If the face has been getting longer at the bottom over the generations, it has been getting shorter (and broader) on top.
( ( If ( ( the face ) ( has ( been ( ( getting ( longer ( at ( the bottom ) ) ) ) ( over ( the generations ) ) ) ) ) ) ) ( , ( it ( ( has ( been ( getting ( ( shorter ( -LRB- ( and ( broader -RRB- ) ) ) ) ( on top ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN If) (S (NP (DT the) (NN face)) (VP (VBZ has) (VP (VBN been) (VP (VBG getting) (ADVP (RB longer) (PP (IN at) (NP (DT the) (NN bottom)))) (PP (IN over) (NP (DT the) (NNS generations)))))))) (, ,) (NP (PRP it)) (VP (VBZ has) (VP (VBN been) (VP (VBG getting) (NP (NP (JJR shorter) (PRN (-LRB- -LRB-) (CC and) (ADJP (JJR broader)) (-RRB- -RRB-))) (PP (IN on) (NP (NN top))))))) (. .)))
The shape of the face doesn't change at all over the span of generations.
( ( ( The shape ) ( of ( the face ) ) ) ( ( ( does n't ) ( ( change ( at all ) ) ( over ( ( the span ) ( of generations ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN shape)) (PP (IN of) (NP (DT the) (NN face)))) (VP (VBZ does) (RB n't) (VP (VB change) (ADVP (IN at) (DT all)) (PP (IN over) (NP (NP (DT the) (NN span)) (PP (IN of) (NP (NNS generations))))))) (. .)))