The first eukaryotes arose as long as 2.9 billion years ago, nearly a billion years earlier than is commonly assumed.
In this process, an archaeal host cell eventually acquired a nucleus and developed into cells with a complete cytoskeleton and the capacity for membrane transport and phagocytosis, only acquiring mitochondria through endosymbiosis 2.3 billion years ago.
This scenario – in which complex life arose under low-oxygen conditions – is published by a team of palaeobiologists, microbiologists and evolutionary biologists in
Nature of 3 December, with NIOZ microbiologist and UvA professor of symbiosis
in evolution Anja Spang as second author. The researchers base this CALM scenario – standing for Complex Archaeon, Late Mitochondrion – on a time-calibrated phylogenetic tree constructed from duplications in 62 marker genes for essential cellular processes across hundreds of species of eukaryotes, bacteria and archaea.
‘Quite an undertaking’, according to molecular biologist and lead author Christopher Kay of the University of Bristol. ‘It required a combination of disciplines: palaeontology to establish the timeline, phylogenetics to produce reliable and usable phylogenetic trees, and molecular biology to place gene families in context.’ A key finding is that mitochondria, arising through the endosymbiotic uptake of a proto-bacterium, appeared late in eukaryotic evolution, and that this timing coincides with the first substantial rise in atmospheric oxygen.
The scenario now outlined conflicts, in any case, with the hydrogen hypothesis for eukaryogenesis, which Chinese microbiologists invoked earlier this year (in Nature, 7 May) in support of their claim that eukaryogenesis took place 2.5 billion years ago.