Complex sentence: On each of the Apollo 17 EVAs, Schmitt was the second out of, but first back into, the Apollo Lunar Module, and is thus the last person to have stepped onto the Moon.
Split into simpler sentences: On each of the Apollo 17 extravehicular activities (EVAs), Harrison Schmitt was the second person out of, and first back into, the Apollo Lunar Module (LM). Schmitt is thus the twelfth and last person to have stepped onto the Moon.