Which observation would indicate that a fossil sequence shows evolutionary change?

• A. The fossils are located in the same layer of rock as modern horses.
• B. The fossils share some structural similarities with modern horses.
• C. The fossils are completely identical to modern horses.
• D. The fossils show a progression of similar structures from ancient to modern horses.

Answer: (D)

The idea being tested is how the fossil record can reveal evolution over time. When fossils show a progression of similar structures from ancient to modern horses, it provides a clear timeline of gradual changes in traits across generations. This pattern indicates that species have transformed over long periods, developing modifications that accumulate and lead toward the modern form, which is how evolution is observed in the real world.

Why this observation fits best: it shows a sequence, not just a snapshot. Seeing a series of fossils that displays a gradual shift in features—such as limb arrangement, tooth shape, or body size—from ancient to modern forms demonstrates change over time and a lineage connecting past and present.

The other options don’t show evolution over time in a sequence. Finding fossils in the same rock layer as modern horses doesn’t reveal that change happened through time. Simply sharing some structural similarities with modern horses shows relatedness but not a timeline of progressive change. Being completely identical to modern horses would indicate no evolutionary change at all.