Final answer:
Plastids evolved from photosynthetic cyanobacteria through endosymbiosis, where an ancestral prokaryotic organism engulfed cyanobacteria, leading to the integral relationship that became plastids.
Step-by-step explanation:
Plastids evolved from photosynthetic cyanobacteria through a process known as endosymbiosis. This event occurred when an ancestral prokaryotic organism engulfed cyanobacteria, which then evolved into what we currently understand as plastids or chloroplasts in modern-day plants and algae. This relationship became so integrated that the cyanobacteria lost the ability to live independently and became a permanent part of the host cell structure, contributing to the photosynthetic capabilities of the host organism. The evidence for this evolutionary process includes similarities in DNA sequences between plastids and cyanobacteria, and also the fact that plastids, like mitochondria, contain their own prokaryote-like DNA, which supports the theory of their endosymbiotic origins. The primary endosymbiosis event is believed to have happened twice in the history of eukaryotes, leading most photosynthetic eukaryotes to be descended from the first event.