According to two papers published in Cell on January 11, 2018, the making of memories and the processes of learning resemble, of all things, a viral infection. It works like this: The shells that transport information between neurons are assembled by a gene called Arc. Experiments conducted by two research teams revealed that the Arc protein that forms a shell, functions much like a Gag, a gene that transports a virus's genetic material between cells during an infection. For example, the retrovirus HIV uses a Gag in exactly this manner.
Viruses are tiny infectious agents that invade cells, bringing foreign genetic material with them. Arc works in a similar way... Arc packages the raw genetic material [and] then visits the other side of the synapse in person. Effectively, Arc ‘infects’ other neurons with the raw materials of memory.
It's hard to fully express how incredible this discovery is. Indeed, the history of the biological sciences has shown that the stranger, the weirder, the more wonderful a finding is, the more likely it is to be true. Recalling a loved dead one is like an infection.
We already know, thanks to the work of Lynn Margulis, that a number of organelles that power the cells of plants and animals descended from bacteria. This is called the endosymbiotic hypothesis. It's now widely accepted as a fact of life. Margulis, who passed away in the winter of 2011, also believed that spiral bacteria evolved into the cilia and flagella that animal cells, such as sperm, use for mobility. (She went as far to suggest that the processes of animal thought or cognition were connected with cilia.) The spiral theory is still controversial. But now that scientists have basically concluded that the processes of memory and learning are closely related to innovations independently developed by viruses and adopted by animals deep in time ("Arc works in a similar fashion right across the tree of life, from flies to humans"), it seems foolish to not seriously re-envision animal and plant biology along Margulian lines.
There is more to think about. Most scientists consider viruses to be non-living. In fact, popular science writer Ed Yong recently made a statement in his piece, "The Viruses That Eavesdrop on Their Hosts," that concerned a new discovery made by another brilliant scientist, Bonnie Bassler, and her student, Justin Silpe (that viruses can listen to the language bacteria uses for what's called quorum sensing): "Viruses are not even technically alive! They’re entirely different entities from bacteria, yet they are intercepting and interpreting the same molecular messages. It’s like a rock eavesdropping on a bird."
If we can compare a rock to a virus—which appears to come alive when infecting a cell, and appears to be dead when doing nothing—then we must begin to suspect that there's something life-like in non-biological things like clouds and hills. Are viruses really the border between life and non-life? Is there a border? A break? The 20th century Russian biochemist, Alexander Oparin, maintained there was a continuum from the inorganic to the organic. This is clearly the case from the stand point of the chemical elements. There is nothing in life that cannot be found in non-living things. We have known this since the 19th century, which experienced the demise of vitalism. But life appears to be not like a rock. And a virus appears to be more like life than a rock. Also life is weird. Viruses are weird. A rock is not. Lastly, it seems that animals and trees are wholly Margulisian—meaning, they are chimerical, or monsters that emerged from and are not separate from the ambiance of microganisms.