The Sama people of the Philippines spend their lives on or near the ocean, and much of their foraging happens underwater. Over generations, something measurable shifted in their biology: their spleens got bigger. The spleen functions partly as a reservoir for red blood cells; more spleen means more oxygen available during a long breath-hold dive. The gene variants that produce larger spleens became more common in the population. Natural selection, working slowly and without intention, found a useful trait and amplified it.
This is what a genuine local adaptation looks like. Herman Pontzer, an evolutionary anthropologist at Duke University, uses examples like this one throughout his new book, Adaptable: How Your Unique Body Really Works and Why Our Biology Unites Us (Penguin Random House, 2025), to illustrate something he treats as central to what our species is. We are, above almost everything else, flexible.
“That’s why there’s 9 billion of us and not 9 billion of some other primate,” Pontzer told Live Science. Adaptability is the mechanism. It’s what allowed Homo sapiens to occupy every biome on the planet, adjusting through culture, technology, and accumulated biological change across generations. No other primate comes close.
The book is a tour of the human body, system by system, with particular attention to how environments have shaped what those systems do. Pontzer has spent years working with the Hadza of Tanzania, a contemporary hunter-gatherer population, and that research gives him a frame of reference for thinking about baseline human physiology that most biomedical research lacks. Working with diverse populations doesn’t just add data points; it changes the questions you know to ask.
What Local Adaptation Actually Requires
The Sama spleen story is clean, but Pontzer is precise about what makes it possible. Local adaptations are real and documented, but the conditions that produce them are narrow. A trait has to help individuals survive and reproduce in one specific place. Not everywhere, just there. It has to persist across enough generations for selection to accumulate. And the environment driving it has to be stable enough, and geographically bounded enough, that gene flow, the constant mixing of alleles through interbreeding between populations, doesn’t dilute the effect before it takes hold.
Most traits don’t survive those criteria.
Skin pigmentation does. The gradient of ultraviolet radiation between the equator and the poles is old and consistent, and so is the gradient of melanin production across human populations. Darker skin offers protection against UV damage; lighter skin permits greater vitamin D synthesis where UV is scarce. Both directions of the trade-off have been advantageous in their respective environments for long enough that selection has had time to work. High-altitude adaptations in Himalayan populations follow a similar structure: the mountains have been high throughout the entire span of human prehistory, and so have the selection pressures they impose.
Other proposed local adaptations collapse under the same scrutiny. In the 1990s, some researchers argued that Black Americans might carry alleles predisposing them to hypertension and heart disease, the implication being that some evolutionary pressure had shaped cardiac function differently in West African populations. Pontzer is skeptical, for reasons that follow directly from the mechanics of local adaptation. Having a heart that functions well is not a localized advantage. It is useful everywhere. Traits that are universally beneficial spread through gene flow. They don’t concentrate in populations.
The same logic applies to claims, still circulating, about population-level differences in cognitive ability having evolutionary roots. Intelligence has been selected for across the entire species, continuously, for as long as Homo sapiens has existed. There is no environment where diminished cognitive ability was adaptive. Any variants that enhance brain function would be expected to spread broadly, not cluster. The framework that makes the Sama spleen story coherent is precisely what makes these other claims incoherent.
A Body Built for Somewhere Else
Pontzer’s work with the Hadza feeds into a second argument in the book: our bodies were shaped in an environment radically different from the one most people now inhabit, and the gap between those environments is doing measurable damage.
Hunter-gatherers are physically active continuously, eating from wild food sources, exposed to a wide range of pathogens. This was the norm for Homo sapiens, and for the hominin lineages that preceded our species, for millions of years. The body we have is a product of that context. Move it into a climate-controlled house with a caloric surplus and minimal required movement, and the same physiology fine-tuned for one environment starts producing maladaptive outcomes in another: heart disease, metabolic disorders, allergies, conditions that appear to have been rare before the agricultural transition and are common now.
This is the evolutionary mismatch, and Pontzer is careful not to let it slide into nostalgia or primitivism. It is a mechanistic observation, not a moral one. The body is responding rationally to its conditions; the conditions just happen to be novel in ways that have outpaced any biological adjustment.
One of the more interesting extensions of this framework involves genetics and development. Pontzer describes the genome as setting a range of possible outcomes rather than a fixed destination. Your genes constrain what you can become but don’t determine it. The environment usually has the larger visible effect on which possibilities actually materialize. Epigenetics adds another layer: environmental stresses can alter how genes are expressed, switching them on or off in ways that persist for a lifetime. In mice, these changes have been shown to transmit to offspring. The environment a mother experiences can affect her children’s biology. In humans, the evidence for transgenerational epigenetic inheritance is suggestive but not yet settled. The studies required take decades to run, and the full picture isn’t in yet.
What Pontzer keeps returning to is that diversity is real but layered in ways that resist simple categorization. Knowing something about a person’s pigmentation tells you essentially nothing reliable about their cardiovascular risk, their cognitive profile, or most other things you might want to know. The systems are largely independent. They evolved under different pressures, respond to different environments, and vary along different axes. The assumption that populations sort cleanly into types, and that a trait in one domain clusters reliably with traits in others, is precisely the error that the correct application of local adaptation logic should prevent.
