I disagree with the statement made in this paper. Humans are not hot, sweaty, natural-born runners -- we are hot, sweaty, natural-born swimmers. Although the theory is speculative, human evolution taking place in a semi-aquatic rather than plains environment explains our upright posture to stay above the water, our near hairless bodies to make us streamlined while swimming, our webbed fingers and toes, the ability of human babies having the innate ability to swim.