Final answer:
Chloroplasts likely originated from cyanobacteria through a symbiotic relationship with a eukaryotic host, a process known as endosymbiosis, as evidenced by their own DNA, ribosomes, and dual membrane structure.
Step-by-step explanation:
The likely origin of chloroplasts is through a process known as endosymbiosis, according to the endosymbiotic hypothesis, which is widely accepted in the scientific community. This hypothesis suggests that chloroplasts evolved from cyanobacteria that entered a symbiotic relationship with a eukaryotic host cell. Lynn Margulis was a significant figure in providing evidence for this theory. Chloroplasts share similarities with bacteria, such as their own DNA and ribosomes, and are also surrounded by two membranes, supporting the idea that these organelles descended from a symbiotic relationship between a eukaryotic host and photosynthetic bacteria.
There's also apparent evidence for a secondary endosymbiosis event, whereby a eukaryote hosting a chloroplast itself became symbiotic with another cell, leading to complex chloroplast structures bounded by multiple membranes in certain lineages, such as chlorarachniophytes. Overall, the endosymbiotic origin of chloroplasts represents a significant step in the evolution of eukaryotes, leading to the wide diversity of photosynthetic life we observe today.