The ancient stone age people believed that semen was marrow. Like the marrow of bones. They believed that the brain too was marrow because it was inside a round bone called the skull just as marrow is in the thiner bones of the legs and fingers and so on.
These people believed that when a man ejaculates that he is losing marrow. They believed that when a person ejaculates marrow from the skull (the marrow of the skull is the brain) would travel down the spine and be spilled out on the ground. Hence they believed that semen was pieces of ones brain that had been spilled on the ground.