Final answer:
Cyanobacteria, through endosymbiosis, evolved into chloroplasts in eukaryotic cells and are essential for photosynthesis. They drastically altered Earth's atmosphere by producing oxygen and now remain a critical part of the photosynthetic machinery in plants, algae, and some protists.
Step-by-step explanation:
Cyanobacteria played a crucial role in the origin of the chloroplasts found in modern plant cells. Through a process known as endosymbiosis, primitive eukaryotic cells ingested photosynthetic cyanobacteria. Over time, these cyanobacteria evolved into chloroplasts, which are now essential for the process of photosynthesis in plants, algae, and some protists. These chloroplasts contain chlorophyll a, similar to that found in cyanobacteria, and the thylakoid membranes, which are integral to the light reactions in photosynthesis.
Furthermore, cyanobacteria are significant due to their role in producing oxygen via photosynthesis, contributing to the transformation of Earth's early atmosphere. These organisms contain internal thylakoid membranes where components necessary for photosynthesis are embedded. It is believed that the chloroplasts of green plants are derived from these ancient cyanobacteria, which could no longer live independently as they became an integral part of eukaryotic cells through endosymbiosis.
Interestingly, evidence from studies of organisms like chlorarachniophytes supports the occurrence of secondary endosymbiosis, where a eukaryotic cell engulfs a photosynthesis-capable eukaryote. The chloroplasts in such cases are surrounded by additional membranes, traces of the complex endosymbiotic relationship that led to their current form.