These cells already had complex interiors, with internal protein trackways traversed by motor proteins that move cargo within the cell. There were structures (lysosomes and peroxisomes) meant to digest proteins within the cells, and all the basics of eukaryotic metabolism, DNA replication, and RNA production. One of the big features that was absent were sets of genes used to determine when a cell should divide and managing the events that need to take place for that to happen. This may suggest that cell division started out as simply limited by metabolic concerns.
How’d this happen?
Roughly a third of gene groups appear to be distinct to eukaryotes and don’t have equivalents in other kingdoms. Some of those may have been present in the lineage that produced the last common ancestor of eukaryotes, and some may have been generated before eukaryotes really started to diversify and branch out.
As expected, many of the other genes came from either the Asgard archaea or Alphaproteobacteria, consistent with the big picture model of our origins. But the researchers also found roughly equal contributions from two other bacterial groups: Planctomycetota and Myxococcota. (All of the bacterial groups involved are diverse and relatively common, in sharp contrast to the Asgard archaea.) These results held up in each of the three different choices of genes they had performed, so aren’t likely to be an artifact of the analysis.
There were also small contributions from a range of different bacterial groups. But species from the group of viruses that includes giant viruses contributed more than any single bacterial group.
The researchers also estimated the timing of when groups of genes were introduced. Asgard archaea represent the earliest contribution, as would be expected. But there was a bacterial lineage that introduced a lot of genes before the mitochondria were present and a second group that made a major contribution afterward. This makes sense if eukaryotes evolved within a microbial mat, where lots of species are in close proximity for long periods of time and may depend on each other for certain metabolites.
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