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I for one have never understood how anyone ever reached the conclusion that
there was an paucity of transitional forms. "Transitional" forms are
common even amongst extant animals. They give taxonomists headaches all
the time. Chitons have partial segmentation. Should they be considered
molluscs or annelids? Boas and pythons have vestigial hind legs. Are they
lizards? Echidnas lay eggs. Are they not mammals?
The very well documented transition from pelycosaurs to therapsids to
mammals not only shows the evolution of the fully erect posture, but quite
clearly shows how some of the jaw bones become modified to form ear
ossicles. I don't see how the transition could be much clearer.
In human evolution, the fossil record is pretty good too, and clearly shows
the evolution of the fully erect stance, increasing cranial capacity, and
changes in tooth morphology, among other things.
Even more dramatic to me is the fact that we can get a "snapshot" as it
were, of the evolution of morphology, by observing development. At certain
stages a fish embryo looks much like a bird embryo looks much like a human
embryo. We can observe the modifications of gill slits and other embryonic
structures to form various derived structures in tetrapods. And the more
closely related species are, the farther along in development we have to
look before they become noticeably distinct.
I would not invest a great deal of energy arguing with a creationist. A
mountain of evidence should make it painfully obvious to anyone with any
intellectual integrity that gorillas, chimpanzees, and orangs are VERY
closely related to humans. If this point cannot be agreed upon, I don't
see what evidence will persuade.