How do scientists theorize that cyanobacteria affect the development of endosymbiosis?

Cyanobacteria are primitive organisms where chloroplasts are believed to be originated. Now, with the theory of endosymbiosis, this suggests that these organelle from cyanobacteria was engulfed by other organisms which has helped them process light with the help of chloroplasts.

The answer is atmosphere. there are four spheres of the earth: 1. geosphere (from greek word 'geo' = ground), 2. hydrosphere (from greek word 'hydro' = water), 3. biosphere (from greek word 'bio' = life), and 4. atmosphere (from greek word 'atmo' = air). a magnetosphere is a region of space around astronomical objects controlled by the magnetic field of that object. through the process of elimination, since lightning is present in the air, it can be found in the atmosphere.