The original human diet had to be some combination of fruit and shellfish. Elaine Morgan is almost certainly correct in thinking humans originally lived in water and whether a person believes in evolution is pretty much irrelevant to that question. Humans would have eaten shellfish which we could deal with with our hands and which fish can't do anything with, and fruit which they went up on shore for. Things like wheat and greens would be tastes acquired much later and it's far from obvious that anything like that is good for us.
The original human diet would have been plants, insects and lots of meat. Before humans hunted they would have scavanged. Especially they would have broken open the bones of large animals for marrow.
Less than 1 million years ago, bipedal - long distance running hominids covered the entire EuroAsian and African continuant's.
All currently living humans are descended from a relatively small tribe of such prehumans who lived on the beach between South Africa and Ethiopia 200.000 years ago. Their lifestyle (up until glacier melting led to inland rivers about 50.000 years ago) probably consisted of running the 5-20 mile lengths of beach and swimming across the dividing channels which range up to a mile or more. Such a beach/outer reef lifestyle would include exploiting the combination of offshore diving, bayside gathering, and a daily jog to beach comb for survival. In other words, humans are evolved to run and swim. Elaine is correct that we evolved to swim but she leaves out the more important long distance running part.
Primary Literature by Jonathan MarksThe Scars of Evolution"The most remarkable aspect of Todaro's discovery emerged when he examined Homo Sapiens for the 'baboon marker'. It was not there... Todaro drew one firm conclusion. 'The ancestors of man did not develop in a geographical area where they would have been in contact with the baboon. I would argue that the data we are presenting imply a non-African origin of man millions of years ago.'"
by Elaine Morgan