How did the female astronaut turn into an ape?

In the original Planet of the Apes, the astronauts get lost in space and crash land on what they believe is an alien planet.

But when they wake up only the female counter part has turned to an ape.

How did this happen?

According to the story line, apes eventually evolved and learned to do all the things that a human was capable of, and the human species lost their capabilities and behaved like primates, but looked a lot like humans. But the astronaut seemingly had turned to an ape.