Q1.
If many organisms share common features, could they also share a common ancestry?
Answer
Yes — that is the standard interpretation, provided the shared feature is a deep structural one and not just a similar solution to a similar problem. As the chapter puts it, similar features in organisms suggest that they have evolved from common ancestors.
Why the inference works: inherited features are passed on through DNA. If two organisms share a complex structure built the same way from the same parts, the simplest explanation is that both inherited it from an ancestor that already had it, rather than that the same complicated structure was invented twice by chance. The forelimb of a bat, a whale and a human contains the same bones in the same order — that is a strong signal of shared ancestry.
But be careful — resemblance can mislead. A shark, a dolphin and a squid are all streamlined, but a shark is a fish, a dolphin is a mammal and a squid is a mollusc. Their body shape is a shared response to water, not a shared ancestor. The wing of an insect (a fold of exoskeleton) and the wing of a bird (feathered forelimb) are similarly unrelated. So the rule is: the deeper and more detailed the shared structure, the stronger the evidence of common ancestry — and the strongest evidence of all is the one the chapter names last, similarity in DNA, which is why genetic data later forced the three domain system.