Animal

Animals are multicellular, eukaryotic organisms belonging to the biological kingdom Animalia.

Unlike plants or algae, animals cannot produce their own food through photosynthesis. Instead, they must ingest organic material and digest it internally. Most animals are also aerobic, meaning they require oxygen to live. They are characterized by having myocytes, which are cells that allow for movement. Most animals can spontaneously move their bodies during at least part of their life cycle. This ability to move is a key distinction from many other life forms.

Animal development follows a specific biological sequence. Most animals begin their lives through sexual reproduction. This process involves the fusion of two types of gametes: small, motile spermatozoa and larger, non-motile ova. When these fuse, they form a zygote. This zygote undergoes mitosis to become a hollow sphere of cells called a blastula.

In many species, the blastula undergoes invagination to form a gastrula. This stage creates a digestive chamber and distinct germ layers. These layers include the ectoderm, the endoderm, and often a third layer called the mesoderm. These layers eventually differentiate into specialized tissues and organs. This complex growth is guided by Hox genes, which signal where body parts like limbs should develop.

Scientists categorize the animal kingdom into several major clades. These include Porifera, Ctenophora, Placozoa, Cnidaria, and Bilateria. Most living animal species belong to the Bilateria clade. Members of Bilateria typically have a bilaterally symmetric body plan and are cephalised, meaning they have a concentrated head region.

Bilaterians are further divided into two large groups: protostomes and deuterostomes. Protostomes include arthropods, molluscs, flatworms, annelids, and nematodes. Deuterostomes include echinoderms, hemichordates, and chordates. The chordate group is particularly significant because it contains all vertebrates.

The history of animals is written in the fossil record. Evidence suggests animals first appeared during the late Cryogenian period. They diversified during the subsequent Ediacaran period in an event called the Avalon explosion.

A massive increase in animal diversity occurred during the Cambrian explosion, which began approximately 539 million years ago. During this time, nearly all modern animal phyla appeared as marine species. This was followed by the Ordovician radiation around 485.4 million years ago, which saw the appearance of most animal classes. Scientists have identified 6,331 genes that may have originated from a single common ancestor living about 650 million years ago.

Our understanding of animal classification has evolved significantly over time. Historically, Aristotle divided animals based on whether they possessed blood. In 1758, Carl Linnaeus established the first hierarchical biological classification system in his work, *Systema Naturae*. Jean-Baptiste Lamarck later expanded this system to include 14 phyla by 1809. In 1874, Ernst Haeckel made a major distinction by dividing the kingdom into multicellular Metazoa and single-celled Protozoa. Today, biologists use molecular phylogenetics to study the evolutionary relationships between different taxa. This modern technique uses genetic data to show how species are truly connected.

Animal diversity is staggering in both number and size.

Scientists have described over 1.5 million living species. This includes approximately 1.05 million insects and 85,000 molluscs. It is estimated that as many as 7.77 million animal species may actually exist on Earth. Sizes vary from the microscopic *Myxobolus shekel*, which is only 8.5 micrometers long, to the massive blue whale. The blue whale can reach lengths of 33.6 meters and weigh up to 190 tonnes.

On land, the African bush elephant is the largest living animal, weighing up to 12.25 tonnes.

Animals play vital roles in global ecosystems and human society. They form intricate food webs through various trophic levels. Herbivores eat producers like plants, while carnivores eat other animals. Some animals are even parasites that live off a host.

Humans rely on animals for food, such as meat, eggs, and dairy. We also use them for materials like wool and leather, or as pets and working animals. Dogs were the first animals to be domesticated by humans.

Beyond survival, animals are deeply embedded in human culture, appearing in everything from ancient cave art to modern religion and sports.