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Eusociality (from Greek εὖ eu "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups.

Did mammals evolve from something with eusociality?

I'm asking because I'm curious about the possibility that mammals evolved from something for which self sacrifice of individuals generally helped their genes survive. Eusociality seems the limit example of that (correct me if I'm wrong).

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No. The only truly eusocial mammals are Heterocephalus glaber (the naked mole-rat) and Fukomys damarensis (the damaraland blesmol). Eusociality doesn't just involve an individual sacrificing their individual fitness for others, altruism is a common behavior among many species, from arthropods and worms to mammals.

Eusociality is specifically a condition where the duties of reproduction are partitioned among individuals in a social group, with some individuals contributing to breeding future generations and others never getting the opportunity to breed. I.e., an ant colony. Only the queen and drone males ever contribute genetically to the future generation, the worker and soldier ants die without leaving offspring. In the naked mole rat and the blesmol, only the queen and some males breed while the other individuals provide them with food, shelter, defense, but never get to breed.

E.O. Wilson suggested that humans can be considered quasi-eusocial because in human society we reward individuals who contribute to society but do not breed (e.g., career individuals), but E.O. Wilson was notoriously kooky with regards to anything involving human society and gender roles.

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  • $\begingroup$ Though with humans (and perhaps other species), a lot of that supposedly self-sacrificing behavior isn't really altruism, so much as it is a a consequence of foresight. E.g. if I risk my life to save the tribe from a man-eating lion, then if I'm successful I'm a hero and get lots of chances to breed. $\endgroup$ – jamesqf Jan 19 at 17:35
  • $\begingroup$ @jamesqf I mean, that's the purpose of altruistic behavior in every species ever. "If I risk my life to save my offspring it will increase my chances to pass on my genes", etc. Even in eusocial species altruism has a selfish basis, or else the behavior wouldn't persist and be passed on. $\endgroup$ – user2352714 Jan 19 at 18:51
  • $\begingroup$ I'm using the dictionary definition of altruism, though: "disinterested and selfless concern for the well-being of others". $\endgroup$ – jamesqf Jan 20 at 2:21

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