Animals are multicellular organisms that eat other organisms for energy, lack rigid cell walls, and develop from a hollow ball of cells called a blastula. That combination of traits separates them from every other kingdom of life. But the full picture is richer than any single checklist, and some of the most fascinating animals break nearly every rule you’d expect.
Cells Without Walls
Every animal cell is fundamentally different from a plant or fungal cell in one immediate way: it has no rigid cell wall. Plant cells are boxed in by tough walls made of cellulose, and fungal cells have walls made of chitin. Animal cells have only a flexible membrane, which is why animal bodies can be soft, stretchy, and capable of rapid shape changes.
That flexibility comes with a tradeoff. Without cell walls to resist water pressure, animal cells can’t rely on the same strategy plants use to stay inflated. Instead, animals maintain a careful balance of salts and water inside and outside their cells. To hold tissues together, animal cells secrete a mesh of proteins and sugar-like molecules called the extracellular matrix. The single most abundant protein in animal tissues is collagen, a rope-like molecule where three protein chains wind tightly around each other. Collagen is what gives skin its strength, tendons their toughness, and bones their framework. No plant or fungus produces it.
Eating by Ingestion
All animals are heterotrophs, meaning they cannot make their own food the way plants do through photosynthesis. But so are fungi, and that’s where an important distinction comes in. Fungi absorb nutrients directly through their cell walls, essentially digesting food externally and soaking it up. Animals take food inside their bodies first and digest it internally. This process of ingestion, whether it’s a whale filtering krill or a spider liquefying an insect before sucking it in, is a hallmark of animal life.
The energy cost of this strategy is steep. A fish, for example, typically converts only about 10% of the carbon it consumes into body mass. The rest fuels movement, body heat, and basic cellular maintenance. That inefficiency is why food chains get shorter at every level: it takes enormous amounts of plant material to support herbivores, and enormous numbers of herbivores to support predators.
The Blastula: A Shared Starting Point
One of the most reliable ways to identify an animal is to look at how it develops. After fertilization, an animal embryo undergoes a rapid series of cell divisions called cleavage. The single fertilized egg splits again and again without growing larger, carving the original cell’s volume into smaller and smaller units called blastomeres. By the end of this process, the blastomeres typically arrange themselves into a hollow sphere: the blastula.
This blastula stage is essentially universal across the animal kingdom. Plants, fungi, and protists don’t form one. From the blastula, the embryo folds inward to create distinct cell layers (a process called gastrulation), which then give rise to all the body’s tissues. Whether you’re looking at a sea urchin, a fruit fly, or a human, development passes through this same hollow-ball stage, even though the adults look nothing alike.
Nervous Systems and Muscle Tissue
No plant or fungus has neurons. No plant or fungus has muscle. These two tissue types are so closely linked to animals that their evolutionary origins trace back over 600 million years, to a time before the first large animal fossils appear in the rock record.
Neurons allow animals to sense their environment and coordinate rapid responses. Muscle cells allow them to move, pump blood, and squeeze food through a gut. Together, these tissues enable the behaviors we most associate with animals: hunting, fleeing, mating, navigating. The rise of predation during the Cambrian period, roughly 540 million years ago, likely drove the evolution of increasingly complex nervous systems and musculature in an arms race between predators and prey.
That said, not every animal has a nervous system. Sponges lack true neurons entirely, and placozoans (tiny, flat organisms that creep along ocean floors) get by without them too. Remarkably, sponge larvae and certain jellyfish larvae that lack neurons can still move and orient themselves with a complexity that rivals relatives who do have nerve cells. The genetic toolkit for building neurons exists in sponges; they simply don’t assemble the finished product.
A Genetic Blueprint for Body Plans
Animals share a powerful set of genes that dictate how bodies are organized from head to tail. The most famous of these are Hox genes, which act like molecular address labels, telling cells where they are along the body’s main axis and what structures to build there. A mutation in a Hox gene can cause a segment to take on the identity of a different segment, like a fly growing legs where antennae should be.
Hox genes work across all animals with bilateral symmetry, from insects to fish to humans. They function in a strikingly consistent way: the order of genes on the chromosome mirrors the order of body regions they control, from front to back. Different combinations of active Hox genes at each position along the body axis create the specific anatomy of that region. Vertebrate Hox genes influence structures from all three embryonic cell layers, shaping everything from vertebrae to ribs to the tissues that fill the spaces between them.
Sexual Reproduction and the Diploid Body
Most animals reproduce sexually, and their life cycle is dominated by the diploid stage, meaning their cells carry two copies of each chromosome. This contrasts with organisms like many fungi, where the body spends most of its life with only one copy. In animals, the only haploid cells (carrying a single chromosome set) are the sperm and egg themselves.
Sexual reproduction is far more common than asexual reproduction across the animal kingdom, even though it’s energetically expensive. The advantage comes from genetic reshuffling during the formation of eggs and sperm, where chromosome segments cross over and swap material. This process improves error correction and generates the diversity that helps populations adapt. In mammals specifically, sexual reproduction isn’t just beneficial, it’s required. Certain genes are chemically silenced depending on whether they came from the mother or father, so an embryo needs both parental contributions to develop normally.
Where Animals Came From
The closest living relatives of animals aren’t plants or fungi. They’re choanoflagellates, single-celled organisms that live in water and bear a striking resemblance to certain cells found in sponges. Some choanoflagellate species can form simple colonies by dividing repeatedly from a single cell, the same basic strategy animals use to build a body. This parallel suggests that the last common ancestor of animals and choanoflagellates may have already been capable of simple multicellularity.
From that ancestor, animals diverged and diversified into a kingdom currently estimated at around 2.6 million species, though only a fraction have been formally described. The true number could be far higher, with some projections reaching into the tens of millions when undiscovered insects, deep-sea organisms, and parasites are factored in.
Sponges: Animals That Break the Mold
Sponges sit at the very base of the animal family tree and challenge nearly every casual assumption about what an animal should be. They have no muscles, no nerves, no organs, and no true tissues. They don’t move as adults. They digest food inside individual cells rather than in a gut. Yet molecular evidence places them firmly within the animal kingdom.
What earns sponges their membership is a combination of traits that align with other animals at the cellular and genetic level. Their cells are eukaryotic and lack cell walls. They develop from embryos. They are multicellular heterotrophs. And genetic analysis of their adhesion molecules and receptors, the proteins cells use to stick together and communicate, confirms they share a common ancestor with all other animals rather than having evolved multicellularity independently. Sponges are the oldest animal group still alive today, a living reminder that “animal” is a much broader category than fur, eyes, and legs might suggest.