Metabolic symbiosis at the origin of eukaryotes

Thirty years after Margulis revived the endosymbiosis theory for the origin of mitochondria and chloroplasts, two novel symbiosis hypotheses for the o rigin of eukaryotes have been put forward. Both propose that eukaryotes aro se through metabolic symbiosis (syntrophy) between eubacteria and methanoge nic Archaea. They also propose that this was mediated by interspecies hydro gen transfer and that, initially, mitochondria:were anaerobic. These hypoth eses explain the mosaic character of eukaryotes (i.e. an archaeal-like gene tic machinery and a eubacterial-like metabolism),as well as distinct eukary otic characteristics (which are proposed to be products of symbiosis). Comb ined data from comparative genomics, microbial ecology and the fossil recor d should help to test their validity.