Animals are one of the basic groups of life on Earth. All animals share a set of fundamental characteristics—they move, they feed on other organisms, they're multicellular and they reproduce sexually. Here we explore 10 facts about animals and find out more about the characteristics that make them different from other groups of living organisms.
FACT: Animals are divided into six basic groups which include amphibians, birds, fishes, invertebrates, mammals and reptiles.
Invertebrates were the first animals to evolve. Invertebrates are animals that lack a backbone. They account for more than 97% of all animal species alive today. Fossil evidence of invertebrates dates back to the late Precambrian, 600 million years ago.
Fishes were among the first vertebrates to evolve. The earliest known fishes were the ostracoderms, a now-extinct group of jawless fish that appeared in the Cambrian Period. Fish later evolved jaws and diversified into a number of lineages including cartilaginous fish, ray-finned fish and lobe-finned fish.
Amphibians were the first vertebrates to make the move from life in water to life on land. The first amphibians evolved from lobe-finned fishes approximately 370 million years ago during the Devonian Period. Despite their early colonization of terrestrial habitats, most lineages of amphibians have never fully severed their ties with aquatic habitats. Amphibians include newts and salamanders, frogs and toads, and caecilians.
Reptiles are cold-blooded vertebrates that diverged from ancestral amphibians about 340 million years ago during the Carboniferous Period. Reptiles include turtles, squamates, crocodiles, and tuataras. There are about 8,000 species of reptiles alive today.
Mammals are vertebrates that evolved from therapsid reptiles during the Jurassic Period about 200 million years ago. There are approximately 5,400 species of mammals alive today. 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.
Birds evolved from reptiles during the Mesozoic Era about 150 million years ago. Birds have a number of characteristics that sets them apart from other vertebrates such as feathers, bills, and a furcula.
FACT: There are between 3 and 30 million species of animals alive today.
Scientists estimate that there are in the ballpark of 3 and 30 million species of animals alive today. Of those species, about 97% are invertebrates and 3% are vertebrates. The insects are the most numerous of all animal groups, with 1 to 29 million species. In addition to insects, invertebrate animals include about 100,000 species of molluscs, 75,000 species of arachnids, 10,000 species of sponges, and more than 20,000 nematotes. Vertebrate animals include 8,000 reptiles, 10,000 birds, 23,000 fish, and 5,000 mammal species.
FACT: The first animals appeared approximately 600 million years ago during the late Precambrian.
The oldest evidence of life, fossilized stromatolites unearthed from Bolivia, is about 3.8 billion years old. It wasn't until the late Precambrian that the first animals appear in the fossil record. Among the earliest animals are those known as the Ediacara biota, an assortment of tubular and frond-shaped creatures that lived between 635 and 543 million years ago. The Ediacara fossils appear to have vanished by the end of the Precambrian.
FACT: Early animals diversified during the Cambrian Explosion.
The Cambrian Explosion (570 to 530 million years ago) refers to an unprecedented and unsurpassed period of evolutionary innovation in the history of our planet. During the Cambrian Explosion, early organisms evolved into many different, more complex forms. During this time period, nearly all of the basic metazoan body plans that persist today were established.
FACT: Animals are multicellular eukaryotes.
All animals have bodies that consist of multiple cells—they are multicellular. In addition to being multicellular, animals are also eukaryotes—their bodies are composed of eukaryotic cells. Eukaryotic cells are complex cells that have membrane-bound nuclei and organelles. The DNA in a eukaryotic cell is linear and organized into chromosomes. With the exception of the sponges, animal cells are organized into tissues that perform different functions. Animal tissues include connective tissue, muscle tissue, epithelial tissue, and nervous tissue.
FACT: Most animals are capable of movement.
Unlike plants, which are fixed to the substrate in which they grow, most animals are motile (capable of movement). One exception is the sponges, which are considered to be sedentary for most of their life cycle, although it has been shown that some species can move at a very slow rate (a few millimeters per day).
FACT: Animals rely on other organisms for food.
All animals are heterotrophs which means they cannot produce their own food. Instead, they must ingest plants and other organisms as a way to get the carbon and energy they need to live.
FACT: Animals undergo sexual reproduction at some point in their life cycle.
Most animals undergo sexual reproduction at some point during their life cycle. Sexual reproduction is a process that combines the genetic material of parent organisms to form the genetic material of offspring. Sexual reproduction augments genetic diversity within a popluation.
FACT: Animal cells are held together by a matrix that contains collagen.
Animal cells are embedded in an extracellular matrix that consists of collagen and glycoproteins. Collagen is a class of proteins that is found exclusively in animals. They are long and fibrous proteins that exhibit great tensile strength and therefore play an important role in structural support. Collagen is prevalent in connective tissue—it occurs in tendons, ligaments, skin, cartilage, bone, blood vessels, and intevertebral disc.
FACT: The largest animal alive today is the blue whale.
The blue whale is the largest animal alive today and is perhaps the largest animal ever to have lived. It weighs in the range of 110 to 160 tonnes and grows to lengths of between 20 and 30 meters. But blue whales break more than just size records. They produce vocalizations at volumes in excess of 180 decibels, earning them the title of the loudest animal on the planet.