These early complex cells contained genes from many different species

These cells already have a complex interior, with internal protein pathways that are crossed by motor proteins that move cargo inside the cell. There were structures (lysosomes and peroxisomes) that are meant to digest proteins inside the cells, and all the essentials of eukaryotic metabolism, DNA replication, and RNA production. One of the main things that was missing was the set of genes that are used to determine when a cell should divide and to control the events that need to happen in order for this to happen. This could mean that cell division started at a slower rate due to metabolic stress.

How did this happen?

About one-third of the gene clusters appear to be unique to eukaryotes and have no equivalents in other kingdoms. Some of them may have existed in the lineage that formed the last ancestor of eukaryotes, and others may have been formed before eukaryotes began to diversify and evolve.

As expected, many other genes came from Asgard archaea or Alphaproteobacteria, in line with the big picture of our origin. But the researchers also found similar contributions from two other bacterial groups: Planctomycetota and Myxococota. (All the groups of bacteria involved are diverse and similar, in contrast to the Asgard archaea.) These results involved three different genetic selections that they did, so they can’t be a snapshot of the analysis.

There were also small contributions from different groups of bacteria. But the types to the group of viruses that include major viruses contributed more than any other bacterial group.

The researchers also estimated when the gene clusters were introduced. Asgard archaea represent the earliest contributions, as might be expected. But there was a line of bacteria that introduced many genes before mitochondria and a second group that made a big contribution after that. This makes sense if eukaryotes evolved within microbial mats, where many organisms live in close proximity for long periods of time and can depend on each other for certain metabolites.