真核生物の進化

The first eukaryotic cell evolved from a primitive prokaryotic cell when its plasma membrane invaginated and pinched off, creating membrane-enclosed organelles like the endoplasmic reticulum. The ER organized around DNA to form the nuclear envelope, and budding ER vesicles fused to form the Golgi apparatus. This compartmentalization transformed the simple prokaryotic structure into a complex eukaryotic cell.

The endosymbiont theory proposes that ancestral eukaryotic cells engulfed aerobic prokaryotes that escaped digestion and evolved into mitochondria. Later, some cells acquired photosynthetic cyanobacteria that became chloroplasts. This theory is supported by organellar similarities to prokaryotes, including circular DNA, fission-based division, and prokaryotic-like ribosomes and genetic machinery.

Mitochondria and chloroplasts contain circular, double-stranded genomes without histones, resembling prokaryotic cells. These organelles divide by fission rather than mitosis, and their inner membranes match bacterial plasma membrane composition. Additionally, their genes, genetic code, and translational machinery are more similar to prokaryotic systems than eukaryotic ones.

The Last Eukaryotic Common Ancestor (LECA) existed 1100 to 2300 million years ago and had already acquired mitochondria and chloroplasts. LECA's efficient organelles enabled evolution of multicellularity and cellular specialization. Its increased genome size and non-coding DNA allowed gene regulation, enabling diverse eukaryotic supergroups to evolve and diverge into the life forms we see today.

Eukaryotic cells' large size and internal compartmentalization allowed their genomes to increase significantly during evolution. Most of this expanded genome consisted of non-coding DNA involved in regulatory processes. The ability to turn genes on and off when required became key to cell specialization and responding to environmental changes, enabling complex multicellular organisms.

The nucleus-first hypothesis suggests the ancestral prokaryote first evolved a membrane around DNA to form the nucleus. The mitochondria-first hypothesis proposes the nucleus formed after mitochondrial endosymbiosis. Conversely, the eukaryote-first hypothesis claims prokaryotic and archaeal life forms evolved from already-existing complex eukaryotic cells, challenging the endosymbiont theory.

Phylogenetic and comparative genomic studies reveal five to six eukaryotic supergroups that evolved from LECA and subsequently diverged into the diverse life forms observed today. These supergroups represent major evolutionary lineages that emerged after LECA acquired its endosymbiotic organelles and developed the capacity for increased genomic complexity and cellular specialization.