Meet three new species of Loricifera
, the first multicellular forms of life found that can live entirely without oxygen (figures and full article
Why is this major? This is the first multicellular
organism discovered without mitochondria
, cellular organelles which facilitate aerobic respiration, i.e. allow life to burn food with the help of oxygen.
There are many forms of bacteria (prokaryotes
) that can do anaerobic respiration, as well as a few single-celled anaerobic eukaryotes like Giardia
and Trichomonas vaginalis
, but no multicellular organisms — until now.
It is widely believed that the ancestor of all eukaryotes
(cells with nuclei) engulfed and retained an Alphaproteobacteria
and that the descendants of that engulfed bacteria became mitochondria — and the descendants of that host
became the group we now call eukaryotes.
Single-celled eukaryotes that live in low- or no-oxygen environments often lack mitochondria but possess a "hydrogenosome
", an organelle that resembles mitochondria and functions like its anaerobic equivalent, helping to turn food into energy (but without the help of oxygen).
There is debate about the evolution of these hydrogenosomes, though it is mostly thought that mitochondria in an anaerobic eukaryote lost the enzymes necessary for aerobic respiration, retaining those needed for anaerobic respiration (such as pyruvate-ferrodoxin oxidoreductase, used for reducing pyruvate which is thought to have been in ample supply in a prebiotic Earth). Alternatively, hydrogenosome-containing eukaryotes may represent a completely distinct
lineage of eukaryotes from the mitochondrial lineage.
One hypothesis is that these organisms share a common ancestor with jellyfish
or some other common multicellular eukaryote. These organisms must then have either undergone endosymbiosis with a single-celled anaerobe (much like what is hypothesized to have happened with mitochondria) or undergone convergent evolution to develop robust hydrogenosome-like functionality.
Another hypothesis, (which, while much less likely, would be by far the most groundbreaking result, if true) is if these organisms represent a completely new multicellular animal that evolved more directly from an anaerobic ancestor with a hydrogenosome.
No matter which evolutionary path these organisms took to get where they are today, studies of loriciferans are likely to alter our understanding of the diversity of life in fundamental ways.