Jeff looked at the screen. His finger was ready. Oh, was it ready. So was his dick. His boner was ready to fucking pop off and get going with his evolution. And once he pressed the start button, the feelings grew even more intense, even as his anatomy modified rapidly.
The first big change was to the skin. It grew thicker, most of the hair follicles vanishing from it aside from on the head. Over 5000 years, humans had evolved to have a tougher hide, keeping them warm without needing fur and making them more resilient to predators and to knives, bullets and other weapons they had created.
His figure slimmed and his muscle mass increased, so Jeff was significantly stronger than humans in his time. As a counterpoint to their modern, sedentary lifestyle, humanity became much more adept at burning fat and converting energy to muscle.
Jeff's body had also shifted to appear more feminine, which he realized was because female sex characteristics were more effective for encouraging procreation. But a bigger change soon followed: his balls moved up inside him and his cock started to shrink! For a moment he freaked, but then it made sense to him: procreation being reliant on two members of opposite sexes was inefficient, so humanity developed to have one sex. His former scrotum opened out into his pussy lips, and his nipples widened and the tips enlarged as he developed mammaries. The vulnerable to physical injury forms of the breasts and balls had been removed, and he could tell from the weird feeling inside him human sperm were now capable of surviving at the body's normal temperature. He also felt his vaginal tract and womb form inside him, and his hips widened, his figure slightly wider than a normal human's so he could give birth without pain or difficulty. At the same time, his penis's shaft had receded to be almost flush with his slit, able to become erect to impregnate a mate but also short enough to trickle its seed into his vagina to make his body self-sufficient.
Jeff had to admit, humanity had kinda ironed out all its flaws now. But what to do with his new form?