Roughly half of the human protein.">genome is made of sequences that can copy themselves and shift to new locations. For decades these transposons were dismissed as genetic junk, or worse, as parasites that occasionally wrecked a gene and caused disease. That verdict was too harsh. The same machinery that lets them jump has been repurposed again and again, and the evidence now suggests they were indispensable to the rise of complex animals.
Consider the placenta. A gene that helps mammals build this temporary organ appears to have been borrowed from a viral sequence that fused cells together. Similar stories keep surfacing: stretches of DNA that once jumped now serve as switches that turn genes on in the brain, the immune system and the developing embryo. In some cases they supply ready-made regulatory elements that evolution can rewire instead of inventing from nothing.
The picture emerging is less about invasion than about negotiation. Transposons supply raw material; the host gradually brings them under control and puts them to work. Half a genome's worth of mobile DNA is not baggage. It is a library of spare parts, most of it quiet, some of it essential.
