The Divergence of Plants and Animals

This is a fascinating evolutionary story. Simply put, the “parting of the ways” between animals and plants did not happen in the forests or grasslands familiar to us, but in the primordial oceans 1.6 billion years ago, and it was completed at the unicellular level.

The core reason for this split can be traced to one key “engulfment” event, followed by the choice of two entirely different survival strategies.

1. The common starting point: LECA (Last Eukaryotic Common Ancestor)

Before they split up, animals and plants had a common ancestor, known to science as LECA (Last Eukaryotic Common Ancestor).

• Life forms at the time: They were all unicellular organisms living in water. They already possessed a nucleus and mitochondria (responsible for providing energy), and survived by engulfing surrounding bacteria or organic fragments.

• They were all “consumers”: At this point, everyone was heterotrophic and had to move around searching for food.

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2. The key divergence: an “engulfment” that changed the world

About 1.5–1.6 billion years ago, the most important “accident” in the history of biological evolution occurred within the eukaryotic family, leading to the emergence of the plant lineage.

The ancestors of plants: swallowed a “solar panel”

A unicellular ancestor somewhere along the plant lineage engulfed a cyanobacterium (Cyanobacteria, a type of photosynthetic bacterium), but strangely, instead of digesting it, it kept the bacterium inside its body to “work” for it. Shutterstock

• Endosymbiosis: The engulfed cyanobacterium later evolved into the chloroplast.

• Consequences: Cells that acquired chloroplasts gained a “superpower”—they could get full simply by basking in sunlight. As a result, they no longer needed to laboriously swim around hunting for food, and instead chose to “lie flat.”

• Evolutionary direction: To make better use of sunlight, they evolved cell walls to support their bodies, becoming sessile plants (Archaeplastida, the archaeplastid organisms).

The ancestors of animals: staying true to their “foodie” nature

The ancestors of animals (and fungi) did not undergo this chloroplast-engulfing event—or, in other words, their lineage missed the opportunity.

• Consequences: They had to continue obtaining energy by “eating” others.

• Evolutionary direction: To capture prey, they needed soft, flexible bodies, so they either discarded or never evolved thick cell walls. To hunt more efficiently, they evolved complex locomotor systems, nervous systems, and muscles, eventually becoming animals (Opisthokonta, the opisthokonts).

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3. Different survival strategies led to differences in form

Once they split due to the presence or absence of chloroplasts, the evolutionary logic of the two lineages became completely different:

FeaturePlant lineage (Plant Lineage)Animal lineage (Animal Lineage)Energy sourceSelf-sufficient (autotrophic): relies on chloroplasts for photosynthesis.Preying on others (heterotrophic): relies on consuming other organisms.Cell structureCell wall: like building a house, there is a hard shell outside the cell. This limits movement but provides support, allowing plants to grow very tall.No cell wall: only a cell membrane. Cells are soft and deformable, allowing muscle contraction and body movement.LifestyleSessile: rooted in one place, with no need to think about chasing prey; evolution focuses on obtaining more sunlight and water.Motile: must move to find food. Evolution focuses on sensory organs (eyes and ears), neural processing (the brain), and locomotion (muscles).Growth patternIndeterminate growth: as long as conditions permit, trees can continue growing taller and branching.Determinate growth: after reaching adulthood, animals’ body size is basically fixed and they do not continue growing indefinitely.

4. An interesting fact about their “close relationship”

Although animals and plants look worlds apart, genetically speaking, they are not actually as distant as you might imagine. They even have some surprising “family relationships”:

• Animals and fungi are closer relatives: You may think mushrooms (fungi) look like plants, but on the evolutionary tree, fungi are actually close relatives of animals. Like animals, they are heterotrophs (they do not photosynthesize), and the component of their cell walls (chitin) is more similar to the exoskeletons of insects than to the cellulose of plants.

• Both have mitochondria: Whether it is one of your cells or the wild grass by the roadside, the cells contain mitochondria. This proves that at an even earlier stage (about 2 billion years ago), we were once one family.

Summary

The parting of the ways between animals and plants was essentially a choice of “whether to make food for yourself”:

• The ancestors of plants chose photosynthesis as a one-time solution, at the cost of being locked inside cell walls and losing the ability to move freely.

• The ancestors of animals were forced to continue hunting, and for survival 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)?

This is a brilliant sequel. If the split between animals and plants happened because of “whether they engulfed chloroplasts,” then the split between animals and fungi was a disagreement over the philosophy of “how to digest.”

Although they are both members of the opisthokont (Opisthokonta) family, about 1 billion years ago they made starkly different choices at an evolutionary crossroads, and thus went their separate ways.

The core of this split was: should food be “swallowed and eaten,” or “digested outside first and then absorbed”?

1. The common starting point: little “swimming champions” waving flagella

Before the split, the common ancestor of fungi and animals was a unicellular organism living in water that looked somewhat like modern choanoflagellates.

• It had a flagellum that swung backward (like the tail of a sperm), used for swimming.

• It had no cell wall, and its body was soft.

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2. The core divergence: a revolution in feeding strategies

This was the key moment when the two paths diverged:

The animal route: sticking with “engulfment” (Phagotrophy)

The ancestors of animals chose a “big-mouth meat-eating” route.

• Strategy: They retained the softness and flexibility of the cell membrane. When they encountered food, they wrapped it with the cell membrane and swallowed it for digestion inside the body (endocytosis).

• Evolutionary consequences:

• No cell wall: To swallow things, the cell had to be soft; with a hard shell, it could not deform to engulf food.

• They had to be mobile: To obtain food, they had to actively pursue and capture it, so they retained and strengthened muscles and nervous systems.

• An internal digestive system: When they later became multicellular, they evolved complex gastrointestinal tracts to process the food they swallowed.

The fungal route: switching to “external digestion” (Osmotrophy)

The ancestors of fungi unlocked a unique skill tree—“chemical warfare.”

• Strategy: Instead of laboriously swallowing food, they secreted digestive enzymes outside the body. These enzymes broke down surrounding organic matter (such as rotten wood) into small-molecule “nutrient soup,” which the fungi then directly absorbed.

• Evolutionary consequences:

• Growing armor (chitinous cell walls): This was a crucial point. When you continuously absorb nutrient liquid from the outside environment, water flows into the cell as well (osmotic pressure). Without a hard shell, the cell would burst like a balloon. Therefore, fungi had to evolve hard cell walls made of chitin, like the shells of shrimp and crabs.

• Giving up movement: With heavy armor, cells could no longer deform and swim like animals. So fungi discarded their flagella and chose sessile growth.

• Evolving hyphae: To contact more food and release enzymes, they needed a huge surface area, so they evolved networks of slender hyphae (Hyphae) that penetrated into the food to consume it.

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3. Why say that fungi “turned themselves into a digestive tract in order to eat”?

There is a vivid metaphor for distinguishing the two:

• Animals digest food inside a bag (the stomach) into which they put it. We put “nature” inside our bodies.

• Fungi insert themselves directly into food. They live in their own food. In a sense, the fungal body is an inverted digestive tract: fungi treat the entire world as their stomach.

4. Independent paths to multicellularity

After the split, both independently invented the “multicellular” mode roughly 600–800 million years ago, but in completely different styles:

FeatureAnimals (Animalia)Fungi (Fungi)Multicellular structureTissues and organs: cells are tightly connected and differentiate into complex three-dimensional organs such as the heart, liver, and lungs.Mycelium: cells connect end to end to form tubes (hyphae). The mushroom you can see is merely their “fruit” during reproduction; the actual body is a huge underground network.Cell wallNone (only a soft membrane).Present (made of chitin, hard yet elastic).Defense methodsEscape or fight: using muscles and nerves.Chemical warfare: because they cannot run away, fungi evolved a powerful capacity for chemical synthesis, producing toxins (poisonous mushrooms) or antibiotics (penicillin) to kill competitors or predators.

Summary

The split between fungi and animals occurred because fungi chose a survival strategy of “high-pressure absorption flow:
To digest food outside the body and absorb nutrients, they were forced to put on chitin armor (cell walls) to withstand osmotic pressure. Although this armor protected them, it also locked them in place, depriving them of the chance to become “beasts,” and they ultimately evolved into Earth’s foremost decomposers.

This is a very profound question. Evolution is not random blind collision; it is the ultimate response of organisms to resource scarcity and survival pressures.

Simply put, the split between animals, plants, and fungi was not about adapting to “different temperatures” or “different geographical locations,” but about competing for three entirely different “ecological niches” (Ecological Niches) in the ocean.

At that time (more than a billion years ago), the ocean was like a crowded gladiatorial arena. Originally, everyone survived by eating bacteria, but resources were limited. To stay alive, they were forced into a “displaced competition”:

1. The ancestors of plants: solving the “energy crisis”

Environmental pressure: The ocean’s surface received abundant sunlight, but the bacteria and organic detritus serving as food were increasingly scarce as everyone competed for them. The risk of going hungry was high.

Evolutionary impetus: “If I can’t get food, I’ll make my own.”
The plant ancestors (archaeplastid organisms) acquired the ability to photosynthesize by engulfing cyanobacteria.

• Environment adapted to: the photic zone (Photic Zone).

• They occupied the ocean’s surface layer, able to survive with nothing but sunlight and carbon dioxide.

• Cost: This lifestyle did not require movement and required a very large surface area to receive sunlight, so they evolved cell walls to support their bodies. This made them the “foundation” of all other life.

2. The ancestors of fungi: making use of “trash nobody eats”

Environmental pressure: As life proliferated, large quantities of dead biological bodies and hard-to-digest cell-wall debris accumulated on the ocean floor. Although these materials contained nutrients, they were too hard and too large for ordinary unicellular organisms to swallow.

Evolutionary impetus: “If I can’t swallow it, I’ll digest it first and then absorb it.”
The fungal ancestors unlocked the “external digestion” skill tree.

• Environment adapted to: substrates rich in organic matter (Substrates), such as seafloor mud and decaying remains.

• They avoided intense predation competition and specialized in “cleaning up the battlefield.”

• Cost: To survive in decaying matter and withstand osmotic pressure, they had to grow thick chitinous cell walls. This also caused them to lose their ability to move, turning them into “decomposers.”

3. The ancestors of animals: adapting to the “arms race”

Environmental pressure: After plants became more abundant, the ocean was filled with energy-rich plant cells; after fungi became more abundant, there were also abundant fungal masses. However, plants had hard shells and fungi had toxins, making them increasingly difficult to eat.

Evolutionary impetus: “To get higher-quality food, I have to become stronger.”
The animal ancestors (one branch of the opisthokonts) embarked on the high-risk, high-reward path of predation.

• Environment adapted to: complex and variable three-dimensional aquatic spaces.

• To bite through the hard shells of plants or capture other moving organisms, animals had to evolve greater speed, sharper senses (eyes and smell), and more complex nervous systems.

• A crucial assist from the oxygen revolution: Around the same time, photosynthesis by plants caused oxygen levels in Earth’s atmosphere and oceans to rise substantially. Oxygen is the “fuel” for efficient combustion, supporting the high-energy muscular movement and brain activity of animals. Without the oxygen boom, this “luxurious” animal lifestyle could not have been sustained.

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Summary: a perfect “division of labor”

This parting of the ways was actually a great division of labor in Earth’s ecosystem:

• Plants (producers): handled the input of energy (converting solar energy into chemical energy).

• Fungi (decomposers): handled the recycling of matter (reducing corpses back into raw materials).

• Animals (consumers): handled the flow of energy (promoting competition and accelerating the pace of evolution).

The most interesting point is:
Their split initially arose to avoid competing with one another (you eat what I do not; I go where you do not). But after splitting up, they formed interdependent relationships: animals cannot do without the oxygen and sugars produced by plants, plants cannot do without the minerals released by fungi’s decomposition, and fungi wait for a feast after animals and plants die.

This exquisite balance is precisely the foundation that has allowed life on Earth to flourish 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?