Its important to distinguish between the myth, one that is entirely a fabrication (and I must reiterate not really one of Rosseau's making, very simplistic interpretation of his work and the brief aside in which he judged uncivilised man to be amoral and unintelligent and is used primarily as a contrast to form his justification for the social contract. You can look to other romance writers for the prevalence of the myth.

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The reality, according to Rousseau, is that man in his initial state thought “good” his rudimentary physical wants and “evil” that which caused him pain. Any knowledge beyond that necessary to satisfy these base desires, and the rationality to obtain such knowledge, was of no consequence.

Thus, Rousseau believes that all prior attempts to frame man’s original condition merely transferred to the state of nature ideas which were acquired in society; so that, in speaking of the savage, they described the social man. This juxtaposition of “savage” and “social” man highlights another of Rousseau’s points of contention: man is by nature a solitary creature.
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and the other myth of the savage uncivilised man in a state of nature aka. Hobbes and Huxley. Neither of these myths were in any way evidence based.

The reality, according to Rousseau, is that man in his initial state thought “good” his rudimentary physical wants and “evil” that which caused him pain. Any knowledge beyond that necessary to satisfy these base desires, and the rationality to obtain such knowledge, was of no consequence.

Thus, Rousseau believes that all prior attempts to frame man’s original condition merely transferred to the state of nature ideas which were acquired in society; so that, in speaking of the savage, they described the social man. This juxtaposition of “savage” and “social” man highlights another of Rousseau’s points of contention: man is by nature a solitary creature.
Kropotkin pointed out the distinction between the direct struggle among individuals for limited resources (generally called competition) and the more metaphorical struggle between organisms and the environment (tending to be cooperative). He therefore did not deny the competitive form of struggle, but argued that the cooperative counterpart has been underemphasized: "There is an immense amount of warfare and extermination going on amidst various species; there is, at the same time, as much, or perhaps even more, of mutual support, mutual aid, and mutual defense...Sociability is as much a law of nature as mutual struggle."[2] However, Kropotkin did consider cooperation as a feature of the most advanced organisms (e.g., ants among insects, mammals among vertebrates) leading to the development of the highest intelligence and bodily organization.

As a description of biology, Kropotkin's work is flawed but not without merit. Stephen Jay Gould admired Kropotkin's observations, but found his grasp of Darwinian theory deficient.[3] Cooperation, if it increases individual survival, is not ruled out by natural selection, and is in fact encouraged. Gould also objected to Kropotkin's reliance on the naturalistic fallacy in refuting Social Darwinism. Modern biology explains Kropotkin's observations in two ways. When different species appear to aid each other, it is a case of mutualism. When individuals within a species aid each other, it is a case of altruism in animals, including kin selection and reciprocal altruism. Douglas H. Boucher places Kropotkin's book as a precursor to the development of mutualism as a theory.[4] In short, Kropotkin's work was successful as observations of nature, but his conclusions are generally suspect because he's not operating within a good scientific framework.