The Forgetting Sarah Marshall star travelled to Kenya's capital Nairobi on behalf of Britain's Sport Relief charity to raise awareness of poverty in the region.
He witnessed dreadful living conditions, bonded with HIV-positive children, and met orphans who have suffered sexual abuse - and he was shocked by the experience.
The funnyman, currently dating pop star Katy Perry, writes in Britain's Daily Mirror newspaper: "Africa is the cradle of humanity, where our species began. The horror we witnessed... made it difficult not to consider that this would be where humanity would end.
"This is the end of mankind, where amidst the medical and chemical waste children forage with pigs and birds and cattle, all animals levelled to the lowest imaginable ebb due to abominable neglect... Whilst here - engulfed by the endless stink and the sound of children choking on dust as cows chewed through plastic and heron swooped down with ghoulish, flesh necks and tarnished feathers - I could not see from where hope could be derived."
And on a visit to a school for orphans, he reflected, "I spent a lot of time hanging about alone at school and I had none of the problems these kids are facing. It was an honour to be even briefly illuminated by their glow."