Evolutionary theory is a little over my head, but there are some obvious problems with your question.
We don't descend from any Chimpanzee or Orangutan because, if we did,
they would also have the cognitive thinking ability that we have.
We aren't closely related to orangutans, period.
The chimpanzee is considered our closest living relative, but that doesn't mean it's a direct ancestor.
Why would an ancestor have to have the same cognitive thinking ability we have? Isn't cognition something that evolves?
I'm going way out on a limb, but who really knows what kind of cognitive abilities apes have? They probably aren't equal to ours, but still...
I am arguing that a high protein diet allowed the first humans to have
the possibility to develop a higher memory, intelligence and other
Gorillas are herbivores, and they rank among the most intelligent species. Come to think of it, a lot of people are vegetarians.
But after all, what would be the real origin of the intelligence?
I'm going out on a limb a bit, but I think many scientists associate the evolution of intelligence in humans with a combination of physical traits - particularly binocular vision, bipedalism and our opposable thumb. Walking upright freed our ancestors' hands so they could manipulate objects, stimulating the brain. Note that octopuses - which can similarly manipulate objects with their tentacles - are considered among the most intelligent invertebrates.
If animals do have intelligence too, why isn't theirs as advanced as
Mammals have more complex brains than birds, herptiles and other groups; that gives us a head start.
We're also unique in being completely bipedal AND having opposable thumbs. Which isn't to say only animals with these features can be intelligent; whales may rival us in intelligence. But we obviously have some very unique physical traits.
The human brain is also relatively large and complex - though some might argue that that's a result of our physical characteristics.