Joseph W. DeVincentis, Dave Tuller, Warren Porter, Edward Brisse, Chris Hays, Prec81ious, Ray, RHW, and Bob Kane sent solutions.
One solution is to treat it as a normal chess problem. In this
case, three moves is best.
1. a8-c6 c4-b4 (only possible move)
2. c1-a2+ b4-c4
3. b2-b3 mate
As a retrograde problem, the bishop on A8 is interesting. It is not promoted, there are 8 white pawns. It is not the original bishop, since it could not have gotten past the pawns. The only way this position could be legal is if the board were upside-down! In this case, the position is unusual, but not impossible. White's last move was to deliver a mate with the pawn.
Answer as a retrograde problem: Mate in 0 moves.