All life on Earth is carbon-based. From bacteria to trees to whales to you, the chemistry of life is built around carbon’s flexible bonding power. But that simple fact can hide a more interesting story. Life did not begin as plants and animals. It began as single-celled organisms, and much of life stayed single-celled because the strategy works.
The mistake is thinking evolution is a ladder: first single-celled life, then multicellular life, then plants and animals, with each step replacing the last. That is not how evolution works. Evolution is a branching tree. Some ancient single-celled lineages eventually led to multicellular plants, animals, and fungi. But many other branches remained single-celled, diversified, and are still evolving today. Bacteria, archaea, protists, algae, amoebas, yeasts, and diatoms are not evolutionary leftovers. They are successful living branches.
This also helps clarify the idea of a last common ancestor, or LCA. When we say humans and chimpanzees share a last common ancestor, we do not mean one modern animal evolved directly from the other. We mean both lines branched from an earlier population. The same is true deeper in life’s story. Animals did not simply “replace” earlier life. They branched from older forms while many other branches kept going in their own direction.
One of the great later turning points was multicellularity. Some cells began living together, communicating, specializing, and forming larger bodies. From there, animal evolution eventually developed bilateral bodies: left and right sides, front and back, direction, motion, sensing, and eventually agency. Bilateral structure did not create agency by itself, but it gave life a geometry that agency could build on. A creature with direction can begin moving through the world in a more organized way.
But single-celled life never stopped being powerful. Diatoms are a beautiful example. Each diatom is one cell, yet many form chains or colonies and help shape entire ecosystems. They are photosynthetic algae wrapped in glass-like silica shells called frustules. They are carbon-based life using silica as armor and architecture. In them, life almost seems to flirt with a science-fiction idea: carbon writes the living chemistry, while silica helps build the house.
So no, life did not simply evolve from single cells into plants and animals as if the old world vanished. Life branched. Some branches became forests, fungi, insects, birds, and humans. Other branches remained microscopic and still run much of the planet. Diatoms bring the story full circle: single-celled carbon life wearing silica glass, sometimes organized with shape, direction, and movement. Not full agency, but perhaps a modest glimpse of proto-agency — life responding to the world through form.
