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.