I can't think how this one could be hoaxed, myself. We've already seen the wonders that Homo Sapiens has given us [Two-headed people, women with 10 breasts, pygmies 14 inches tall, etc...], and I don't see any reason to doubt that this might be another example.
As a variation on Disgruntled's theory, what if the local tribes had a fear of very tall people, and banished anyone who was abnormally tall [
a la The Chrysalids], and then these outcasts got together? The tribe thus created would gradually get taller, since all of its members has the "tall gene", shall we say...
Evolution at work, almost...
j/k