The Wrong Button

Jerry Anderson was an astronaut faced with an impossible choice: die of asphyxiation in a few hours or see if the alien pod he was transporting really was an escape pod and find out if it could actually save him.

When he enters it, he finds that the controls are unreadable, lacking anything to go on, and rapidly running out of air. He presses the blinking green button.

The next thing he knows, he isn't human anymore, and he finds himself on a seashore, next to some birds feasting on a body that looks very similar to his new body.

He is alive, but staying alive will be a challenge, and he will be able to communicate with the locals, assuming the next ones he finds don't kill him on sight.