Around 1.4 billion years ago, a jump in evolutionary complexity may have occurred when a simple single cell engulfed another simple single cell through phagocytosis. These two cells likely had some genetic variability between them and a more complex cell was born. Through phagocytosis, the ingested cell is not chewed up and digested, but rather forms a beneficial relationship with its devourer. Endosymbiotic theory suggests that post-phagocytosis, the resulting symbiotic relationship was so successful over generations that the engulfed cell eventually relied on the host cell to provide the resources it needed to survive and the host cell likely gained the benefit of any advantageous features its prey may have evolved such as the ability to generate energy or photosynthesize.
The mitochondria in human cells and the chloroplasts required for photosynthesis in plant cells may have started out as bacteria with extraordinary abilities that were engulfed by other cells. These compartments still have minimal circular genomes that look like bacterial genomes, evidence for the hypothesis that they were once independent. Although this approach to symbiosis is limited to simple, single celled organisms, there is evidence that more complex genomes can still benefit from contact with foreign genes. One example of this is a protein called Arc that works very similarly to the viral coat proteins of retroviruses like HIV. It assembles together to create an envelope that carries RNA between cells — this is how retroviruses infect their hosts. In mice and possibly in humans, this behavior is essential for memory function. It’s likely that the human version of Arc came from a virus that infected a cell long ago and now we can’t form memories without it.