Os cientistas acreditam que o segredo de nossas vidas longas são nossos ancestrais.
Os pesquisadores acreditam que os idosos contribuíram para a longa vida humana.
A seleção natural é impiedosamente egoísta, de acordo com um cânone de longa data na biologia evolutiva, preferindo características que aumentam a probabilidade de reprodução bem-sucedida. Isso geralmente indica que a chamada “força” de seleção está bem equipada para eliminar mutações prejudiciais que se manifestam durante a infância e ao longo dos anos reprodutivos.
Michael Gurven, professor de antropologia da UC Santa Barbara. Crédito: Matt Perko
No entanto, diz-se que a seleção perde o interesse em nosso bem-estar físico quando a fertilidade diminui. Nossas células são mais suscetíveis a mutações perigosas após a menopausa. Isso geralmente significa que a mortalidade ocorre rapidamente após a fertilidade parar na esmagadora maioria dos animais.
Isso coloca os humanos (assim como certas espécies de baleias) em um clube exclusivo: criaturas que vivem muito depois de suas carreiras reprodutivas terminarem. Como podemos suportar viver à sombra da seleção por décadas?
“Do ponto de vista da seleção natural, a longa vida pós-menopausa é um quebra-cabeça”, disse Michael Gurven, professor de antropologia da Universidade da Califórnia em Santa Bárbara.
Essa relação entre fertilidade e longevidade é bastante evidente na maioria das espécies, incluindo os chimpanzés, nossos parentes vivos mais próximos entre os primatas, onde a probabilidade de sobrevivência diminui em proporção direta à capacidade de procriar. Em contraste, apesar de perder a capacidade de ter filhos, as mulheres podem viver décadas em humanos.
“Nós não ganhamos apenas alguns anos extras – temos um verdadeiro estágio de vida pós-reprodutivo”, disse Gurven.
Em um estudo publicado no Anais da Academia Nacional de Ciênciaso autor sênior Gurven e o ecologista populacional e ex-pesquisador de pós-doutorado da UCSB Raziel Davison desafiam a sabedoria convencional de que o poder da seleção natural em humanos deve desaparecer completamente após a reprodução sexual.
Eles afirmam que uma longa vida pós-reprodutiva não se deve apenas aos recentes avanços na saúde e na medicina. “O potencial para uma vida longa faz parte de quem somos como humanos, uma característica evoluída do curso da vida”, disse Gurven.
O segredo para o nosso sucesso? Nossos avós.
“As ideias sobre o valor potencial dos adultos mais velhos estão circulando há algum tempo”, disse Gurven. “Nosso artigo formaliza essas ideias e pergunta qual pode ser a força da seleção quando você leva em conta as contribuições dos adultos mais velhos”.
Por exemplo, uma das principais ideias para a longevidade humana é chamada de Hipótese da Avó – a ideia de que, por meio de seus esforços, as avós maternas podem aumentar sua aptidão ajudando a melhorar a sobrevivência de seus netos, permitindo que suas filhas tenham mais filhos. Tais efeitos de aptidão ajudam a garantir que a[{” attribute=””>DNA is passed down.
“And so that’s not reproduction, but it’s sort of an indirect reproduction. The ability to pool resources, and not just rely on your own efforts, is a game changer for highly social animals like humans,” Davison said.
In their paper, the researchers take the kernel of that idea — intergenerational transfers, or resource sharing between old and young — and show that it, too, has played a fundamental role in the force of selection at different ages. Food sharing in non-industrial societies is perhaps the most obvious example.
“It takes up to two decades from birth before people produce more food than they’re consuming,” said Gurven, who has studied the economy and demography of the Tsimané and other indigenous groups of South America. A lot of food has to be procured and shared to get kids to the point where they can fend for themselves and be productive group members. Adults fill most of this need with their ability to obtain more food than they need for themselves, a provisioning strategy that has sustained pre-industrial societies for ages and also carries over into industrialized societies.
“In our model, the large surplus that adults produce helps improve the survival and fertility of close kin, and of other group members who reliably share their food, too,” Davison said. “Viewed through the lens of food production and its effects, it turns out that the indirect fitness value of adults is also highest among reproductive-aged adults. But using demographic and economic data from multiple hunter-gatherers and horticulturalists, we find that the surplus provided by older adults also generates positive selection for their survival. We calculate all this extra fitness in late adulthood to be worth up to a few extra kids!”
“We show that elders are valuable, but only up to a point,” contends Gurven. “Not all grandmothers are worth their weight. By about their mid-seventies, hunter-gatherers and farmers end up soaking up more resources than they provide. Plus, by their mid-seventies, most of their grandkids won’t be dependents anymore, and so the circle of close kin who stand to benefit from their help is small.”
But food isn’t everything. Beyond getting fed, children are also taught and socialized, trained in relevant skills and worldviews. This is where older adults can make their biggest contributions: While they don’t contribute as much to the food surplus, they have the accumulation of a lifetime of skills they can deploy to ease the burden of childcare on parents, as well as knowledge and training that they can pass on to their grandchildren.
“Once you take into account that elders are also actively involved in helping others forage, then it adds even more fitness value to their activity and to them being alive,” Gurven said. “Not only do elders contribute to the group, but their usefulness helps ensure that they also receive from the surpluses, protections, and care from their group. In other words, interdependence runs both ways, from old to young, and young to old.”
“If you’re part of my social world, there might be some kickback,” Davison explained. “So to the extent that we’re interdependent, I’m vested in your interest, beyond just simple kinship. I’m interested in getting you to be as skilled as possible because some of your productivity could help me down the road.”
Gurven and Davison found that rather than our long lifespans opening up opportunities that led to a human-like foraging economy and social behavior, the reverse is more likely — our skills-intensive strategies and long-term investments in the health of the group preceded and evolved with our shift to our particular human life history, with its extended childhood and unusually long post-reproductive stage.
In contrast, chimpanzees — who represent our best guess as to what humans’ last common ancestor may have been like — are able to forage for themselves by age 5. However, their foraging activities require less skill, and they produce minimal surplus. Even so, the authors show that if a chimpanzee-like ancestor would share their food more widely, they could still generate enough indirect fitness contributions to increase the force of selection in later adulthood.
“What this suggests is that human longevity is really a story about cooperation,” said Gurven. “Chimpanzee grandmothers are rarely observed doing anything for their grandkids.”
Though the authors say their work is more about how the capacity for long life came to first exist in the Homo lineage, the implication that we owe it to elders everywhere is an important reminder looking forward.
“Despite elders being far more numerous today than ever before in the past, there’s still much ageism and underappreciation of older adults,” Gurven said. “When COVID seemed to be most deadly just for older adults, many shrugged their shoulders about the urgency of lockdown or other major precautions.
“Much of the huge value of our elders goes untapped,” he added. “It’s time to think seriously about how to reconnect the generations, and harness some of that elder wisdom and expertise.”
Reference: “The importance of elders: Extending Hamilton’s force of selection to include intergenerational transfers” by Raziel Davison and Michael Gurven, 6 July 2022, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2200073119
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