Origin of eukaryotic organelles: which best explains how eukaryotic cell organelles first emerged?

Introduction / Context:
The endosymbiotic theory explains the emergence of membrane-bound organelles such as mitochondria and chloroplasts. Understanding this concept is central to evolutionary cell biology.

Given Data / Assumptions:

• Eukaryotes possess mitochondria (nearly universal) and, in plants/algae, chloroplasts.
• These organelles contain their own DNA and 70S ribosomes.
• We seek the best explanation for the initial emergence of such organelles.

Concept / Approach:
Endosymbiosis posits that ancestral prokaryotes entered into stable, mutually beneficial relationships: a host engulfed aerobic bacteria that became mitochondria; later, a eukaryote engulfed cyanobacteria-like cells that became chloroplasts. Genomes were reduced and interdependence evolved.

Step-by-Step Solution:

Verification / Alternative check:
Phylogenies place mitochondrial genes close to alpha-proteobacteria and chloroplast genes near cyanobacteria, supporting endosymbiosis.

Why Other Options Are Wrong:
A: Protista is a paraphyletic grouping; not an explanation. C: Reproduction attempts do not explain organelle origins. D: Organelles arose much earlier than animal/fungal divergence.

Common Pitfalls:
Assuming organelles arose de novo within eukaryotes rather than through symbiosis; confusing chronology of eukaryotic diversification.

Final Answer:
when prokaryotes engulfed each other and became interdependent