The 17th-century French Millenarian Isaac La Peyrère, because of his influence on subsequent thinkers and movements, is usually credited with formulating the pre-Adamite theory. In his Prae-Adamitae, published in Latin in 1655, La Peyrère argued that Paul’s words in Romans 5:12-14 should be interpreted to mean that “if Adam sinned in a morally meaningful sense there must have been an Adamic law according to which he sinned. If law began with Adam, there must have been a lawless world before Adam, containing people.”[11] Thus, according to La Peyrère, there must have been two creations; first the creation of the Gentiles and then the creation of Adam, who was the father of the Hebrews. The existence of pre-Adamites, La Peyrère argued, explained Cain’s taking of a wife and the building of a city after Abel’s murder in the Book of Genesis.
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I don’t remember why Isaac La Peyrere is not considered an integral part of the timeline found in another thread… especially since he titled his work “Pre-Adams” (see Romance title above).
Here is the link to the main thread for the Timeline (a work in progress… first we are collecting the individuals…)