vendian - Recent

Evolution of the Metazoa

Metazoa are generally considered to have evolved from organisms similar to extant choanoflagellata, collared flagellates that have the same structure as certain sponge cells do. Molecular phylogeny place them in a larger clade called opisthokonta, which also include fungi and a number of small parasitic protista. 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.

From a simple colonial choanoflagellate ancestor, animals developed through increasing grades of specialization and complexity, first sponges, then coelenterates, and finally bilateral animals (possessing a head and front and rear). A recent interpretation of this monophyletic animal kingdom theory is the phylogenetic scheme of Wainright et al. 1993 that shows choanoflagellates contained within the monophyletic assemblage Metazoa (= "animals"), and Fungi as the closest sister group to Metazoa.

The problem here is that although choanoflagellates seem clearly related to sponges, it is not clear how closely related sponges are to the rest of the Metazoa. It is also difficult to see how such a poorly organized organism as a sponge (essentially nothing but a glorified colonial protozoan) can develop into organisms with a proper body structure and internal organs. The most widely held theory seems to be that a colonial choanoflagellate evolved into a hollow spherical ball of cells, the blastula, which constitutes the earliest embryonic stage of development, and even occurs in sponges. The 'blastula model' of metazoan evolution goes all the way back to the famous 19th century German Lamarkist Ernst Haeckel.

However, it is not certain that such a blastaea animal ever even existed. The theory that "ontogeny recapitulates phylogeny," championed by Haeckel (according to which the growing embryo passes through all its past evolutionary stages - e.g. the early human fetus possesses gill slits and a tail), was enormously popular for some time. It was incorporated into the late 19th century and early twentieth century esoteric and occult speculations of Blavatsky and Rudolph Steiner, and more recently, the "Up from Eden" transpersonal psychology of Ken Wilber. However, the theory has been shown to have very little scientific basis. In fact it is now known that Haeckel faked a number of his images, chopping up the embryos so they would more closely resemble his theoretical predictions.

All we can say for certain is that, some time during the Late Proterozoic era, an unknown protozoan (or protistan) organism developed into a tiny colonial form, which eventually became the common ancestor of the Metazoa. The actual nature of this organism is not known, as it was soft-bodied and left no trace. It used to be thought that sponges evolved from a different single celled organism to higher animals (in which case the Metazoa are a polyphyletic taxon), but recent molecular phylogenetic evidence indicates this is not the case.

The first fossils that might represent animals appear towards the end of the Proterozoic, around 575 million years ago, and are known as the Vendozoa. There relationship is modern phyla is problematic, and it is possible they are not really metazoan animals at all. Together with various simple trace fossils and some intriguing phosphatic embryos they make up what is known of the Ediacaran biota. Most known extant and extinct animal phyla suddenly appear at the dawn of 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.


Credits: Bill 060928, MAK060929, evolution of the Metazoa ATW050815 Palaeos com - Metazoa; material added or modified from Wikipedia MAK061210

Personal tools