If his path hadn't crossed with that scientist, he might never have made the discovery.

crossed 是动词 cross 的过去分词形式。cross 本意是"穿过、横跨"，在这里引申为"相遇、交叉"的意思。crossed with 表示"与...相遇/交叉"，指人生轨迹的偶然交汇。