Nelson Lund--the author of Rousseau's Rejuvenation of Political Philosophy--has responded in the comments on a previous post by saying that I have misinterpreted both Rousseau and the orangutan studies. Rousseau's "nascent man" is not "utterly solitary," Lund says, because Rousseau acknowledges that mothers care for their offspring, and individuals in the state of nature do "perhaps" recognize one another individually. The life of orangutans looks "much like" this--"an original asocial state of nature in which our ancestors lacked speech and did not form families."
After thinking more about this, I will now say that while there is evidence that some primates--lemurs, galagos, tarsiers, and orangutans--live largely "solitary" lives, they are still social animals, and therefore there is no evidence for "an original asocial state of nature." Moreover, there is some evidence that the earliest form of social organization in primate evolution was solitary but social.
Primatologists distinguish three basic categories of social organization--solitary, pair-living, and group-living. All primates--nonhuman and human--are unusually social animals. So even "solitary" primates are still "social." One primatologist explains:
"By definition, the adults of solitary primate species are typically encountered alone during their period of activity . . . . Because mothers may associate with their offspring, because some individuals may form sleeping groups regularly, because communication among neighbors by sounds and odors is widespread, and because individuals meet conspecifics regularly, 'solitary' should not be confused or equated with 'asocial.' Instead, this term simply reflects the fact that, unlike those of group- and pair-living species, activities of individuals ae not synchronized in a manner that results in permanent spatial association, and thus primatologists encounter solitary primates as single individuals most of the time. A solitary lifestyle is therefore a highly complex and rather challenging system to study" (Kappeler 2012, 24).About a third of all primate species are solitary in this sense. Most of these solitary species are lemurs, galagos (also known as bushbabies), or tarsiers. All of these small primates are nocturnal. Orangutans are the only solitary ape species, and they are the only solitary species that is diurnal.
Lemurs are endemic to Madagascar. Galagos are found in Africa and Asia. Tarsiers are limited to the Indonesian archipelago and the Philippine islands. Orangutans in the wild are found only in Borneo and Sumatra.
Shultz et al. (2011) studied the genetic distances and phenotypic social-structural similarities of 217 living primate species, which showed that social organization tends to be similar among closely related species--and thus suggesting that social structure is genetically determined. They also concluded that the earliest primates lived some 72 Mya as solitary foraging individuals who came together only for mating. Then, multimale/multifemale groups appeared first some 52 Mya.
One could infer from this that the earliest hominids lived in multimale/multifemale promiscuous social bands, as is true for chimpanzees today (Gintis et al. 2015). But still the earliest primate ancestors of these hominids would have been a solitary but social species. If Rousseau's "nascent man" is this kind of animal, then the evidence of primatology can be seen as confirming Rousseau's speculation about the pure state of nature.
Gintis, Herbert, Carel van Schaik, and Christopher Boehm. 2015. "Zoon Politikon: The Evolutionary Roots of Human Political Systems." Current Anthropology 56:327-353.
Kappeler, Peter M. 2012. "The Behavioral Ecology of Strepsirrhines and Tarsiers." In The Evolution of Primate Societies, eds. John C. Mitani, Joseph Call, Peter M. Kappeler, Ryne a. Palombit, and Joan B. Silk, pp. 17-42. Chicago: University of Chicago Press.
Shultz, Susanne, Christopher Opie, and Quentin D. Atkinson. 2011. "Stepwise Evolution of Stable Sociality in Primates." Nature 479:219-222.