Eukaryogenesis

Eukaryogenesis, the process which created the eukaryotic cell and lineage, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The process is widely agreed to have involved symbiogenesis, in which archaea and bacteria came together to create the first eukaryotic common ancestor (FECA). This cell had a new level of complexity and capability, with a nucleus, at least one centriole and cilium, facultatively aerobic mitochondria, sex (meiosis and syngamy), a dormant cyst with a cell wall of chitin and/or cellulose and peroxisomes. It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor (LECA), gaining capabilities along the way, though the sequence of the steps involved has been disputed, and may not have started with symbiogenesis. In turn, the LECA gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms. Life arose on Earth once it had cooled enough for oceans to form. The last universal common ancestor (LUCA) was an organism which had ribosomes and the genetic code; it lived some 4 billion years ago. It gave rise to two main branches of prokaryotic life, the bacteria and the archaea. From among these small-celled, rapidly-dividing ancestors arose the Eukaryotes, with much larger cells, nuclei, and distinctive biochemistry. The eukaryotes form a domain that contains all complex cells and most types of multicellular organism, including the animals, plants, and fungi. According to the theory of symbiogenesis (also known as the endosymbiotic theory) championed by Lynn Margulis, a member of the archaea gained a bacterial cell as a component. The archaeal cell was a member of the Asgard group. The bacterium was one of the Alphaproteobacteria, which had the ability to use oxygen in its respiration. This enabled it – and the archaeal cells that included it – to survive in the presence of oxygen, which was poisonous to other organisms adapted to reducing conditions.