1. Even though the whale-resembling Basilosaurus ostensibly looked different from Mesonychid, scientists have discovered from fossil records that beneath Basilosaurus's flippers were fingers that are anatomically similar to those of a Mesonychid. Such existence of a vestigial organ (trace of homologous organ in other species) serves as a powerful evidence that an evolution had occurred.
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3. The three organisms exemplify a convergent evolution, a process by which unrelated organisms independantly evolve similar trait(s); in this case, wings. We may assume that the three organisms, despite fundamental differences, were pressured into adapting to similar environments where flying was necessary for survival.
4. Organisms which share a more recent common ancestor are more closely related in terms of their DNA structures; to be more precise, in terms of their genetic sequence. By comparing the DNAs of different organisms in the Common Descent Lab, we can get a glimpse about the so-called 'evolutionary tree', a branching diagram that accounts for the degree of change of a certain specie. Many primates we know including monkeys are in the same evolutionary branch as humans. This means that humans are not direct descents of monkeys, but share a common ancestor with them.
5. Structures that have different mature forms but develop from the same embryonic tissues are called homologous structures, and homology refers to the appearance of such homologous structures among different organisms. For example, the limbs of the four modern vertebrates: turtle, aligator, bird and mammal are homologous structures that consist of same basic bones.
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