The deeper into evolution you get, the more an uncanny feeling of continuity grows. This is especially true when considering viruses and the origins of life. On some level it is easy to think of a virus as a pre-cell; it is smaller, more compact, in some ways simpler. But without a cell, and without access to the extensive machinery required for replication and release of its progeny, the virus is as dead as a ping pong ball floating through space. So while it’s easy to think of evolution linearly – that over time life has grown more and more complex – I think it more likely that the virus came from us, or whatever we were back then (maybe a simple unicellular prokaryote).
Our cells make virus-like particles all the time – little packets of membrane and genetic information which can be taken up by neighboring cells. These exosomes can go a long way along the spectrum of what we think of as self sustaining or infective. Some are just little blebs of membrane and cytoplasm, harmless and useless. Some contain messenger RNA, which when taken up by a neighboring cell will instruct it to make more of this or that protein. Is it a bridge too far to think one of these messenger packets might have gone rogue, carrying instructions for the hijacking of the next cell, instructions for the endless propagation of self?
When you accept the intertwined nature of our fates, the fossils of viruses start popping up all over our own genome. These artifacts are in every cell in our body – approximately 8% of our genome consists of retroviral DNA, genetic information which was stuck in during some ancient infection and has been passed down generation to generation. There’s the Arc protein found in neurons, which could have been co-opted from an ancient virus to allow for better intracellular signaling. There is the HAP2 protein of sperm, which looks an awful lot like a protein many viruses use to invade cells. This protein is so ancient it is found in all living things that mix up their genetic information through gamete fusion, including plants, animals, and parasites such as malaria’s causative agent, P. falciparum. When you take a step back, the two processes are similar – both the sperm and the virus need to breach a cell, inserting its own DNA. Is it possible that the origins of this process resembled a virus invading a cell, and was only later co-opted for the purposes of genetic mixing?
When you disappear down this rabbit hole, viruses start to feel like extensions of us. They’ve intermingled in our genetic ancestry almost since the beginning, giving and taking some of our best biological innovations. Evolution is a tangled web, affected both by our surrounding environment and by the viruses we’ve caught.