Animal

• 1. Introduction

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  An animal can be as small as 8.5 micrometres or as long as 33.6 metres. The blue whale, Balaenoptera musculus, stretches across that upper limit, while one of the tiniest known species, Myxobolus shekel, never grows past 8.5 micrometres when fully formed. Both belong to the same biological kingdom, Animalia. Over 1.5 million living animal species have been described, and one estimate puts the true number as high as 7.77 million. Around 1.05 million of the described species are insects. What ties a microscopic parasite to a whale the weight of dozens of elephants? How did this kingdom arise, and how did it spread into nearly every habitat on Earth? And why were thinkers from Aristotle to Carl Linnaeus so uncertain about where its boundaries lay?

• 2. What Makes An Animal

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  Animals cannot make their own food, a trait they share with fungi but not with plants or algae. Instead they ingest organic material and digest it internally. They are eukaryotic, multicellular, and aerobic, breathing oxygen to fuel their bodies. With few exceptions they have myocytes and can move, and they reproduce sexually. The word itself comes from a Latin noun meaning 'having breath or soul'. In everyday speech, the term often refers only to nonhuman animals, though the biological definition takes in every member of Animalia. Several structural features set animals apart from all other living things. Their cells sit within an extracellular matrix built from collagen and elastic glycoproteins, rather than being locked in place by cell walls. They can spontaneously move their bodies during at least part of their life cycle. And during embryonic development they pass through a blastula stage, a hollow sphere of cells. Inside most animals lies a digestive chamber, with either one opening, as in Ctenophora, Cnidaria, and flatworms, or two openings, as in most bilaterians. That flexible matrix does more than hold cells together. It can be calcified into shells, bones, and spicules, the hard architecture that builds an animal's form.

• 3. From Egg To Embryo

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  Nearly all animals reproduce sexually, producing haploid gametes through meiosis. The smaller, motile gametes are spermatozoa and the larger, non-motile ones are ova. When these fuse, they form a zygote that divides by mitosis into the hollow blastula. In sponges, blastula larvae swim off, attach to the seabed, and grow into a new sponge. In most other groups the blastula folds inward, or invaginates, to form a gastrula with a digestive chamber and two germ layers, an outer ectoderm and an inner endoderm. A third layer, the mesoderm, usually develops between them, and these layers then differentiate into tissues and organs. Hox genes govern this process, signalling when and where to build structures such as body segments and limbs. Mating repeatedly with a close relative tends to cause inbreeding depression, as harmful recessive traits become more common. Animals have evolved many mechanisms to avoid breeding too closely. Some skip sex entirely. Asexual reproduction often yields a genetic clone of the parent, through fragmentation, through budding as in Hydra and other cnidarians, or through parthenogenesis, where fertile eggs form without mating, as in aphids.

• 4. Webs Of Eating

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  Predation is, at its core, a consumer-resource interaction, where one organism feeds on its prey. Animals are sorted into ecological groups by how they feed: carnivores, herbivores, omnivores, fungivores, scavengers and detritivores, and parasites. Almost all multicellular predators are animals. Prey evolve anti-predator adaptations, and the pressure each side imposes drives an evolutionary arms race between them. Some species mix their methods. In parasitoid wasps, the larvae feed on the living tissues of their hosts and kill them, while the adults mainly sip nectar from flowers. Others are highly specialised, like hawksbill sea turtles, which mainly eat sponges. Most animals ultimately depend on the biomass and bioenergy that plants and phytoplankton produce through photosynthesis. Herbivores, as primary consumers, eat plant material directly, while carnivores higher up the chain take their nutrients second-hand. Animals oxidise carbohydrates, lipids, and proteins in cellular respiration to grow and sustain themselves. Not every animal relies on sunlight. Some benthic animals living near hydrothermal vents and cold seeps on the dark sea floor consume organic matter made through chemosynthesis, produced by archaea and bacteria oxidising compounds such as hydrogen sulfide.

• 5. Conquering The Habitats

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  Animals originated in the ocean, and every extant phylum except Micrognathozoa and Onychophora still has at least some marine species. The move to land came in stages. Several lineages of arthropods began colonising land around the same time as land plants, between 510 and 471 million years ago, during the Late Cambrian or Early Ordovician. Vertebrates followed much later. The lobe-finned fish Tiktaalik started moving onto land in the late Devonian, about 375 million years ago. Other groups that reached land environments include Mollusca, Platyhelmintha, Annelida, Tardigrada, Onychophora, Rotifera, and Nematoda. Today animals fill nearly every habitat and microhabitat on Earth, with faunas suited to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, deserts, air, and even the insides of other organisms. There are limits, though. Animals are not especially heat tolerant. Very few can survive at constant temperatures above 50 degrees Celsius, or in the most extreme cold deserts of continental Antarctica. The collective influence of animals on the processes shaping Earth's surface remains largely understudied, with most work confined to individual species and well-known examples.

• 6. Counting The Kingdom

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  Among described animals, the arthropods dwarf every other phylum, with roughly 1,257,000 species, including about a million insects and more than 40,000 malacostracans. Molluscs number around 85,000 and chordates more than 70,000, of which around 65,000 are vertebrates. Other major phyla follow: Platyhelminthes near 29,500, Nematoda around 25,000, Annelida about 17,000, Cnidaria 16,000, Porifera 10,800, Echinodermata 7,500, Bryozoa 6,000, Rotifera 2,000, Nemertea 1,350, and Tardigrada 1,335. The described counts hide deep uncertainty. For nematodes, around 25,000 to 27,000 species have been described, yet published estimates of the true total range from 10,000 to figures as high as 100 million. Using patterns in the taxonomic hierarchy, researchers calculated in 2011 that about 7.77 million animal species exist in all. Size spans the same enormous range. The blue whale, the largest animal ever to live, weighs up to 190 tonnes. The largest living land animal, the African bush elephant, Loxodonta africana, reaches up to 12.25 tonnes and 10.67 metres. The biggest land animals of all time were titanosaur sauropods like Argentinosaurus, perhaps 73 tonnes, and Supersaurus, perhaps 39 metres long.

• 7. Tracing The First Animals

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  A molecule called 24-isopropylcholestane turns up in rocks roughly 650 million years old, and it is made only by sponges and pelagophyte algae. Molecular clock estimates point to sponges as its likely source, placing animal origins deep in the Cryogenian period. The first body fossils appear later, in the Ediacaran, in forms such as Charnia and Spriggina. Doubts about whether these were truly animals eased when the animal lipid cholesterol was found in fossils of Dickinsonia. Many phyla then surface during the Cambrian explosion, starting about 539 million years ago, in beds like the Burgess Shale, alongside extinct forms such as the predatory Anomalocaris. That apparent suddenness may be an artefact of the fossil record. Auroralumina attenboroughii, the earliest known Ediacaran crown-group cnidarian, dates to 557 to 562 million years ago, some 20 million years before the Cambrian, and comes from Charnwood Forest in England. It is thought to be one of the earliest predators, catching small prey with its nematocysts. Some palaeontologists push animal origins even further back, possibly to 1 billion years ago. Trace fossils in the Tonian period may hint at worm-like animals about 5 millimetres wide, though the giant single-celled protist Gromia sphaerica makes similar tracks, leaving the question open.

• 8. Naming The Branches

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  Aristotle divided animals through his own observations into those with blood, roughly the vertebrates, and those without. He arranged them on a scale from man down through live-bearing tetrapods, crustaceans, and finally creatures like sponges. He was unsure whether sponges were animals or plants, knowing they could sense touch and contract, yet were rooted and never moved. Carl Linnaeus built the first hierarchical classification in 1758 in his Systema Naturae, sorting animals into the classes Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Jean-Baptiste de Lamarck called Vermes 'a chaotic mess' and, by his 1809 Philosophie Zoologique, had created nine phyla beyond the vertebrates. Georges Cuvier, in his 1817 Le Regne Animal, used comparative anatomy to group animals into four embranchements, a division later followed by Karl Ernst von Baer, Louis Agassiz, and Richard Owen. In 1874, Ernst Haeckel split the kingdom into Metazoa and Protozoa, the single-celled animals later moved into the kingdom Protista. Modern classification leans on molecular phylogenetics. That same approach still fuels live debate, as a 2017 study by Roberto Feuda supported sponges as sister to other animals, while a 2023 study by Darrin Schultz used ancient gene linkages to argue for comb jellies in that position.