Animals are familiar creatures to most of us. We are, after all, animals ourselves. Beyond that, we share the planet with a remarkable diversity of other animals, we rely on animals, we learn from animals, and we even befriend animals. But do you know the finer points of what makes one organism an animal and another organism something else, such as an plant or a bacterium or a fungus? In this slideshow, you'll find out more about animals and why they are unlike the other lifeforms that populate our planet.
The oldest evidence of life dates back some 3.8 billion years. The earliest fossils are of ancient organisms called stromatolites. Stromatolites were not animals—animals would not appear for another 3.2 billion years. It was during the late Precambrian that the first animals appear in the fossil record. Among the earliest animals are those of the Ediacara biota, an assortment of tubular and frond-shaped creatures that lived between 635 and 543 million years ago. The Ediacara biota appear to have vanished by the end of the Precambrian.
Animals need energy to power all aspects of their lives including their growth, development, movement, metabolism, and reproduction. Unlike plants, animals are not capable of transforming sunlight into energy. Instead, animals are heterotrophs, which means they cannot produce their own food and must instead ingest plants and other organisms as a way to get the carbon and energy they need to live.
Unlike plants, which are fixed to the substrate in which they grow, most animals are motile (capable of movement) during some or all of their life cycle. For many animals, the ability to move is obvious: fish swim, birds fly, mammals scamper, climb, run and mosey. But for some animals, movement is subtle or restricted to a short period of their life. Such animals are described as being sessile. Sponges, for example, are sedentary for most of their life cycle but do spend their larval stage as free-swimming animals. Additionally, it has been shown that some species of sponges can move at a very slow rate (a few millimeters per day). Examples of other sessile animals that move only very minimally include barnacles and corals.
All animals have bodies that consist of multiple cells—in other words, they are multicellular. In addition to being multicellular, animals are also eukaryotes—their bodies are composed of eukaryotic cells. Eukaryotic cells are complex cells, inside which internal structures such as the nucleus and the various organelles are enclosed in their own membranes. The DNA in a eukaryotic cell is linear and it is organized into chromosomes. With the exception of the sponges (the simplest of all animals), animal cells are organized into tissues that perform different functions. Animal tissues include connective tissue, muscle tissue, epithelial tissue, and nervous tissue.
The evolution of animals, since their first appearance 600 million years ago, has resulted in an extraordinary number and diversity of lifeforms. As a result, animals have evolved many different forms as well as a multitude of ways of moving, obtaining food, and sensing their environment. Throughout the course of animal evolution, the numbers of animal groups and species has increased and, at times, decreased. Today, scientists estimate that there are more than 3 million living species.
The Cambrian Explosion (570 to 530 million years ago) was a time when the rate of diversification of animals was both remarkable and rapid. During the Cambrian Explosion, early organisms evolved into many different and more complex forms. During this time period, nearly all of the basic animal body plans developed, body plans that are still present today.
Sponges are the simplest of all animals. Like other animals, sponges are multicellular, but it is here the similarities end. Sponges lack the specialized tissues that are present in all other animals. The body of a sponge consists of cells that are embedded within a matrix. Tiny spiny proteins called spicules are scattered throughout this matrix and form a support structure for the sponge. Sponges have many small pores and channels distributed throughout their body that serve as a filter-feeding system and enables them to sift food from the water current. Sponges diverged from all other animal groups early in the evolution of animals.
All animals with the exception of the sponges have specialized cells in their bodies called neurons. Neurons, also called nerve cells, send electrical signals to other cells. Neurons transmit and interpret a wide range of information such as the animal's well being, movement, environment, and orientation. In vertebrates, neurons are the building blocks of an advanced nervous system that includes the animal's sensory system, brain, spinal cord, and peripheral nerves. Invertebrates have nervous systems that are made up of fewer neurons than those of vertebrates, but this does not mean the nervous systems of invertebrates are simplistic. Invertebrate nervous systems are efficient and highly successful at solving the problems of survival these animals face.
Most animals, with the exception of sponges, are symmetrical. There are different forms of symmetry in the various animal groups. Radial symmetry, present in cnidarians such as sea urchins, and also in some species of sponges, is a type of symmetry in which the animal's body can be divided into similar halves by applying more than two planes that pass through the length of the animal's body. Animals that exhibit radial symmetry are disk-shaped, tube-like or bowl-like in structure. Echinoderms such as sea stars exhibit a five-point radial symmetry called pentaradial symmetry.
Bilateral symmetry is another type of symmetry present in many animals. Bilateral symmetry is a type of symmetry in which the animal's body can be divided along a sagittal plane (a vertical plane that extends from head to posterior and divides the animal's body into a right and left half).