I heard that Rousseau (died in the 1770s) once speculated that chimpanzees might be humans in the “state of nature” that classical liberal philosophy discusses. The original humans before language, technology, social organization, and property.
I also remember hearing once of an ancient Greek explorer who went to Africa and saw apes for the first time (might have been gorillas or chimps or something, I don’t remember) and described them as a “tribe”, as if he were under the impression these were some kind of human, albeit very different than any he’d ever met.
Apparently throughout ancient, medieval and early modern history there was some understanding of apes being a kind of animal that was closer to humans than any other, with eerie similarities like their body shape and their ability to grasp with hands.
Were Lamarck and Darwin truly the first to develop theories of evolution, was there any awareness before them that humans evolved from apes? It would seem strange for Rousseau to suggest what he suggested, for example, if no one had ever heard of the idea that humans descended from apes.
Before Darwin and Lamarck, there was argumentation about natural laws. To boil it down, the argument involved if humans could scientifically determine the ways the world worked. For example, if I were to shoot this cannon ball, I can determine how far it will go with a certain angle, and velocity. Before the ideas of natural law, most intellectuals believed it was impossible to determine, what were seen as, acts of divine power. One of the main individuals to perceive the world this way was Issac Newton with the establishment of the Three Laws of Motion. Charles Darwin even pays lip service to this idea of natural laws when he uses William Whewell's quote in Origin of Species that “events are brought about not by insulated interpositions of Divine power, exerted in each particular case, but by the establishment of general laws.” This "establishment of general laws" carried over into the biological realm with naturalists attempting to explain life through evolution.
In evolutionary terms, the ideas of evolution had existed before Lamarck and Darwin. In 1735, Carl Linnaeus established a hierarchical relationships between plants,with Systema Naturae, and with the tenth edition in 1758, he organized animals. While not an evolutionist, it highlighted the differences and relationships between species, something that would be crucial to evolutionary theory. It is important to note that Rousseau admired Linnaeus so much that he wrote "I know no greater man on earth". On top of Linnaeus, there was Buffon, a French naturalist who lived in the same time period of Rousseau, exhibited pre-Darwinian thought. For Buffon, creatures have been "improved" and or "degenerated", indicating some level of evolutionary thought that might have possibly influenced Rousseau. Despite, Buffon's thoughts, he refused to the concept that humans were related to apes. This denial led to debate with James Burnett who argued that apes were the "brother of man". In addition, Buffon and Burnett, there was also Erasmus Darwin, Charles Darwin's grandfather who argued that every creature had originated from a common ancestor in the mid 1790s. So even before Lamarck and Darwin, there were concepts of evolutionary thought. The only thing that separated Lamarck and Darwin was how the process of evolution occurred. For Lamarck, it was Lamarckism, and for Darwin, it was the process of natural selection.
With that in mind, it would make sense that Rousseau was reading and following people like Linnaeus, Buffon, and Burnett, and by following these naturalists, he could have been influenced by evolutionary ideas.