I was watching this video: Crash Course Big History #6- Human Evolution. In the video, John Green says: "The classic view of foraging life is best described by Thomas Hobbes, who wrote: 'No arts, no letters, no society, and which is worst of all continual fear and danger of violent death and the life of man, solitary, poor, nasty, brutish, and short.'" And then he offers another view on foraging life: Jean-Jacques Rousseau's: "The first person, who having enclosed a plot of land, took it in his head to say, 'this is mine' and found people simple enough to believe him, was the true founder of civil society. What crimes, murders, what miseries and horrors would the human race had been spared, had someone pulled up the stakes or filled in the ditch and cried out to his fellow men: 'Do not listen to this impostor, you are lost if you forget that the fruits of the earth belong to all, and the Earth to no one." However, Thomas Hobbes lived from 1588-1679 and Jean-Jacques Rousseau lived from 1712-1778. "On the Origin of Species" was published by Charles Darwin in 1859, and before this, humanity had no knowledge of evolution. Didn't the world believe that the Earth was 6,000 years old before Darwin published "On the Origin of Species?" How did Thomas Hobbes and Jean-Jacques Rousseau know that humans were hunter-gatherers for 200,000 years? I would greatly appreciate any answers.
What Hobbes and Rousseau were using to make their argument is known in philosophy a thought experiment. Simply put, a thought experiment is an imaginative device that allows the author to work out a hypothesis or a theory. Both Hobbes and Rousseau were state of nature theorists, which meant that their arguments hinged on developing a theory for how humans naturally act and how society came to be. To do this, they created thought experiments to describe how humans acted before they entered into civil society and how they entered into civil society. This "pre society) period is called the State of Nature (SoN).
The Hobbesian SoN describes humans as naturally egoist and self-serving, and where man has the liberty to preserve himself by any means necessary, and who's primary focus is self preservation above all else (preservation of others is secondary). These are known as the Laws of Nature. Hobbes describes this state of unchecked personal liberties as a State of War. The State of War doesn't mean never ending and continuous violence, but the continuous threat of violence. "For WAR consisteth not in battle only or the act of fighting; but in a tract of time, wherein the will to contend by battle is sufficiently known." Hobbes does not see humans in the state of nature as communal beings, but rather extremely hostile and selfish. Such an existence Hobbes famously remarked, would define the life of man as, "solitary, poor, nasty, brutish, and short." Understandably so, Hobbes argues that no one wants to life in such conditions.
The solution to the SoN for Hobbes is in the social contract. The social contract is a process whereby people lay down their liberties to absolute self preservation to a single person, with the understanding that this person (whom still retains their natural rights) will protect you and enforce any trade/agreements made with others. This act of giving up ones natural rights in order to engage in such activities is the moment when man exits the state of nature and enters civil society. This theoretic understanding of human nature was the basis for Hobbes' larger argument for the existence of absolute monarchs, who were that singular person who retained their natural god given rights.
Rousseau's state of nature was different to the Hobbesian theory. Rousseau argued that humans lived in state where human interaction is largely at a minimum. They will join together to mate, but then go their separate ways. Eventually, Rousseau argues, the perils of individual reliance become too great and men form social pacts (contracts) to combat the forces of nature. Rousseau makes clear that this communal formation is different from Hobbesian monarch, "[man] in giving himself to all, gives himself to nobody." The general will of the people, Rousseau argues, is the supreme ruler. Once again, the formation of this social contract is the condition for exiting the state of nature, and entering civil society.
Rousseau's theory of the general will was the basis for his argument in favor of representative sovereign. Since no one man could control the genera, the government must have its power derived from the people. This sort of rhetoric should be familiar with Americans, as we see this type of language and logic used extensively in arguments over the formation of the Constitution.
Now, to answer your question, "How did Thomas Hobbes and Jean-Jacques Rousseau know that humans were hunter-gatherers for 200,000 years?" Frankly, they didn't. For Hobbes, it would have been a difficult task to philosophically justify the existence of a monarch without making claims about the fundamentals of human existence, and the emergence of civil society. The task of justifying the representative system requires just the same. These thinkers were crafting thought experiments and laying out frameworks for the nature of humans in order to build a foundation for their larger arguments. Whether or not they were "correct" is in consequential to these larger theses.
Sorry for any poor grammar or sentence structure, its a tad late in the evening.