IB Biology Exam Prep

Answer: E, Protozoon. The protozoon is the simplest and oldest organism among the choices, and is the only unicellular organism listed.

Key Takeaway: A key principle of evolution is that simpler beings evolve and give rise to more complex organisms. Organisms with simple cellular structure give rise to organisms with more complex structure, tissues, and organization. On that principle alone, the protozoon, a unicellular eukaryote, is most likely to be the oldest organism among the choices.

Photo: Protozoon from cow's rumen, Sharon Franklin, Stephen A. Ausmus