Scientists have discovered a microorganism that may bridge the gap between simple and complex cellular life forms.
The discovery, reported in the journal Nature, will have far-reaching implications in our understanding of the evolution of life on Earth, including humans.
Despite life's diversity, all living things can be classified into two basic groups: simple cell organisms known as prokaryotes, which include bacteria and archaea; and eukaryotes, which are complex cellular organisms that contain nuclei, other internal structures or organelles, and cytoskeletons.
Eukaryotes include all animals including humans, as well as all the plants, fungi, and protists, which are things like algae, moulds, protozoa, diatoms and dinoflagellates.
Understanding how complex eukaryotes evolved from simple prokaryotes has been a major challenge for evolutionary palaeobiology.
Recent studies have hinted that eukaryotic cells may have emerged from archaea prokaryotic cells, but no intermediate organism had been found.
Now, Dr Thijs Ettema of Uppsala University in Sweden and colleagues have identified new archaea they've named Lokiarchaeota, which could be the closest prokaryote known to the eukaryote state.
Ettema and colleagues sequenced samples of marine sediments enriched in deep sea archaea that hadn't previously been cultured in the laboratory.
They found Lokiarchaeota have genes which code for proteins only otherwise found in eukaryotes, such as parts of the cytoskeleton.
"Our results provide strong support for hypotheses in which the eukaryotic host evolved from a bona fide archeon, and demonstrate that many components that underpin eukaryote-specific features were already present in that ancestor," they write.
"This provided the host with a rich genomic 'starter-kit' to support the increase in the cellular and genomic complexity that is characteristic of eukaryotes."
Amazing find
The "technically outstanding" research is "convincing", write evolutionary biologists Professor Martin Emberley and Dr Tom Williams of Newcastle University in the UK, in a separate commentary in Nature.
"Making evolutionary trees for ancient relationships is difficult, especially given that the split between eukaryotes and the Lokiarchaeota may have occurred more than two billion years ago," they write.
But, they say, the genome hints of an organism that has specific traits that could allow eukaryotes to engulf bacterial symbionts, which could eventually evolve into mitochondria, the vital organelle used by eukaryotes as cell power sources.
The find is "very significant", agrees Professor Malcolm Walter of the Australian Centre for Astrobiology at the University of New South Wales
"We have long thought that eukaryotes arose from archaea in some sort of distant relationship, but now it seems it's not so distant at all."
"These are an intermediate organism between the formally known archaea and eukaryotes, so they're filling a gap in the biological record, and that's a very exciting thing."
"I'll be looking for a geological record of these things in the near future."
Stuart Gary
ABC
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