Animals are multicellular organisms that eat other organisms for energy, lack rigid cell walls, and develop from an embryo through a specific sequence of cell divisions found nowhere else in nature. That combination of traits sets every animal, from a sea sponge to a blue whale, apart from plants, fungi, bacteria, and every other form of life. But the deeper you look, the more surprising the answer gets.
Cells Without Walls
The most fundamental difference starts at the cellular level. Bacteria, fungi, algae, and plants all surround their cells with rigid cell walls. Animal cells don’t have them. Instead, animal cells sit within a flexible scaffold called the extracellular matrix, a mesh of proteins and sugar-based molecules that fills the spaces between cells, binds tissues together, and helps regulate cell behavior. This matrix gives animal tissues their flexibility. It’s why your skin can stretch, your joints can bend, and a jellyfish can pulse through water.
Plant cells need their rigid walls to survive. Because plants don’t actively balance the water concentration inside and outside their cells, water constantly flows inward through osmotic pressure. The stiff wall keeps the cell from swelling and bursting. Animal cells solve this problem differently, actively pumping salts and water to maintain balance, which means they can stay soft and flexible. That flexibility is what allows animals to move in ways no plant ever could.
Collagen: The Signature Protein
If you strip away every trait that animals share with at least one other group of organisms, what’s left? According to taxonomists at UC Berkeley, the answer is a specific type of extracellular matrix built from collagen, proteoglycans, adhesive glycoproteins, and a class of receptor proteins called integrins. Collagen is one of the most abundant proteins in any animal body. It forms the structural fibers in skin, tendons, cartilage, bone, and blood vessels. Every animal, from a coral polyp to a human, produces some version of it. No plant, fungus, or bacterium does. This shared molecular toolkit is inherited from a common multicellular ancestor, and it remains the single most reliable marker that unites all animals as a group.
Eating by Ingestion
Plants make their own food from sunlight. Fungi dissolve food externally and absorb the nutrients. Animals do something different: they take solid food into their bodies and break it down internally. Biologists call this holozoic nutrition, and it follows a specific sequence. First, food is ingested (taken in). Then it’s digested, broken into molecules small enough to pass through cell membranes. Those molecules are absorbed into cells, used for energy or building materials, and whatever can’t be digested is expelled as waste.
This style of feeding shapes almost everything about animal life. It’s why animals have mouths, stomachs, intestines, and the sensory systems needed to find food in the first place. Whether an animal eats plants (herbivore), other animals (carnivore), or both (omnivore), the underlying process is the same. Fungi are also heterotrophs, meaning they can’t make their own food, but they secrete digestive enzymes outward and absorb nutrients through their cell surfaces. Animals internalize the whole process.
A Unique Path Through Embryonic Development
Every animal begins life as a single fertilized cell that divides repeatedly to form a hollow ball of cells called a blastula. That blastula then undergoes gastrulation, a dramatic reorganization in which cells fold inward to create distinct layers of tissue. This process is universal across the animal kingdom and does not occur in any other group of organisms.
In most animals, gastrulation produces three primary tissue layers. The innermost layer (endoderm) eventually forms the gut lining and internal organs. The outermost layer (ectoderm) becomes skin and the nervous system. And the middle layer (mesoderm) gives rise to muscle, bone, and the circulatory system. Simpler animals like jellyfish have only two of these layers, but the gastrulation process itself is shared. It’s the developmental event that makes complex animal body plans possible.
A Genetic Toolkit for Body Plans
Animals share a set of master-switch genes called Hox genes that control how the body is organized along its head-to-tail axis. These genes act as transcriptional regulators, essentially telling cells where they are in the body and what structures to build. They’re found in all animals with bilateral symmetry, from insects to fish to humans.
The effects of Hox genes are startlingly specific. In mice, inactivating one group of Hox genes causes the lumbar spine to sprout ribs as though it were the rib cage. Knocking out another group eliminates the sacrum entirely. Different Hox gene clusters determine whether ribs are attached to the sternum or float freely. The same basic logic applies across species: the genes are arranged in order along the chromosome, and that order corresponds to their position of action along the body. A fruit fly’s Hox genes and a human’s are recognizably related, despite more than 500 million years of separate evolution.
Reproduction: Mostly Sexual, With Exceptions
Most animals reproduce sexually, combining genetic material from two parents. This is the ancestral mode of reproduction for the group, and it generates the genetic diversity that helps populations adapt to changing environments. But the animal kingdom is full of exceptions. Some insects, reptiles, and fish can reproduce through parthenogenesis, in which females produce offspring from unfertilized eggs. Among mealybugs alone, about 10% of species reproduce partially or entirely without males. These asexual strategies appear to have evolved independently multiple times from sexual ancestors, suggesting that while sexual reproduction is the default, animals have repeatedly found workarounds when conditions favor it.
Where Animals Came From
The closest living relatives of animals are choanoflagellates, a group of single-celled organisms that look remarkably like the feeding cells of sponges. Genome sequencing of the choanoflagellate species Monosiga brevicollis revealed roughly 9,200 genes, many of which encode the same cell-adhesion and signaling protein domains found in animals. The last common ancestor of choanoflagellates and animals lived more than 600 million years ago in the late Precambrian period and was either single-celled or capable of forming simple colonies.
What’s striking is that the genetic raw materials for multicellular life were already present before animals existed. The choanoflagellate genome contains protein domains associated with cell adhesion and extracellular matrix construction. But the way those domains are linked together differs between choanoflagellates and animals, suggesting that extensive reshuffling of existing genetic parts followed the split between the two lineages. Animals didn’t invent multicellularity from scratch. They repurposed a toolkit that was already there, then built on it to produce the staggering diversity of body forms alive today.