There's a nice discussion of this by Daniel Dennett Darwin's Dangerous Idea (1995, pp 243-245). Apparently Elaine Morgan has taken considerable establishment heat for these ideas not so much because the ideas are not worthy but because she is a science writer, not a scientist, and is therefore without the "proper" credentials. However, she seems to have followed up on her ideas with testable predictions in a variety of fields, and has revised her theory as needed. Dennett cites a newer book (Roede, et al, "The Aquatic Ape; Fact or Fiction," 1991) that provides a collection of pro and con essays.
My own view is that the theory is fascinating and, whether or not it turns out to be true, is currently a solid competitor with other theories.