Pc7 = Ph2. White made 7 P captures and Bc8 was captured at home.
Pd6 = Pe7. Black made 3 P captures.
Black's only got a few moves left to retract, so the start is forced.
-1.Bg2-h1 a5xBb4 -2.Bf1-g2 a6-a5 -3.g2xRf3!
A screen on the 8th rank is needed, but it is not yet clear why it must be a Rook.
-3...Rf7-f3 -4.R~-h7 Re7-f7 -5.R~ Re8-e7 -6.Kh7-h8 Ng6-f8+
The rest of the sequence is easy enough to see without going move by move. wR, K, B return to a1, e1, c1; f7xQe6 puts wQ back, she returns to d1, d2xBe3 puts bB back, he returns to f8; e7xNd6 clears the way for wPc7, and the next uncapture must be d6xPc7, because bPc could not have left the file.
So why was the 2nd uncapture a Black Rook? Answer: An R cannot get back to a8 unless he does so before all the Black pawns retract.