Endosymbiosis hypothesis: What does the endosymbiotic theory explain about the origin of complex cells?

Introduction / Context:
The endosymbiotic theory proposes a pivotal step in the evolution of complex life. It explains how organelles such as mitochondria and chloroplasts originated from once free-living prokaryotes that entered into a stable, mutually beneficial relationship with an ancestral host cell.

Given Data / Assumptions:

• We focus on the directionality of evolution described by endosymbiosis.
• Key organelles: mitochondria (alpha-proteobacterial ancestry) and chloroplasts (cyanobacterial ancestry).
• Evidence is molecular, structural, and functional.

Concept / Approach:

Endosymbiosis holds that certain prokaryotes became internal symbionts, eventually transforming into organelles within a host cell. Genomic remnants (circular DNA), bacterial-like ribosomes, double membranes, and gene phylogenies support this prokaryote-to-eukaryote trajectory.

Step-by-Step Solution:

Verification / Alternative check:

Comparative genomics and phylogenies place mitochondrial genes close to alpha-proteobacteria and plastid genes close to cyanobacteria. Antibiotic sensitivities and ribosomal properties resemble bacteria more than eukaryotic cytosolic systems.

Why Other Options Are Wrong:

Prokaryotes evolving from eukaryotes contradicts both fossil record and molecular data.

Algae/protozoa directional claims do not encapsulate the organelle-origin mechanism described by endosymbiosis.

Common Pitfalls:

Thinking endosymbiosis explains every eukaryotic trait; ignoring that many eukaryotic features derive from the host lineage and subsequent innovations beyond the organelles themselves.

Final Answer:

Eukaryotes evolved from prokaryotes via symbiotic events