Eukaryotic Cell

The eukaryotic cell is the fundamental unit of organization for all complex life on Earth: animals, plants, fungi, and protists. It is defined by the presence of a membrane-bound nucleus enclosing the genome and by internal membrane-bound organelles — mitochondria in all eukaryotes, chloroplasts in photosynthetic lineages, endoplasmic reticulum, Golgi apparatus, lysosomes. The eukaryotic cell is not a prokaryotic cell that evolved internal compartments. It is a chimera: a fusion of an archaeal host and a bacterial symbiont, whose genomes merged and whose metabolisms integrated to produce an organism with capabilities neither ancestor possessed. The eukaryotic cell appeared approximately two billion years ago, after nearly two billion years of exclusively prokaryotic life. Its appearance was discontinuous — a phase transition, not an incremental elaboration — and it opened the possibility space for multicellularity, nervous systems, consciousness, language, and culture. Every human thought occurs in eukaryotic cells.