Background: The tree of life has been proposed to consist of three major groups (domains) of species: Bacteria, Archaea, and Eukaryotes. Under this proposed arrangement, each group arose from a single common ancestor (all species in that group diverged/evolved from a single ancestor species), and those common ancestors arose from a shared common ancestor way before that (thus proposing that all life on Earth as we know it evolved from a single species). This doesn’t necessarily mean that there was only one original living cell and all life descended from that, but that’s a whole other can of worms that I won’t go into today. What I want to focus on is that the current arrangement of the tree of life has Archaea (single-celled organisms, many of which live in extreme conditions) and Eukaryotes (animals, plants, etc.) as separate groups. Today’s paper by Spang et al. identifies and sequences genes from previously undiscovered species of Archaea that support an origin for Eukaryotes within the group of Archaea.
Problem/Question: Due to the length of time since the divergence of early life on Earth, and the vast number of species we have yet to identify, it is very difficult to accurately determine the deep relationships in the tree of life.
Results: The authors sampled water near deep sea vents in the Arctic Mid-Ocean Spreading Ridge and sequenced DNA found in their samples. From this, they identified new sequences that were similar to sequences of the same gene (16S rRNA) from Archaean species. Additional sequences were amplified and eventually a nearly complete genome (~92%) was sequenced for one of the new species (names Lokiarchaeum after the sample collecting location near a landmark called “Loki’s Castle”). Others were not abundant enough in the samples from the deep sea vents to get enough DNA for full genome sequencing.
When phylogenetic analyses were performed (comparisons of gene or protein sequences between different species to determine their relationships), Lokiarchaeum was found to be related to species of Archaea from one of the largest, and most studied groups (known as TACK after names of species within the group). Additionally, when they compared 36 protein sequences that are present in Archaeans and Eukaryotes, the Eukaryotic sequences grouped with Lokiarchaeum with strong support. This was surprising in that some scientists have hypothesized that Eukaryotes arose from within Archaea, but it has been very difficult to find support for this hypothesis.
In order to further support their findings, and better ensure that it was not due to an error or a quirk of the phylogenetic analyses used, they compared many other genes throughout the genome of Lokiarchaeum. While they found most genes were most closely related to Bacterial or Archaean genes (as expected for an Archaean species), there were 175 that were most similar to Eukaryotic gene sequences. The authors performed numerous tests to confirm that this finding is not due to contamination. Additionally, some of these genes suggest that Lokiarchaeum and its relatives had some features more similar to Eukaryotic cells than previously identified in Archaeans. The authors point out a few, but I won’t go into detail here as that would take a lot of extra explanation.
Big Picture: This study identified new species of Archaea that appear to be the most similar to Eukaryotes found to date. These findings provide support for the origin of the Eukaryotic species group within Archaea instead of next to it. They also narrow down where within Archaea Eukaryotes may have arisen and lay the groundwork for identifying more species that are closely related to Lokiarchaeum that could provide additional detail into the origin of Eukaryotes. By determining the origin and relationship of Eukaryotes (which we are part of) compared to other species, we can better understand how life evolved on planet Earth and how humans and other species became what they are today.
The Ribosome 