Complex sentence: In late 1974, she used locatives after an object phrase for the first time; the lateness with which she did this surprised the scientists, as she had already been using locatives to modify subject noun phrases for a long time.
Split into simpler sentences: In late 1974, she used locatives after an object phrase for the first time. The lateness with which she did this surprised the scientists, as she had already been using locatives to modify subject noun phrases for a long time.