How did animals and plants diverge?
This is a fascinating evolutionary story. In simple terms, the "divergence" of animals and plants did not occur in the familiar forests or grasslands we know, but in the primordial oceans 1.6 billion years ago, and it happened at the single-cell level.
The core reason for this split can be attributed to a crucial "engulfment" event and the subsequent choice of two completely different survival strategies.
Before the split, animals and plants shared a common ancestor, which scientists call LECA (Last Eukaryotic Common Ancestor).
Life form at that time: They were all single-celled organisms living in water. They already possessed a nucleus and mitochondria (responsible for providing energy), surviving by engulfing surrounding bacteria or organic debris.
They were all "consumers": At this stage, everyone was a heterotroph, having to swim around to find food.
About 1.5-1.6 billion years ago, one of the most important "accidents" in biological evolutionary history occurred within the eukaryotic family, leading to the birth of the plant lineage.
The Ancestor of Plants: Swallowed a "Solar Panel"
A single-celled ancestor of the plant lineage engulfed a cyanobacterium (a type of photosynthetic bacteria). Strangely, it did not digest this bacterium but kept it inside to "work." Shutterstock
Endosymbiosis: This engulfed cyanobacterium later evolved into chloroplasts.
Consequence: The cell that acquired chloroplasts gained a "superpower"—it could get its fill just by basking in sunlight. Thus, they no longer needed to swim laboriously to hunt and chose to "lie flat."
Evolutionary Direction: To better utilize sunlight, they evolved cell walls to support their bodies, becoming sessile plants (Archaeplastida).
The Ancestor of Animals: Stuck to Its "Foodie" Nature
The ancestors of animals (and fungi) did not experience this chloroplast-engulfing event, or perhaps their lineage missed this opportunity.
Consequence: They had to continue relying on "eating" others to obtain energy.
Evolutionary Direction: To capture prey, they needed soft and flexible bodies, so they discarded or never evolved thick cell walls. To hunt more efficiently, they evolved complex motor systems, nervous systems, and muscles, eventually becoming animals (Opisthokonta).
Once they diverged due to the presence or absence of chloroplasts, the evolutionary logic of the two paths became entirely different:
Feature Plant Lineage Animal Lineage Energy Source Self-sufficient (Autotrophic): Relies on chloroplasts for photosynthesis. Plundering others (Heterotrophic): Relies on preying on other organisms. Cell Structure Has cell walls: Like building a house, cells have a hard outer shell. This limits movement but provides support, allowing plants to grow tall. No cell walls: Only a cell membrane. Cells are soft and deformable, allowing muscle contraction and body movement. Lifestyle Sessile: Rooted in one place, no need to use brains to chase prey. Evolutionary focus is on how to obtain more sunlight and water. Motile: Must move to find food. Evolutionary focus is on senses (eyes, ears), neural processing (brain), and motor abilities (muscles). Growth Pattern Indeterminate growth: As long as the environment allows, trees can keep growing taller and branching. Determinate growth: Animal size is largely fixed after adulthood, with no indefinite growth. Export to Sheets4. Interesting "Close Relative" Facts
Although animals and plants seem vastly different, genetically they are not as distant as one might imagine, and there are even some surprising "family relationships":
Animals are closer to fungi: You might think mushrooms (fungi) look like plants, but from the evolutionary tree, fungi are actually close relatives of animals. Like animals, they are heterotrophs (do not perform photosynthesis), and their cell wall component (chitin) is more similar to insect exoskeletons than plant cellulose.
Both have mitochondria: Whether it's your cells or wild grass by the roadside, cells contain mitochondria. This proves that we were once one family at an earlier stage (about 2 billion years ago).
Summary
The divergence of animals and plants essentially boils down to a choice of "whether to make your own food":
The ancestors of plants chose the one-and-done method of photosynthesis, at the cost of being locked within cell walls and losing the ability to move freely.
The ancestors of animals were forced to continue hunting, and to survive, they had to evolve smarter brains and more flexible bodies, ultimately giving rise to humans.
Would you like me to explain more about how fungi fit into this picture (since they are often confused with plants but are closer to animals)?
Then how did fungi and animals diverge?
This is a brilliant sequel. If the split between animals and plants was about "whether to swallow a chloroplast," then the split between animals and fungi was a disagreement over **"digestion methods."
Although they are both members of the Opisthokonta family, around 1 billion years ago, they made drastically different choices at an evolutionary crossroads, leading them to diverge.
The core of this split lies in: Should food be "swallowed and eaten inside," or "digested outside and then absorbed"?
Before the split, the common ancestor of fungi and animals was a single-celled organism living in water, somewhat resembling modern choanoflagellates.
It had a single posterior flagellum (like a sperm's tail) used for swimming.
It did not have a cell wall; its body was soft.
This was the crucial moment when the two paths diverged:
Animal Route: Sticking to "Phagotrophy"
The ancestors of animals chose a path of "big-mouthed meat-eating."
Strategy: They maintained the softness and flexibility of their cell membranes. When encountering food, they would wrap it with their cell membrane and swallow it for internal digestion (phagocytosis).
Evolutionary Consequences:
No cell walls allowed: Because they needed to swallow things, cells had to be soft. A hard shell would prevent deformation for swallowing.
Must be motile: To get food, they had to actively hunt, so they retained and enhanced muscles and nervous systems.
Internal digestive system: Later, during multicellularization, they evolved complex guts to process swallowed food.
Fungal Route: Switching to "Osmotrophy"
The ancestors of fungi unlocked a unique skill tree—"chemical warfare."
Strategy: They stopped laboriously swallowing food. Instead, they secreted digestive enzymes externally. These enzymes broke down surrounding organic matter (like rotten wood) into small-molecule "nutrient soup," which the fungi then directly absorbed.
Evolutionary Consequences:
Grew armor (chitin cell wall): This is a key point. When constantly absorbing external nutrient fluid, water also floods into the cell (osmotic pressure). Without a hard shell, the cell would swell and burst like a balloon. Therefore, fungi had to evolve a rigid cell wall (made of chitin, the same material as shrimp and crab shells).
Abandoned motility: With heavy armor, cells could no longer deform and swim like animals. So fungi lost their flagella and chose a sessile growth lifestyle.
Evolved hyphae: To contact more food and release enzymes, they needed a huge surface area, so they evolved a network of slender hyphae to burrow into their food.
There's a very vivid metaphor to distinguish the two:
Animals put food into a bag (stomach) inside their bodies for digestion. We bring "nature" into our bodies.
Fungi insert themselves directly into their food. They live inside their food. In a sense, the fungal body is an inverted intestine; they use the whole world as their stomach.
After diverging, both independently invented "multicellularity" around 600-800 million years ago, but in completely different styles:
Feature Animalia Fungi Multicellular Structure Tissues and organs: Cells tightly connect, differentiating into complex three-dimensional organs like heart, liver, lungs. Mycelium: Cells connect end-to-end forming thin tubes (hyphae). The mushroom you see is just their reproductive "fruit"; the main body is a vast underground network. Cell Wall None (only a soft membrane). Yes (composed of chitin, rigid yet elastic). Defense Mechanisms Flee or fight: Using muscles and nerves. Chemical warfare: Because they can't run away, fungi evolved strong chemical synthesis capabilities, producing toxins (poisonous mushrooms) or antibiotics (penicillin) to kill competitors or predators. Export to SheetsSummary
The divergence of fungi and animals occurred because fungi chose a survival strategy of "high-pressure absorption flow":
To digest and absorb nutrients externally, they had to don chitin armor (cell walls) to withstand osmotic pressure. This armor, while protecting them, also locked them in place, depriving them of the chance to become "beasts," ultimately evolving into Earth's ultimate decomposers.
What are the evolutionary reasons? What environments were they adapting to?
This is a very profound question. Evolution is not random stumbling but organisms' ultimate response to resource scarcity and survival pressure.
Simply put, the divergence of animals, plants, and fungi was not to adapt to "different temperatures" or "different geographical locations," but to compete for three distinctly different "ecological niches" in the ocean.
The ocean at that time (over a billion years ago) was like a crowded arena. Originally, everyone survived by eating bacteria, but resources were limited. To survive, they were forced into a "misaligned competition":
Environmental Pressure: The ocean surface was sunny, but the bacteria and organic debris used as food were becoming scarcer due to competition. The risk of starvation was high.
Evolutionary Driver: "If I can't grab food, I'll make my own."
The ancestors of plants (Archaeplastida) acquired photosynthetic ability by engulfing cyanobacteria.
Adapted Environment: The Photic Zone.
They occupied the ocean's surface layer, surviving wherever there was sunlight and carbon dioxide.
Cost: This lifestyle didn't require movement and needed a large surface area to capture sunlight, so they evolved cell walls to support their bodies. This made them the "foundation" for all other life.
Environmental Pressure: As life proliferated, the ocean floor accumulated vast amounts of dead organism carcasses and indigestible cell wall residues. These materials were nutritious but too hard and large for ordinary single cells to swallow.
Evolutionary Driver: "If I can't swallow it, I'll digest it first and then absorb."
The ancestors of fungi unlocked the "external digestion" skill tree.
Adapted Environment: Substrates rich in organic matter, such as seabed silt and decaying debris.
They avoided intense predatory competition, specializing in "cleaning up the battlefield."
Cost: To survive in decaying matter and withstand osmotic pressure, they had to develop thick chitin cell walls. This also cost them motility, turning them into "decomposers."
Environmental Pressure: After plants became abundant, the ocean was filled with energy-rich plant cells; after fungi proliferated, there were also abundant fungal masses. However, plants had hard shells, and fungi had toxins, making them increasingly difficult to eat.
Evolutionary Driver: "To eat higher-quality food, I must become stronger."
The ancestors of animals (a branch of Opisthokonta) embarked on the high-risk, high-reward path of predators.
Adapted Environment: The complex, three-dimensional aquatic space.
To crush plant shells or capture other motile organisms, animals had to evolve greater speed, sharper senses (eyes, smell), and more complex nervous systems.
Key Assist from the Oxygen Revolution: Around the same period, due to plant photosynthesis, Earth's atmospheric and oceanic oxygen levels rose dramatically. Oxygen is the "fuel" for efficient combustion, supporting animals' high-energy muscle movement and brain activity. Without this oxygen explosion, such a "luxurious" lifestyle for animals would be unsustainable.
Summary: A Perfect "Division of Labor"
This divergence was essentially a major division of labor in Earth's ecosystem:
Plants (Producers): Handled energy input (converting solar energy into chemical energy).
Fungi (Decomposers): Handled material cycling (breaking down corpses into raw materials).
Animals (Consumers): Handled energy flow (promoting competition and accelerating evolutionary speed).
The most interesting point is:
Their initial split was to avoid competition with each other (I'll eat what you don't, I'll go where you don't). But after splitting, they formed interdependent relationships: animals depend on the oxygen and sugars produced by plants, plants depend on minerals released by fungal decomposition, and fungi wait for the feast after animals and plants die.
This delicate balance is the foundation for the prosperity of life on Earth to this day.
Would you like to know how this split eventually led to the "Cambrian Explosion" (when animals suddenly became huge and complex), or how they faced the challenge of moving onto land?