What observation led researchers to propose that chloroplasts evolved from cyanobacteria?

Researchers proposed that chloroplasts evolved from cyanobacteria based on several key observations:

1. Structural Similarities: Chloroplasts and cyanobacteria share similar structural features, such as the presence of thylakoid membranes where photosynthesis occurs.

2. Genetic Evidence: Chloroplasts contain their own DNA, which is circular and similar to the DNA found in cyanobacteria. This suggests a common ancestry.

3. Biochemical Pathways: The photosynthetic processes and pigments (like chlorophyll) in chloroplasts are similar to those in cyanobacteria, indicating a shared evolutionary origin.

4. Endosymbiotic Theory: The theory suggests that an ancestral eukaryotic cell engulfed a cyanobacterium, leading to a symbiotic relationship. Over time, the engulfed cyanobacterium evolved into the modern chloroplast.

These observations collectively support the hypothesis that chloroplasts originated from cyanobacteria through an endosymbiotic event.