A Level Biology Practice Exam

Which theory explains the evolution of eukaryotic cells from prokaryotic cells?

1. Cell theory

2. Biogenesis

3. Endosymbiosis

4. Evolution by natural selection

The correct answer is: Endosymbiosis

The correct answer is indeed the endosymbiosis theory, which explains how eukaryotic cells evolved from prokaryotic cells. This theory proposes that certain organelles within eukaryotic cells, specifically mitochondria and chloroplasts, originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Over time, these engulfed organisms and their host cells developed a symbiotic relationship, leading to a more complex cellular structure. The endosymbiosis theory is supported by various pieces of evidence, such as the presence of double membranes around these organelles, their own circular DNA similar to bacterial DNA, and the fact that they replicate independently of the cell cycle through a process resembling binary fission. This theory highlights a critical step in evolutionary history that led to the diversity of life forms we see today. In contrast, other theories like cell theory fundamentally describe the building blocks of life and how they function, while biogenesis pertains to the origin of life from pre-existing life, neither of which specifically addresses the transition from prokaryotic to eukaryotic cell structures. Natural selection explains how species evolve over time through adaptive traits, but does not specifically detail the origin of euk