From RD's first link:
In 1924, the Russian plant biochemist and evolutionary biologist Aleksandr I. Oparin questioned Haeckel’s scheme. Oparin could not reconcile his Darwinian view—that simple organisms had gradually evolved into more complex ones—with the prevalent belief that life had suddenly appeared on Earth with a self-sustaining metabolism. So he proposed an alternative scenario. He posited that a long period of abiotic synthesis on early Earth had caused organic compounds to accumulate in a prebiotic soup, which had preceded life.