The short answer: through kin selection.
According to the new paper from the Proceedings of the National Academy of Sciences (PNAS) shown below, and in general in evolutionary biology, altruism is defined as “a behavior decreasing the expected survival and/or reproduction (fitness) of the actor while increasing the fitness of the recipient.”
The simplest example of such altruism involves parental care. A human mother taking care of her child is using resources (milk, time, effort) that in fact reduces her chance of survival or of having future kids. But the kid itself, the recipient, benefits. Parental care evolves because the cost to the mom is less than the benefit to the kid she tends.
Likewise with any sacrifice people make for their relatives. The reason this has evolved is that genes promoting parental behavior do entail a cost to their carriers, but they more than repay that cost by helping the perpetuation of the same genes (“genes identical by descent”) in the offspring, which has a 50% of getting a parental-care gene from the parent. Thus the gene gets a net boost from the behavior it produces.
So there’s a calculus involved for genes that reduce your fitness but help that of the recipient. This calculus is expressed in “Hamilton’s rule,” introduced by the great evolutionary biologist W. D. Hamilton. In general, a gene producing altruistic behavior—reducing the fitness of its carrier but helping others who carry copies of the same gene—will evolve by natural selection (i.e., increase in frequency) if it satisfies this equation:
r x b > c
where c is the fitness cost to the donor of performing the act, b is the benefit to the recipient, and r is the “degree of relationship”, i.e., the chance that the recipient actually carries a copy of the altruism-producing gene because it’s related to the donor (“identity by descent”).
So, for example, r for parents vs. offspring is 0.5: the chance that an offspring will inherit an altruism gene (gene form, actually: an “allele”) from a parent is 50% due to segregation and assortment during reproduction. One can conclude that a gene that makes you expend effort to help your kid will be favored by natural selection if the fitness benefit to your kid is at least twice the cost to you. r for siblings is also 50% (brothers and sisters share half their genes), so a gene could be favored that causes you to help your siblings if the cost to you is also less than half the benefit to your siblings. r for uncles compared to nieces and nephews is 25% (therefore, for Uncle Joe, his altruism will evolve if the cost to him is less than a quarter of the benefit to niece Sarah, and so on.
The interaction between relatives, close or distant gentically, is the way that most evolutionists think that altruism has evolved. For a gene that incurs fitness costs in its bearer, but doesn’t give a benefit to those carrying other copies of the same gene, will go extinct. This is why when we observe self-sacrifice in nature, it’s nearly always to help relatives. (Think of the “broken wing” display in which a mother bird, feigning injury but risking her life, lures a predator away from her chicks.)
And when animals have a way to recognize and avoid taking care of unrelated organisms, they can. Here’s a note evolutionist Bruce Lyon sent me about the work of him and his colleagues on coots:
American coot females lay eggs in each others’ nests and they recognize and the host parents deal with the brood parasitic eggs/offspring at two stages: they recognized about a third of parasitic eggs and reject them by burying them down in the nest and they can also learn to recognize some parasitic chicks, and if they recognize the chicks they kill them.
. . . Lots of other birds have been shown to be able to recognize their own chicks, as in colonial seabirds, but they don’t use this to kill other chicks but instead insure that they feed their own kids.
This makes no sense unless parental care involves relatedness. (If you have questions about this, I’ll ask Bruce to answer them in the comments.)
It doesn’t have to be direct relatedness, either. If a population is viscous, with individuals not moving around much, people will become related simply because they mate more often with nearby individuals. That’s why there’s a high degree of relatedness in small religious communities like the Dunkers and Amish, who don’t marry their siblings or cousins but marry those in the community. Over time, this causes an increase in relatedness in such communities.
Hamilton proposed his “rule” in 1964, but others hit on it as well, including J. B. S. Haldane, who was reported to say that he’d lay down his life for two brothers or eight cousins (you’d have to save all the relatives’ lives for this to work), and the idea was also worked out mathematically by the eccentric biologist George Price.
But in the last two decades, several biologists have claimed that altruism could evolve without this kind of kin selection—without individuals behaving in a way to favor their relatives. Most prominent among these contrarian biologists is Martin Nowak at Harvard, who has said that altruism doesn’t need relatedness to evolve, simply requiring a particular population structure. Other biologists have said that kin selection could work, but so could population structure alone.
It turns out that in all these cases, the population structure proposed in fact causes individuals to be related and favors altruism because of that relatedness. While many biologists recognize the mathematical equivalence of “population structure” and “kin selection models”, Nowak has denied this, stating that geographic population structure alone (his model of “spatial selection”), even if it doesn’t create a web of relatedness, could favor the evolution of altruism.
Nowak is wrong. This is demonstrated in the new paper in PNAS by Kay, Keller, and Lehmann (click on link to get it, pdf here, and reference at bottom). The upshot: you can’t get the evolution of altruism with population-structure alone, unless that population structure creates kin relationships that satisfy Hamilton’s rule. Kin selection remains the sine qua non for the evolution of altruism.
What the authors did is simple: they looked up all the scientific papers that showed the evolution of altruism, including those that ignored kin selection as well as those that denied kin selection was operating, and then analyzed whether the models indeed created a structure in which relatedness was important to determine whether kin selection—even if ignored or denied—was crucial for evolving altruism. They found 89 papers of theoretical models in which altruism evolved. The authors parsed them this way (my emphasis):
Among the 89 altruism models, 46 adopted Hamilton’s conceptual framework, attributing the evolution of altruism to positive relatedness. The remaining 43 all claimed alternative mechanisms. To evaluate the veracity of their claims, we first subdivided these 43 papers into those where the role of relatedness was denied (17 cases; SI Appendix, Table S3), and those which made little or no mention of relatedness (26 cases; SI Appendix, Table S4).
Among the 17 papers where the presence/role of relatedness was denied (SI Appendix, Table S3), our analysis of the life cycles of the models showed that the proposed scenario led to positive relatedness between interacting agents in every case. Moreover, in most of these models, agents reproduced clonally (e.g., “parents pass on their type to their offspring”) with interactions occurring among nearest neighbors, as in the stepping-stone model of Fig. 1, with only one individual per node/group. This represents the tritest instance of kin selection.
As for the remaining 26 models which proposed non-kin mechanisms for the evolution of altruism but didn’t mention relatedness, this is what Kay et al.’s analysis showed:
The 26 papers which make little or no mention of relatedness attribute the evolution of altruism to diverse alternative mechanisms including “social diversity,” “social viscosity,” “topological heterogeneity,” “network heterogeneity,” “network reciprocity,” “spatial reciprocity,” “spatial structure,” and “multiplex structure” (SI Appendix, Table S4). Analysis of these models revealed that in every case interacting individuals are related, relatives benefit from each other’s altruism, and kin selection therefore operates.
So none—zero, zip, bukes—of the “alternative” models could evolve altruism without kin selection and relatedness. This isn’t so mendacious when the authors just ignore kin selection, proposing models that nevertheless produce the interactions that allow kin selection. But it IS bad behavior when authors like Nowak claim that the evolution of altruism has nothing to do with kin selection and relatedness. That is a form of careerism—proposing some new mechanism when you haven’t done the scientific legwork (like Kay et al. did) to show that the new boss is the same as the old boss.
The upshot: the evolution of biological altruism, in which individuals sacrifice their own fitness to help others, cannot proceed without kin selection. There would be no selection on parents to help adopted children, since they aren’t related. (The fact that they do, in both humans and animals, is certainly a case of misplaced parental instinct. Warblers feeding cuckoo chicks, who aren’t even in the same species, is a prime example of hijacking of parental impulses.)
Why do so many authors ignore kin selection or say it isn’t operating in the evolution of altruism? (Nowak isn’t the only one of the latter.) Kay et al. give three suggestions. The first is careerism, as I’ve mentioned above: you don’t get famous by just showing what’s already been demonstrated. But they also note that some models are made by people who aren’t evolutionists and thus may be unaware of Hamilton and Price’s work (these people are economists, physicists, and so on). Finally, some authors know and understand Hamilton’s rule but are so steeped in it that they simply don’t bother to bring it up explicitly in their models.
So don’t believe claims that altruism can evolve without kin selection.
BUT, what about those cases in which, say, humans help others, risking their lives for people who aren’t related to them? I often use as examples volunteer firemen, who risk their lives for people they don’t even know. Or, in war, soldiers have died by throwing themselves on a grenade to save their platoon. This is certainly altruism, but it doesn’t involve kin. This kind of sacrifice is almost completely unknown in other species, where individuals aren’t seen to risk their lives for non-relatives. That alone gives you a clue that there is some cultural aspect to this kind of altruism in humans. But you are as good as I am in speculating about this, and I’ll leave it as a thought exercise.
Coda: I never met Hamilton even though we overlapped in time (he lived from 1936-2000, dying at only 63 from what may have been a combination of an ulcer and malaria). But I know many people who knew Hamilton well, and without exception they paint him as an unassuming and genial man—an all-around nice guy as well as a scientific genius (he was also a keen naturalist and spent a lot of time in the tropics). He had some bizarre ideas, but also some ideas that became foundational in the evolution of behavior. Here he is (I just noticed that he looks a bit like me, but with longer hair).
Kay, T., L. Keller, and L. Lehmann. 2020. The evolution of altruism and the serial rediscovery of the role of relatedness. Proceedings of the National Academy of Sciences 117:28894-28898.