The original O's were hunters and had acidic stomachs for digesting meat, says D'Adamo.

Type O-
People with type O blood fare best intense
physical exercise and animal proteins and less well on
dairy products and grains, says Dr. D'Adamo. The leading
reason for weight gain among Type O's is the gluten found in
wheat products and, to a lesser extent, lentils, corn, kidney beans, and cabbage, Dr. D'Adamo explains. Ideal exercises for Type O's include aerobics, martial arts, contact sports, and running. 

D'Adamo admits that even he was initially skeptical about the blood-type diet theory. His first inkling that "By Jove!
Dad might be on to something" came when he discovered that
type O's tend to get more ulcers. You remember, type O's are
the ones with the acidic stomachs for digesting prime rib and Quarterpounders. Type A's, on the other hand, have a lower
ulcer rate but a higher rate of stomach cancer, linked to low
levels of stomach acid production.
Eureka!