ANIMALIA

Animals are a major group of multicellular, eukaryotic organisms of the kingdomAnimalia 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 aremotile, meaning they can move spontaneously and independently. All animals must ingest other organisms or their products for sustenance.

Most known animal phyla appeared in the fossil record as marine species during theCambrian explosion, about 542 million years ago. Animals are divided into various sub-groups, including birds, mammals, reptiles, fish and insects.Animals are all the creatures belonging to the kingdom Animalia, ranging in complexity from simple organisms like sponges to highly developed human beings. (Some of the other kingdoms are Plantae, which encompasses things like grass, trees, and flowers, and Fungi, which are things like mold and mushrooms.) Animals make up at least three-quarters of all the species on Earth, and they are distinguished from plants and other organisms by their ability to move. Even tiny animals have muscles and therefore can get around-to find food or a mate, or to get away from enemies.

While humans are animals, often when people use the word "animal" they are referring to all animals except humans. Sometimes people are referring specifically to mammals-warm-blooded creatures like dogs, cows, or lions-as opposed to birds, reptiles, or fish.

Experts estimate that over one million different kinds of animals have been identified in the world. There may be millions more, particularly insect species, that have not yet been identified or discovered by scientists. Hundreds of years ago scientists began dividing the animal kingdom into categories based on certain characteristics like body type, ways of reproducing, and what the animals can do (fly, swim, walk on two legs, and so on). The animal kingdom (and every other kingdom as well) is divided and subdivided into numerous other categories. If animal classification categories were viewed as an upside-down pyramid, kingdom-the largest and broadest classification-would be at the top. The animal kingdom is divided into several different parts, called phyla (the singular form of that word is "phylum"); each phylum is further divided into classes. The other levels of division are order, family, genus, and species, with species being the tip of that upside-down pyramid, or the most specific way to categorize. When scientists give the official name for a type of animal, they use the genus and species names. Human beings are part of the genus Homo and the species sapiens; therefore, our scientific name is Homo sapiens.

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The first animals to evolve were invertebrates. Fossil evidence of invertebrates dates back to the late Precambrian, 600 million years ago. Invertebrates evolved from single-celled microorganisms. Since then, invertebrates have diversified into countless forms. An estimated 97 percent of all species are alive today are invertebrates.

Invertebrates are united more by what they lack (a backbone) than by shared characteristics. Invertebrates include animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.

Fishes were among the first vertebrates to evolve. The earliest known fishes were the ostracoderms, a now-extinct group of jawless fishes that appeared in the Cambrian Period, about 510 million years ago. Other early fish include the conodonts and the agnanthans (the hagfish and the lamprey). Fishes later evolved jaws and diversified into a number of lineages including cartilaginous fishes, ray-finned fishes and lobe-finned fishes.

The ray-finned fishes are the most diverse of all vertebrate groups, with some 24,000 species. There are about 810 species of cartilaginous fishes and 8 species of lobe-finned fishes.

Amphibians were the first vertebrates to make the move from life in water to life on land. Despite their early colonization of terrestrial habitats, most lineages of amphibians have never fully severed their ties with aquatic habitats. The first amphibians evolved from lobe-finned fishes approximately 370 million years ago during the Devonian Period.

Amphibians include newts and salamanders, frogs and toads, and caecilians. There are between 5000 and 6000 species of known amphibians alive today. Amphibian species are in decline around the world due to a variety of threats including of invasive species, habitat destruction, disease, climate change and toxins.

Reptiles are cold-blooded vertebrates that diverged from ancestral amphibians about 340 million years ago during the Carboniferous Period. Early reptiles included organisms such as Hylonomus, Petrolacosaurus, Archaeothyris and Paleothyris. The oldest evidence of reptiles is a set of footprints found in Nova Scotia. Two characteristics distinguish early reptiles from amphibians: scales and the ability to lay hard-shelled amniotic eggs.

Reptiles include turtles, squamates, crocodiles and tuataras. There are about 8,000 species of reptiles alive today. Of the four reptile groups, the squamates (amphisbaenians, lizards and snakes) are the most diverse with nearly 7,600 species.

Birds evolved from reptiles during the Mesazoic Era about 150 million years ago. Today, more than 9,000 species inhabit virtually every terrestrial habitat on the planet. Birds have a number of characteristics that sets them apart from other vertebrates such as feathers, bills and a furcula.

Birds, best known for their ability to fly, are unmatched in their command of the skies. Albatrosses glide over the vast open sea, hummingbirds hover motionless in mid-air, and birds of prey capture prey with pinpoint accuracy. But not all birds are aerobatic experts. Some species such as ostriches, kiwis and penguins, lost their ability to fly long ago in favor of lifestyles suited more for land or water.

Mammals are vertebrates that evolved from therapsid reptiles during the Jurassic Period about 200 million years ago. The first mammals, known as morganucodontids, were nocturnal insect eaters that resembled modern-day shrews. For the better part of 130 million years, mammals remained small and lived in a world dominated by the dinosaurs. But around 65 million years ago, a drastic shift in climate caused the extinction of more than two-thirds of the animal species on the planet, including the dinosaurs. Mammals survived this bout of climate change and in its wake, diversified and colonized the many newly-available habitats. Today, mammals are remarkably varied, with some 5,400 species occupying every continent on the globe.

Mammals display a remarkable array of adaptations that enable them to inhabit a wide range of habitats. Mammals range in size from the minute bumblebee bat which measures a mere three centimeters in length, to the magnificent blue whale, which can measure 33 meters head-to-tail. Some of the better-known mammal groups include carnivores, rodents, elephants, marsupials, rabbits, bats, primates, seals, anteaters, cetaceans, odd-toed ungulates and even-toed ungulates.