There’s no definitive theory yet on how viruses evolved. Are they cellular parasites which have lost most of their unnecessary living functions, or are they genetic assemblages derived from the “jumping genes” of some organism?
Possibly both: simple viruses with just a few genes may be the latter, while complex viruses may be descended from free-living bacteria-like organisms.
I’ve been fascinated by the T4 bacteriophage ever since I saw an Open University programme about it. (Remember those?)
It looks like some sort of space module, or, at its true scale, a kind of tiny molecular machine.
And that’s exactly what it is. The basic components — the icosahedral head, the stem, the legs — are all minute pieces of molecular engineering. T4 only infects one species of bacterium: the escherichia coli which lives in our gut. When a T4 encounters an e. coli cell, it “lands” on the surface like a lunar module. The legs grip the surface, and pull the head downwards, causing the stem to pierce the cell wall, and the virus’s DNA to be injected.
Within ten minutes, the viral DNA has been spliced into the cell’s DNA, and the cell begins to generate virus components instead of the normal proteins it needs to live.
Remarkably, the different virus components are produced separately within the cell, and “self-assemble” into complete viruses. Self-assembly is a big topic in nanotechnology, with engineers attempting to manufacture their own molecular machines. T4 is way ahead of them.
Afer about half an hour, the doomed bacterium is stuffed full of newly-hatched viruses, and it bursts open, releasing them into the environment (your intestine, in this case) to infect new hosts.