What are Animals and Why are They Important?
Animals are multicellular, eukaryotic organisms that belong to the kingdom Animalia. They have many different forms, functions, and behaviors, but they share some common characteristics, such as consuming organic material, breathing oxygen, moving, reproducing sexually, and developing from a hollow sphere of cells called a blastula.
Animals are important for many reasons. They are part of the biodiversity of life on Earth, which provides essential ecosystem services, such as pollination, decomposition, nutrient cycling, and climate regulation. They are also sources of food, clothing, medicine, and companionship for humans and other animals. They play important roles in culture, religion, art, and science. They inspire curiosity and wonder, and help us learn more about ourselves and the world around us.
In this article, we will explore some of the amazing diversity and adaptations of animals, as well as some of the threats and challenges they face in the modern world.
The Diversity of Animals
Animals are one of the most diverse groups of living organisms on Earth. As of 2022, there are about 2.16 million described animal species, but it is estimated that there could be up to 7.77 million animal species in total. Animals range in size from microscopic rotifers to gigantic blue whales. They live in almost every habitat imaginable, from the deepest oceans to the highest mountains, from the hottest deserts to the coldest polar regions.
Animals can be classified into different groups based on their body plan, symmetry, embryonic development, and evolutionary relationships. The most basic division is between animals with and without a backbone (vertebrates and invertebrates). Vertebrates include mammals, birds, reptiles, amphibians, and fish. Invertebrates include insects, arachnids, crustaceans, molluscs, annelids, nematodes, cnidarians, sponges, and many more.
Another way to classify animals is based on their symmetry. Symmetry refers to how an animal’s body parts are arranged around a central axis. Most animals have bilateral symmetry , meaning they can be divided into two equal halves along a single plane. Some animals have radial symmetry , meaning they can be divided into multiple equal parts along any plane that passes through the center. Examples of radially symmetrical animals are starfish and jellyfish. A few animals have no symmetry at all , such as sponges.
A third way to classify animals is based on their embryonic development. Embryonic development refers to how an animal’s body forms from a single cell (the zygote) to a multicellular organism (the adult). One of the key stages in embryonic development is gastrulation , which is when the blastula folds inward to form a hollow tube (the gut) with two layers of cells (the endoderm and ectoderm). Some animals undergo a further process called neurulation , which is when a third layer of cells (the mesoderm) forms between the endoderm and ectoderm.
Based on these processes, animals can be divided into two major groups: protostomes and deuterostomes . Protostomes are animals whose mouth develops from the first opening in the gut (the blastopore), while deuterostomes are animals whose anus develops from the blastopore. Protostomes include arthropods , molluscs , annelids , nematodes , and many more. Deuterostomes include echinoderms , chordates , hemichordates , and a few others.
A fourth way to classify animals is based on their evolutionary relationships. Evolutionary relationships refer to how closely related different animals are based on their common ancestry. Scientists use various methods to reconstruct the evolutionary history of animals , such as comparing their DNA sequences , morphology , fossils , and behavior . One of the most widely used tools for depicting evolutionary relationships is a phylogenetic tree , which is a branching diagram that shows how different groups of animals diverged from each other over time.
The most recent common ancestor of all living animals is thought to have lived about 650 million years ago. This ancestor was probably a single-celled eukaryote that had some features of animals , such as a membrane-bound nucleus , mitochondria , flagella , and genes for cell ad