Final answer:
Vestigial structures are body parts that have lost their original function through evolution, serving as evidence for common ancestry and evolutionary changes in organisms.
Step-by-step explanation:
The structures that serve no apparent purpose in a living organism, seemingly remnants from a previous ancestor, are known as vestigial structures. These structures include examples like the wings on flightless birds, the leaves on some cacti, the hind leg bones in whales, and the human appendix. These vestigial structures are significant because they provide evidence for evolution, suggesting that organisms have changed from using the structure to not using the structure or repurposing it for different activities.
It is important to note that not all similarities between organisms are based on common ancestry; some are the result of convergent evolution and thus are called analogous structures. Homologous structures, on the other hand, are similar structures in different organisms that reflect a common ancestor.
Vestigial structures are a fascinating aspect of evolutionary biology and help to trace the lineage and adaptive history of species.