All life on Earth is carbon-based. 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: single-celled life followed by multicellular life, then plants and animals. 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 of single-celled life.
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, direction, and agency. 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. In them, life flirts with a science-fiction idea: silica-based life.
All this 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 earlier populations.
