What observation led Charles Darwin to propose that organisms originate from a common ancestor?

• A. The lack of variation within populations
• B. The presence of homologous structures in different species
• C. The uniformity of environmental conditions across the globe
• D. The identical genetic makeup of all living organisms

Answer: (B)

Verified step by step guidance

1. Understand the concept of homologous structures: Homologous structures are anatomical features that are similar in structure but may serve different functions in different species. These similarities suggest a shared evolutionary origin.
2. Recognize Darwin's observation: Darwin noticed that different species, even those adapted to vastly different environments, often had similar anatomical structures, such as the forelimbs of mammals, birds, and reptiles.
3. Connect homologous structures to common ancestry: The presence of homologous structures implies that these species inherited these features from a common ancestor, which underwent evolutionary changes over time to adapt to different environments.
4. Eliminate incorrect options: The lack of variation within populations, uniformity of environmental conditions, and identical genetic makeup are not observations that directly led Darwin to propose common ancestry. These ideas either misrepresent biological diversity or oversimplify genetic differences.
5. Conclude the reasoning: Darwin's observation of homologous structures in different species was a key piece of evidence supporting the idea that all organisms share a common ancestor, as these structures reflect evolutionary relationships.