Perfectly Deprecated

Some time in the distant past, a flock of pigeons crossed the Indian Ocean and reached an island with no cats, no dogs, no snakes, and no mammals on the ground at all. The only native mammals on Mauritius were bats. There was fruit, there were seeds, and there was nothing that hunted large birds on the forest floor.

Flight is one of the most expensive things an animal can do. A flying bird carries large breast muscles, a deep keeled breastbone to anchor them, long feathers that must be grown and replaced, and a metabolism fast enough to power it all. That machinery earns its keep when it carries a bird away from a predator or across open water to new food. On an island where nothing needs escaping, and where the food is already underfoot, much of it earns nothing.

Nothing on Mauritius decided that flight was unnecessary. What happened was slower and more ordinary. Birds varied, as birds always do, in how much of their body went into wings and flight muscle. On a predator-free island, individuals that invested a little less in flight paid no survival penalty for it, and the energy they saved could go into growth, eggs, and chicks. Over many generations, those small differences in reproduction shifted the population. Physiologists have linked flightlessness to lower energy expenditure, ^[1] and a comparative study of island rails and other birds found that island lineages repeatedly and predictably shrink their flight muscles relative to their legs. ^[2]

The descendants of those pigeons became the dodo: large, heavy, ground-nesting, and unable to fly. Genetic work places it inside the pigeon family, close to the living Nicobar pigeon, which still flies. ^[3]

In its world, the dodo was not a failed bird.

It was a finished one.

When the World Arrived

Dutch ships reached Mauritius in 1598. Sailors described birds that did not flee. They were hunted, but hunting was probably not the decisive blow. The ships also carried pigs, crab-eating macaques, and rats, and these animals found eggs and chicks on the ground, unguarded by any instinct built for them. Historians of the species now tend to weigh introduced animals and habitat change more heavily than sailors with clubs. ^[4] The last widely accepted sighting was in 1662; a statistical analysis of the sighting record suggests the species was probably gone by around 1690. ^[5]

Less than a century. The body plan that made the dodo well suited to Mauritius had taken far longer than that to form.

New Zealand tells the same story more slowly, and with a survivor. Before humans arrived, it had no land mammals except bats. Its most dangerous predators were birds, including Haast's eagle, the largest eagle known, which hunted by sight from above. The kākāpō, a nocturnal, flightless, moss-green parrot and the heaviest parrot alive, evolved a defense perfectly fitted to that threat: when alarmed, it freezes. Motionless against the vegetation, it is very hard for a hunter using its eyes to find. ^[6]

Then people arrived, first Polynesian settlers with the Pacific rat and dogs, and later Europeans with cats, ship rats, and stoats. These predators hunt largely by smell, and the kākāpō has a strong, musty scent. A freezing kākāpō is still well camouflaged. It is also sitting perfectly still in front of an animal that does not need to see it. Females raise chicks on the ground and leave the nest at night to feed, leaving eggs and chicks exposed to rats. By 1995 the known population had fallen to about fifty birds, and the species survives today only on predator-free islands under intensive management. ^[6]

The kākāpō's defense did not stop working. It still does exactly what it evolved to do. What changed is who it is being used against.

These two birds are not isolated cases. Across the Pacific, archaeological bone deposits show that human colonization of island after island was followed by the disappearance of many bird species and populations, a large share of them flightless or weak-flying rails. ^[7] The reasons birds lost flight were not identical everywhere. Energy savings, the absence of predators, the lack of any need to disperse, and a ground-feeding way of life combined differently in different lineages. ^[1,2] What they shared was a match between the animal and its island that took a long time to build and very little time to break.

A Signal That Changed Its Meaning

The birds lost an ability. The next case is about an instinct that remained perfectly intact.

Sea turtle hatchlings dig out of their nests mostly at night and must reach the sea quickly, before crabs, birds, dehydration, or sunrise find them. They cannot see the ocean from the sand. They are not following the Moon either, as is often said: hatchlings orient correctly on moonless nights too.

What they respond to is the shape and brightness of the horizon. On a natural beach, the seaward direction is open, low, and slightly brighter, because the sky over water reflects starlight, moonlight, and skyglow across a wide, unobstructed horizon. Landward, dunes and vegetation form a darker, taller silhouette. Experiments show that hatchlings move toward the lower, brighter part of the horizon and away from high dark shapes, with beach slope playing a smaller role. ^[8] Once in the water, they switch cues, orienting into the waves and later using the Earth's magnetic field. ^[9]

For a very long time this rule, "go toward the low, bright horizon and away from the dark, high one," pointed to the sea on nearly every nesting beach in the world.

Then coastlines were lit. Hotels, houses, roads, streetlights, and resorts put bright point sources behind the dunes. In experiments with loggerhead hatchlings, artificial light drew them away from the water, and they were especially sensitive to short-wavelength light, such as the bluish-white light common in modern lamps. ^[10] Hatchlings that crawl toward a road can die from exhaustion, predation, dehydration, or traffic. Later work showed the natural cues still matter: a dark dune silhouette can partly counteract artificial light, meaning the hatchling is weighing competing information rather than simply malfunctioning. ^[11]

The turtle's orientation has not become defective. It is doing what it always did. The environment changed the meaning of the signal. Brightness used to mean ocean. On many beaches, it now means town.

Biologists call this an evolutionary trap: a situation in which rapid environmental change causes an organism to make a choice, based on cues that were once reliable, that now leads to a worse outcome than an available alternative. ^[12] When the choice specifically concerns where to live or breed, the narrower term ecological trap is used: animals prefer a habitat that is actually poorer than others they could choose. ^[12,13] Mayflies and other aquatic insects that lay eggs on dry asphalt or dark car paint, because these surfaces reflect horizontally polarized light the way water does, are a well-studied example. ^[14] The hatchling's crawl toward streetlights is an evolutionary trap but not an ecological one: it is a movement decision, not a choice of habitat.

The flightless birds were not trapped in this sense. Nothing lured the dodo toward a pig. Their case belongs to a broader category, evolutionary mismatch: a trait that evolved under one set of conditions now produces poorer outcomes under new ones. Every trap is a kind of mismatch. Not every mismatch is a trap. ^[15]

The Biology of Being Right Too Early

Behind both stories is a simple fact about natural selection: it has no foresight.

Selection is not a designer preparing organisms for the future. It is the accumulated result of which individuals survived and reproduced in the conditions they actually lived in. A trait becomes common because it helped under past conditions. Whether it will help under future ones is not part of the process.

Put as a sequence:

a trait is beneficial → the environment changes → the trait remains → its effect changes.

The organism does not have to change at all for the outcome to reverse.

When environments change slowly relative to generation time, populations can often track them. When they change quickly, a gap opens between the traits a population has and the traits that would now serve it best. Evolutionary biologists call this gap adaptive lag; John Maynard Smith formalized a related quantity, the lag load, as the reduction in a population's fitness caused by its failure to keep pace with a changing environment. ^[16] Traits that lower fitness in the current setting are described as maladaptive, and maladaptation turns out to be common in nature, not a rare exception. ^[17,18]

The terms overlap, so it helps to see them side by side.

The first column names established concepts in evolutionary biology; the examples are documented cases, although how each case is classified can vary between authors. ^{[12,13,15,18]}

None of this means evolution has "failed." Evolution sometimes responds with surprising speed. Along roads in Nebraska, cliff swallows nesting under bridges evolved shorter wings over about thirty years, apparently because shorter-winged birds were better at taking off steeply and avoiding cars; road kills declined. ^[19] But selection can only work with the variation a population already has, and only as fast as generations turn over. For a long-lived parrot that breeds only in certain years, or for a dodo with no predator-wariness to draw on, the world moved faster than any population could.

How Much of This Applies to Us?

Anatomically modern humans have existed for roughly 300,000 years. ^[20] Agriculture is about 12,000 years old. Electric light, cars, industrial food, cities of millions, and constant contact with strangers are, on that scale, recent. Humans have not stopped evolving. Lactase persistence, the ability to digest milk into adulthood, spread in several populations within the last several thousand years. ^[21] But the pace of cultural and technological change has far exceeded the pace of genetic change.

So the same question arises: are some human traits well adapted to conditions that no longer exist?

The evolutionary mismatch hypothesis in human biology and psychology proposes exactly that, and it has become an influential framework. ^[22,23] It is also one that is easy to abuse. The most careful versions are those that make testable claims. Several areas are plausible candidates.

Food. A preference for sweet, fatty, energy-dense food would have been useful when such food was scarce and unpredictable. In environments where it is cheap and constant, the same preferences can contribute to overeating. There is experimental support for the modern half of this story: in a controlled inpatient trial, people on an ultra-processed diet ate about 500 more calories a day than on a minimally processed diet matched for presented nutrients, and gained weight. ^[24] The ancestral half is shakier. The famous "thrifty gene" idea, that humans were selected to store fat against famine, has been seriously challenged, and alternative explanations remain in play. ^[25]

Stress. The body's stress response mobilizes energy, sharpens attention, and suppresses long-term processes like digestion and repair. That is well suited to short threats. Physiological research shows that when these systems are activated repeatedly or chronically, as they can be by ongoing social, financial, or psychological pressures, the same mediators that protect in the short term contribute to damage in the long term. ^[26] The physiology is well established. Calling it a mismatch is an interpretation, but a reasonable one.

Belonging. For most of human history, survival depended on staying in a group. Sensitivity to exclusion would plausibly have been valuable. Today, social connection still matters enormously: a meta-analysis of 148 studies found that people with stronger social relationships had about a 50% greater likelihood of survival over the study periods. ^[27] What has changed is the scale and form of social life: large anonymous societies, and networks where approval and rejection can come from thousands of strangers. Whether ancient sensitivities misfire in these settings is a live question, not a settled one.

Reward. Brain systems that respond to food, sex, social approval, and novelty evolved in environments where those rewards were intermittent and came in natural forms. Pure psychoactive drugs, and other stimuli concentrated far beyond anything ancestral, act directly on these systems. Randolph Nesse and Kent Berridge argued that drug use can be understood partly in this way: substances that stimulate emotional and reward mechanisms directly, bypassing the situations those mechanisms evolved to track. ^[28]

Threat. Human attention is drawn quickly to possible danger. In a world of predators and hostile strangers that was plainly useful. Surrounded by traffic, news, and endless reports of distant catastrophes, the same vigilance may be activated very often by things a person can neither flee nor fight.

These examples need a firm caveat. Obesity, anxiety, depression, addiction, and loneliness are multifactorial. Genes, development, economics, culture, and individual history all shape them. Evolutionary explanations of human behavior are notoriously easy to construct and hard to test, and critics have long pointed out that a plausible ancestral story is not evidence on its own. ^[23,29] Mismatch is best treated as one lens that can generate hypotheses, not a master explanation.

The Angry Body

Aggression is where this question becomes most uncomfortable.

It is tempting to say that humans are "naturally violent" and leave it there. The evidence does not support anything so simple. Aggression is not one instinct. Richard Wrangham has argued that human aggression includes at least two quite different forms: reactive aggression, the hot, impulsive response to provocation, which humans show at unusually low rates compared with many primates; and proactive aggression, planned and goal-directed, which humans are unusually capable of. ^[30] Violence also varies enormously between societies. A comparative analysis placed the level of lethal violence expected for early humans at roughly 2% of deaths, based on our position among mammals, and found that actual rates in human populations rose and fell dramatically with social and political organization. ^[31] Among mobile hunter-gatherer bands, most recorded killings arose from personal disputes between individuals rather than organized war between groups. ^[32]

And humans, like other primates, also evolved powerful tools for limiting conflict: reconciliation, alliance-building, mediation, caregiving, and restraint. ^[33] Our lineage is at least as marked by cooperation as by violence.

Still, there is a question worth asking. Impulses such as retaliation, defense of territory, status competition, and fast responses to perceived threat are likely shaped, at least in part, by life in small groups, where their consequences were limited by what one body, or a few bodies, could do. This is a hypothesis, not a finding.

An angry person once had a body.

An angry person today may have a car, a firearm, a global audience, a bureaucracy, an army, or millions of people ready to react within minutes. The impulse may be no stronger than it was. What has changed is the distance between an impulse and its consequences, and how far those consequences travel.

If there is a mismatch here, it may lie less in the impulse itself than in how easily technology and institutions can amplify it.

It is also worth being careful about what such explanations mean. To say a behavior may have evolved is to say something about its history. It says nothing about whether the behavior is good. Treating "natural" as though it meant "right" is a well-known error, the naturalistic fallacy. ^[29] Many things natural selection produced, including parasites, infanticide in some species, and disease, are natural in exactly this sense.

An Answer Without Its Question

A flightless island bird cannot understand why predators suddenly appeared. A kākāpō cannot know that the animal in front of it hunts by smell. A hatchling cannot know that the brightest horizon may now lead to a parking lot.

Humans are in a different position, not because our instincts are better, but because we can sometimes notice them. We can observe an impulse without obeying it. We can see a cue as a cue. We have learned to shield beach lights, restore predator-free islands, and move kākāpō to places where their old defense works again. We can build norms and institutions that restrain some impulses and encourage others, and we can change environments rather than waiting for our bodies to change.

That ability has limits. Recognizing a mismatch does not dissolve it, and many human mismatches remain hypotheses rather than demonstrated facts. What is well established is the pattern in other species: rapid environmental change, much of it caused by humans, has left many organisms carrying traits that once served them and no longer do. ^[12,15,18] What remains open is how far the same pattern reaches into human bodies, minds, and societies.

Evolution does not produce traits that are good in general. It produces traits that worked in particular conditions. Every organism alive carries answers to the problems its ancestors faced. Usually the questions change slowly enough that the answers can change with them.

Sometimes the world changes first.

And an answer that was once perfectly correct becomes wrong without changing at all.

References

[1] McNab, B. K. (1994). “Energy Conservation and the Evolution of Flightlessness in Birds.” The American Naturalist, 144(4), 628–642. https://doi.org/10.1086/285697

[2] Wright, N. A., Steadman, D. W., & Witt, C. C. (2016). “Predictable evolution toward flightlessness in volant island birds.” Proceedings of the National Academy of Sciences, 113(17), 4765–4770. https://doi.org/10.1073/pnas.1522931113

[3] Shapiro, B., Sibthorpe, D., Rambaut, A., et al. (2002). “Flight of the Dodo.” Science, 295(5560), 1683. https://doi.org/10.1126/science.295.5560.1683

[4] Hume, J. P. (2006). “The history of the Dodo Raphus cucullatus and the penguin of Mauritius.” Historical Biology, 18(2), 69–93. https://doi.org/10.1080/08912960600639400

[5] Roberts, D. L., & Solow, A. R. (2003). “When did the dodo become extinct?” Nature, 426, 245. https://doi.org/10.1038/426245a

[6] Powlesland, R. G., Merton, D. V., & Cockrem, J. F. (2006). “A parrot apart: the natural history of the kakapo (Strigops habroptilus), and the context of its conservation management.” Notornis, 53(1), 3–26. https://doi.org/10.63172/605353vxtvye

[7] Steadman, D. W. (1995). “Prehistoric Extinctions of Pacific Island Birds: Biodiversity Meets Zooarchaeology.” Science, 267(5201), 1123–1131. https://doi.org/10.1126/science.267.5201.1123

[8] Salmon, M., Wyneken, J., Fritz, E., & Lucas, M. (1992). “Seafinding by Hatchling Sea Turtles: Role of Brightness, Silhouette and Beach Slope as Orientation Cues.” Behaviour, 122(1–2), 56–77. https://doi.org/10.1163/156853992X00309

[9] Lohmann, K. J., & Lohmann, C. M. F. (1996). “Orientation and Open-Sea Navigation in Sea Turtles.” Journal of Experimental Biology, 199(1), 73–81. https://doi.org/10.1242/jeb.199.1.73

[10] Witherington, B. E., & Bjorndal, K. A. (1991). “Influences of artificial lighting on the seaward orientation of hatchling loggerhead turtles Caretta caretta.” Biological Conservation, 55(2), 139–149. https://doi.org/10.1016/0006-3207(91)90053-C

[11] Tuxbury, S. M., & Salmon, M. (2005). “Competitive interactions between artificial lighting and natural cues during seafinding by hatchling marine turtles.” Biological Conservation, 121(2), 311–316. https://doi.org/10.1016/j.biocon.2004.04.022

[12] Schlaepfer, M. A., Runge, M. C., & Sherman, P. W. (2002). “Ecological and evolutionary traps.” Trends in Ecology & Evolution, 17(10), 474–480. https://doi.org/10.1016/S0169-5347(02)02580-6

[13] Robertson, B. A., & Hutto, R. L. (2006). “A framework for understanding ecological traps and an evaluation of existing evidence.” Ecology, 87(5), 1075–1085. https://doi.org/10.1890/0012-9658(2006)871075:AFFUET2.0.CO;2

[14] Horváth, G., Kriska, G., Malik, P., & Robertson, B. (2009). “Polarized light pollution: a new kind of ecological photopollution.” Frontiers in Ecology and the Environment, 7(6), 317–325. https://doi.org/10.1890/080129

[15] Robertson, B. A., Rehage, J. S., & Sih, A. (2013). “Ecological novelty and the emergence of evolutionary traps.” Trends in Ecology & Evolution, 28(9), 552–560. https://doi.org/10.1016/j.tree.2013.04.004

[16] Maynard Smith, J. (1976). “What Determines the Rate of Evolution?” The American Naturalist, 110(973), 331–338. https://doi.org/10.1086/283071

[17] Crespi, B. J. (2000). “The evolution of maladaptation.” Heredity, 84(6), 623–629. https://doi.org/10.1046/j.1365-2540.2000.00746.x

[18] Brady, S. P., Bolnick, D. I., Barrett, R. D. H., et al. (2019). “Understanding Maladaptation by Uniting Ecological and Evolutionary Perspectives.” The American Naturalist, 194(4), 495–515. https://doi.org/10.1086/705020

[19] Brown, C. R., & Bomberger Brown, M. (2013). “Where has all the road kill gone?” Current Biology, 23(6), R233–R234. https://doi.org/10.1016/j.cub.2013.02.023

[20] Hublin, J.-J., Ben-Ncer, A., Bailey, S. E., et al. (2017). “New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens.” Nature, 546, 289–292. https://doi.org/10.1038/nature22336

[21] Tishkoff, S. A., Reed, F. A., Ranciaro, A., et al. (2007). “Convergent adaptation of human lactase persistence in Africa and Europe.” Nature Genetics, 39(1), 31–40. https://doi.org/10.1038/ng1946

[22] Manus, M. B. (2018). “Evolutionary mismatch.” Evolution, Medicine, and Public Health, 2018(1), 190–191. https://doi.org/10.1093/emph/eoy023

[23] Li, N. P., van Vugt, M., & Colarelli, S. M. (2018). “The Evolutionary Mismatch Hypothesis: Implications for Psychological Science.” Current Directions in Psychological Science, 27(1), 38–44. https://doi.org/10.1177/0963721417731378

[24] Hall, K. D., Ayuketah, A., Brychta, R., et al. (2019). “Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomized Controlled Trial of Ad Libitum Food Intake.” Cell Metabolism, 30(1), 67–77. https://doi.org/10.1016/j.cmet.2019.05.008

[25] Speakman, J. R. (2008). “Thrifty genes for obesity, an attractive but flawed idea, and an alternative perspective: the ‘drifty gene’ hypothesis.” International Journal of Obesity, 32(11), 1611–1617. https://doi.org/10.1038/ijo.2008.161

[26] McEwen, B. S. (1998). “Protective and Damaging Effects of Stress Mediators.” New England Journal of Medicine, 338(3), 171–179. https://doi.org/10.1056/NEJM199801153380307

[27] Holt-Lunstad, J., Smith, T. B., & Layton, J. B. (2010). “Social Relationships and Mortality Risk: A Meta-analytic Review.” PLoS Medicine, 7(7), e1000316. https://doi.org/10.1371/journal.pmed.1000316

[28] Nesse, R. M., & Berridge, K. C. (1997). “Psychoactive Drug Use in Evolutionary Perspective.” Science, 278(5335), 63–66. https://doi.org/10.1126/science.278.5335.63

[29] Downes, S. M. “Evolutionary Psychology.” Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/evolutionary-psychology/

[30] Wrangham, R. W. (2018). “Two types of aggression in human evolution.” Proceedings of the National Academy of Sciences, 115(2), 245–253. https://doi.org/10.1073/pnas.1713611115

[31] Gómez, J. M., Verdú, M., González-Megías, A., & Méndez, M. (2016). “The phylogenetic roots of human lethal violence.” Nature, 538, 233–237. https://doi.org/10.1038/nature19758

[32] Fry, D. P., & Söderberg, P. (2013). “Lethal Aggression in Mobile Forager Bands and Implications for the Origins of War.” Science, 341(6143), 270–273. https://doi.org/10.1126/science.1235675

[33] de Waal, F. B. M. (2000). “Primates—A Natural Heritage of Conflict Resolution.” Science, 289(5479), 586–590. https://doi.org/10.1126/science.289.5479.586