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This is related to How does "be altruist to those who are similar to you" evolve?
Altruism that is
- Not reciprocal
- Not familiar
has little explanation. One possible explanation is that the trait itself may correlate well with genetics. One great answer there is that often the cost of altruism is small anyway. It can explain why people vote. Here the expense is small anyway.
Still there seems to be some factors that are even bigger.
Let's take a look at people that die for their ideology. Christian martyrs, Muslim suicide bombers, or Communist guerilla fighters. They seem to get so little and well, die.
And that's pretty common. It seems pretty easy for a leader or pedagogue to rouse men to be soldiers. Of course, becoming a soldier is a pretty shitty job, yet most men don't mind.
These people make a huge sacrifice for the sake of their country, ideology, or people that are not even genetically related to them.
Your question is quite broad and asks for explanations for various behaviours which can lead to self-sacrifice.
Religious reasons: The genetic influence here may be a predisposition to let others influence you. This is what gives rise to culture in the first place, in other words: the predisposition to at some point maybe sacrifice yourself because you are taught to do so can only die out if the basic behaviour which gives rise to culture dies out. Cultures which lead their members to die may decimate their own numbers, but they will not wipe out culture itself because in the bigger scale, those with culture do better than those without. (Plus, those who sacrifice themselves may have children as well, so any genetic influence on their behaviour can be carried on.) This also goes far into the field of memetic evolution, which is disputed but may be interesting to read about.
Political reasons: As in, dying for one's nation or country rather than because of teachings. This probably has a defensive behaviour towards one's own group as the genetic influence. Also, reputation plays a big role: see shigeta's excellent answer.
You also mention willingness to subordinate oneself which is a very common pattern not only in humans. Richard Dawkins touches on this in The Selfish Gene, but there are probably papers more focussed on this particular topic out there as well.
I think it is far-fetched to assume a genetic inheritance of in general "willingness to die for something" (which would be required for genetic evolution to work on it). Each thing that some people may be willing to die for can have completely different reasons. I think in these terms the willingness to die is usually an exaggeration of a behaviour which evolved for different reasons.
There isn an effect called "Indirect reciprocity" where individuals just give to everyone they meet without direct requirement of reciprocity.
This sort of benefit to others is common - hospitality to strangers, general politeness, good customer service all fall along these lines. You hope they will come back and benefit you again, but maybe they will tell someone else who will know you are a good community member.
It is only sustainable in a system where the cost/benefit ratio is less than the reputation benefit of the act. It sounds as if this is only good for public acts but if the benefit is transferred to a social entity that outlasts the individual (like your children, a relative's children, a religion or a corporation say), the result could still hold.
If you think about typical morality/ethics really it still makes sense to think that what we call altruism must still have a net positive benefit. If there is no benefit long term or to anyone, it really isn't useful or even good, its random. What we usually call altruism is usually some sort of reciprocal cooperation.
A soldier who dies in combat or someone who dies for their beliefs but everyone knows about it as a public statement benefits from their act indirectly. I don't think its altruism in the pure sense of the word. Defending the nation, ones' beliefs or whatever is, in its sense its own reward. Veterans come back from a war are hopefully respected for their work. Having a purple heart can be a good thing to show people. I'm not saying these people are adequately compensated for what they have been through, but just trying to draw a distinction between pure biological altruism and 'indirect reciprocity'.
Examples of Indirect reciprocity might be the use of tax money to build highways and build power and water infrastructure. Its important - its the glue that holds a nation or a group together. If you got punished for doing these things we wouldn't be hanging as a nation very long!
A martyr with no family at all who would benefit would still count as an altruism I think, but most acts of public piety and sacrifice do benefit the individual by reputation. Something to think about.
Sexual reproduction was likely an early evolutionary innovation after the appearance of eukaryotic cells. It appears to have been very successful because most eukaryotes are able to reproduce sexually and, in many animals, it is the only mode of reproduction. And yet, scientists also recognize some real disadvantages to sexual reproduction. On the surface, creating offspring that are genetic clones of the parent appears to be a better system. If the parent organism is successfully occupying a habitat, offspring with the same traits should be similarly successful. There is also the obvious benefit to an organism that can produce offspring whenever circumstances are favorable by asexual budding, fragmentation, or by producing eggs asexually. These methods of reproduction do not require another organism of the opposite sex. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, in asexual populations, every individual is capable of reproduction. In sexual populations, the males are not producing the offspring themselves, so hypothetically an asexual population could grow twice as fast.
However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why are meiosis and sexual reproductive strategies so common? These are important (and as yet unanswered) questions in biology, even though they have been the focus of much research beginning in the latter half of the 20th century. There are several possible explanations, one of which is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population. Thus, on average, a sexually reproducing population will leave more descendants than an otherwise similar asexually reproducing population. The only source of variation in asexual organisms is mutation. Mutations that take place during the formation of germ cell lines are also the ultimate source of variation in sexually reproducing organisms. However, in contrast to mutation during asexual reproduction, the mutations during sexual reproduction can be continually reshuffled from one generation to the next when different parents combine their unique genomes and the genes are mixed into different combinations by crossovers during prophase I and random assortment at metaphase I.
Vaccine science is brow-furrowingly complicated, but the underlying mechanism is simple. A vaccine exposes your body to either live but weakened or killed pathogens, or even just to certain bits of them. This exposure incites your immune system to create armies of immune cells, some of which secrete antibody proteins to recognize and fight off the pathogens if they ever invade again.
That said, many vaccines don’t provide lifelong immunity, for a variety of reasons. A new flu vaccine is developed every year because influenza viruses naturally mutate quickly. Vaccine-induced immunity can also wane over time. After being inoculated with the shot for typhoid, for instance, a person’s levels of protective antibodies drop over several years, which is why public health agencies recommend regular boosters for those living in or visiting regions where typhoid is endemic. Research suggests a similar drop in protection over time occurs with the mumps vaccine, too.
Vaccine failures caused by vaccine-induced evolution are different. These drops in vaccine effectiveness are incited by changes in pathogen populations that the vaccines themselves directly cause. Scientists have recently started studying the phenomenon in part because they finally can: Advances in genetic sequencing have made it easier to see how microbes change over time. And many such findings have reinforced just how quickly pathogens mutate and evolve in response to environmental cues.
Viruses and bacteria change quickly in part because they replicate like mad. Three days after a bird is bitten by a mosquito carrying West Nile virus, one milliliter of its blood contains 100 billion viral particles, roughly the number of stars in the Milky Way. And with each replication comes the opportunity for genetic change. When an RNA virus replicates, the copying process generates one new error, or mutation, per 10,000 nucleotides, a mutation rate as much as 100,000 times greater than that found in human DNA. Viruses and bacteria also recombine, or share genetic material, with similar strains, giving them another way to change their genomes rapidly. Just as people — with the exception of identical twins — all have distinctive genomes, pathogen populations tend to be composed of myriad genetic variants, some of which fare better than others during battles with vaccine-trained antibodies. The victors seed the pathogen population of the future.
The bacteria that cause pertussis, better known as whooping cough, illustrate how this can happen. In 1992, recommendations from the U.S. Centers for Disease Control and Prevention (CDC) began promoting a new vaccine to prevent the infection, which is caused by bacteria called Bordetella pertussis. The old vaccine was made using whole killed bacteria, which incited an effective immune response but also caused rare side effects, such as seizures. The new version, known as the “acellular” vaccine, contained just two to five outer membrane proteins isolated from the pathogen.
The unwanted side effects disappeared but were replaced by new, unexpected problems. First, for unclear reasons, protection conferred by the acellular vaccine waned over time. Epidemics began to erupt around the world. In 2001, scientists in the Netherlands proposed an additional reason for the resurgence: Perhaps vaccination was inciting evolution, causing strains of the bacteria that lacked the targeted proteins, or had different versions of them, to survive preferentially.
Studies have since backed up this idea. In a 2014 paper published in Emerging Infectious Diseases, researchers in Australia, led by the medical microbiologist Ruiting Lan at the University of New South Wales, collected and sequenced B. pertussis samples from 320 patients between 2008 and 2012. The percentage of bacteria that did not express pertactin, a protein targeted by the acellular vaccine, leapt from 5 percent in 2008 to 78 percent in 2012, which suggests that selection pressure from the vaccine was enabling pertactin-free strains to become more common. In the U.S., nearly all circulating viruses lack pertactin, according to a 2017 CDC paper. “I think pretty much everyone agrees pertussis strain variation is shaped by vaccination,” Lan said.
Hepatitis B, a virus that causes liver damage, tells a similar story. The current vaccine, which principally targets a portion of the virus known as the hepatitis B surface antigen, was introduced in the U.S. in 1989. A year later, in a paper published in the Lancet, researchers described odd results from a vaccine trial in Italy. They had detected circulating hepatitis B viruses in 44 vaccinated subjects, but in some of them, the virus was missing part of that targeted antigen. Then, in a series of studies conducted in Taiwan, researchers sequenced the viruses that infected children who had tested positive for hepatitis B. They reported that the prevalence of these viral “escape mutants,” as they called them, that lacked the surface antigen had increased from 7.8 percent in 1984 to 23.1 percent in 1999.
Some research suggests, however, that these mutant strains aren’t stable and that they may not pose much of a risk. Indeed, fewer and fewer people catch hepatitis B every year worldwide. As physicians at the Icahn School of Medicine at Mount Sinai in New York summarized in a 2016 paper, “the clinical significance of hepatitis B surface antigen escape mutations remains controversial.”
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What Colors do Animals See?
Many differences exist between what humans and other animals can see. Some animals, among them frogs, fish, some rodents, and many marsupials, can see ultraviolet (for reviews see Bennett et al. 1994). However, the differences between what humans see and what other animals see are likely to go well beyond our inability to see ultraviolet.
Research suggests that birds may have the most sophisticated visual system of any vertebrate. They likely see hues that we cannot imagine. Many birds have five classes of cones, and some species of birds see ultraviolet. In addition, a bird’s visual system includes oil droplets thought to act as filters of light entering individual cones (Bennett et al. 1994). Scientists do not know how this increased dimensionality enhances the colors that birds perceive. Many reptiles have color vision, and researchers have learned that diurnal lizards have four types of cones and colored oil droplets, suggesting they probably have tetrachromatic color vision (reviewed by Roth and Kelber 2004).
Old World monkeys, apes, and humans all enjoy trichromatic color vision (Jacobs 1993), but most terrestrial mammals are cone dichromats, and like the small percentage of humans with dichromatic vision, they likely can distinguish only a limited number of colors (Peichl et al. 2001 reviewed by Jacobs 1993). Of the ungulates studied to date, such as horses, pigs, goats, cows, sheep, and deer, all have the photopigment basis for dichromatic color vision (Carroll et al. 2001). Similarly, researchers studying the color vision of dogs have confirmed they are dichromats. In addition, they have found that the cones present in the central area of a dog’s retina are low in number, probably representing less than 10% of the total photoreceptors present. Interestingly, the same area of the human retina consists predominately of cones. Presumably, the canine visual system’s greater reliance on rod photoreceptors allows a dog to function well in dim light, making it a more effective predator in its ecological niche (Neitz et al. 1989 Miller and Murphy 1995).
Even among humans, men and women may perceive colors differently. For instance, using genetic analysis and behavioral tests, researchers at the University of California San Diego explored gender-linked differences in color perception, in particular, the sensory experience of women who have an extra photopigment in their retinas. Some estimates suggest four-photopigment females represent up to 50% of the female population 8% of males are presumed to have four-photopigment retinas (Neitz et al. 1998). Their study suggested that women with four-photopigment retinas perceived more chromatic bands in the typical rainbow spectrum than either men or women with trichromatic retinas (Jameson et al. 2001).
Other scientists’ work on the genes involved in human color vision suggests that even among those of us who can discriminate red colors, we may be perceiving different reds. Researcher Samir Deeb and colleagues at the University of Washington have been studying how the genetic makeup of an individual’s retina affects color perception. In one study, they learned that approximately 40% of men with normal color vision had the amino acid alanine in their red pigment, whereas 60% had a different amino acid, serine, at the same spot (Winderickz et al. 1992, Fig. 1b). The pigment with serine is shifted to red, the one with alanine to green, explained Deeb in a phone interview. “It’s not a big shift but still really quite exciting,” he said. “It affects not only normal color vision, but also the severity of abnormal color vision.”
Disadvantages of hibernation for animals
Hibernation is shown to impose costs on animals. These costs are both in the harmful physiological effects of hibernating, as well as in the costs of being unable to respond to stimuli.
Harmful physiological effects
It has been determined that animals show poor memory retention after hibernating. When ground squirrels (Spermophilus citellus) are trained to complete a task involving spatial memory and an operant condition test, they perform poorly when tested again after hibernation compared to a control group which did not hibernate. This shows that hibernation has a negative effect on memory retention (Millesi et al. 2001).
Immunocompetence is also reduced when hibernating, making the animal vulnerable to infections and parasites during and shortly after hibernating (Luis and Hudson, 2006).
It has also been shown that nutrient absorption in the digestive tract is slowed at low body temperatures (Carey, 1989) and in reproductive females hibernation can result in slowed growth of young (Racey, 1982). Daily torpor could also be responsible for accumulation of sleep debt (Daan et al., 1991) and reduced synaptic efficacy (Strijkstra et al., 2003). In males sperm cell production is inhibited during hibernation because of low body temperatures and low MR (Racey, 1982).
It is thought that the periodic arousals are used to restore the maintenance of cells, restore immunological processes to some degree and remove deleterious substances in the brain. Since the harmful physiological effects of hibernation can still be measured, in spite of the fact that all hibernating species show arousals, those harmful effects must be quite large and pervasive.
Hibernating animals have a decreased sensitivity to stimuli and therefore cannot quickly respond to threats. Arousal from hibernation can take several minutes to several hours. This makes them more vulnerable for predation. (Radzicki et al., 1999).
Animals that store food are susceptible to hoard pilferage because of their decreased sensitivity to stimuli and inability to quickly respond to the intruder. When the hoard is being stolen, the hibernator will almost certainly die.
Reliability on the location where hibernation takes place
A hibernator is highly reliable on the integrity and suitability of its hibernaculum (place of hibernation). When a hibernator is dispelled from its hibernaculum, exposure and the inability to find a suitable new hibernaculum soon enough cause it to almost inevitably die. Unexpected adverse circumstances in the hibernaculum can have the same effect.
Sex Essential Reads
The Real Reasons Why People Have Affairs
When Partners Stop Having Sex, Whose Choice Is It?
I would argue that although the Pham and Shackelford study is an interesting one, the results are an inconclusive test of their hypotheses because there are alternative explanations for their findings. Some of these alternative explanations, such as those involving female satisfaction, and my own hypothesis that female attractiveness generally increases male interest in sexual activity seem like more obvious explanations. This does not necessarily mean that the authors are incorrect, only that more research is needed to test these different explanations. For example, studies might assess whether men with more attractive partners are also more interested in other activities associated with sexual foreplay, such as kissing and so on. It would then be possible to test whether interest in oral sex is independent of interest in these other activities. It is also possible that oral sex might serve a combination of functions and that all of these hypotheses have a grain of truth.
Finally, it might be a good idea to consider the woman’s perspective. A possible problem with Pham and Shackelford’s approach to understanding this subject is that they seem to portray women as passive recipients of male interest and that female agency is not considered. For example, if the function of cunnilingus was to detect whether a woman had been sexually active with another male, it would seem reasonable that if she had in fact been unfaithful she might try to avoid receiving cunnilingus to avoid detection. Furthermore, Pham and Shackelford consider “recurrent risk of sperm competition” purely in terms of the woman’s attractiveness to other men. While it may be true that men are more likely to target attractive women for affairs, it is also the case that the woman actually has a say in the matter. Some women are more likely to be unfaithful than others and this may be related to her character and choices as much as her looks. Future studies might consider whether men are more likely to perform oral sex on a woman who may present a “recurrent risk of sperm competition” due to her own desires and her actual willingness to be unfaithful.
© Scott McGreal. Please do not reproduce without permission. Brief excerpts may be quoted as long as a link to the original post is provided.
 Stephen Jay Gould seems to have originated this particular canard about evolutionary psychologists being “pan-adaptationists” who are too blind to see that many features of the human psyche have no evolutionary function in themselves. See this article by Tooby and Cosmides, leading figures in the field, which shows how Gould completely misrepresented their work, in which they explicitly stated that most human behaviours are probably by-products without an evolutionary function.
 It is also worth noting that the Baker and Bellis study has been strongly disputed by Elisabeth Lloyd on the basis that the sample size was too small to draw any reliable conclusions. The findings by Baker and Bellis do not appear to have been replicated so their claims might be taken with a grain of salt.
Other posts discussing sexuality and evolutionary psychology
Backstrom, L., Armstrong, E. A., & Puentes, J. (2011). Women's Negotiation of Cunnilingus in College Hookups and Relationships. Journal of Sex Research, 49(1), 1-12. doi: 10.1080/00224499.2011.585523
The three main problems with sexual reproduction, as explained by science
Over at PLoS Biology, biologist Denis Roze has a fascinating article introducing a question that may at first seem obvious, but is in fact one of the enduring mysteries of biology: Why bother to have sex? People interested in evolutionary biology may already be familiar with the reasons why sexual intercourse is an improbable evolutionary development, but Roze does a great job summing them up like so:
Many important costs are associated with sexual reproduction, in particular:
The cost of males (or "2-fold cost of sex"): in many species, males do not provide any resource to the next generation, yet sexual females typically invest half of their resources into the production of males. Everything else being equal, this generates a 2-fold advantage for asexual females (producing only female offspring).
The cost of breaking favourable genetic combinations: genotypes that are able to survive to adulthood and reproduce prove that they are relatively fit in their own environment. Reproducing sexually may disrupt beneficial genetic combinations and lower the mean fitness of offspring.
Costs associated with the mating process: finding a mate can be costly in time and energy and may also increase risks of predation and parasite transmission. Furthermore, in some species mating may harm the female and affect her future reproductive success.
Ultimately, however, Roze explains why recent evidence from the real world bolsters the idea that sexual reproduction helps organisms adapt to new environments. He writes:
In particular, several classical biological models proved very useful to explore the benefits of sex during adaptation, with different experimental evolution studies on Chlamydomonas reinhardtii, Saccharomyces cerevisiae, and Escherichia coli, demonstrating that sexual (or recombining) lines adapt faster to new environments than asexual lines. Can this translate into a net benefit for sexuals when competing against asexuals? Evidence for this has been recently provided by experimental populations of the nematode Caenorhabitis elegans, showing that this mostly self-fertilizing organism evolves towards higher rates of biparental sex when adapting to a new environment (or coevolving with a pathogen).
A new study in PLoS Biology shows how sexual reproduction is useful in this way, too. Read more of Roze's article on PLoS Biology .
The Evolution of Prejudice
Update (1/24/14): The study reported in this article was retracted from the Journal of Personality and Social Psychology in December 2013 at the request of the authors. The reason for the retraction was the researchers&rsquo discovery that the results could not be independently replicated by lab members due to inaccurate coding performed by one of the co-authors.
For more information about the retraction, including an explanation by principal investigator Laurie Santos, please visit this link: http://retractionwatch.com/2013/12/24/doing-the-right-thing-yale-psychology-lab-retracts-monkey-papers-for-inaccurate-coding/.
Psychologists have long known that many people are prejudiced towards others based on group affiliations, be they racial, ethnic, religious, or even political. However, we know far less about why people are prone to prejudice in the first place. New research, using monkeys, suggests that the roots lie deep in our evolutionary past.
Yale graduate student Neha Mahajan, along with a team of psychologists, traveled to Cayo Santiago, an uninhabited island southeast of Puerto Rico also known as &ldquoMonkey Island,&rdquo in order to study the behavior of rhesus monkeys. Like humans, rhesus monkeys live in groups and form strong social bonds. The monkeys also tend to be wary of those they perceive as potentially threatening.
To figure out whether monkeys distinguish between insiders (i.e. those who belong to their group) and outsiders (i.e. those who don&rsquot belong), the researchers measured the amount of time the monkeys stared at the photographed face of an insider versus outsider monkey. Across several experiments, they found that the monkeys stared longer at the faces of outsiders. This would suggest that monkeys were more wary of outsider faces.
However, it is also possible that outsiders simply evoke more curiosity. To rule this out, the researchers took advantage of the fact that male rhesus monkeys leave their childhood groups once they reach reproductive age. This allowed the researchers to pair familiar outsider faces (monkeys that had recently left the group) with less familiar insider faces (monkeys that had recently joined the group). When presented with these pairs, the monkeys continued to stare longer at outsider faces, even though they were more familiar with them. The monkeys were clearly making distinctions based on group membership.
Mahajan and her team also devised a method for figuring out whether the monkeys harbor negative feelings towards outsiders. They created a monkey-friendly version of the Implicit Association Test (IAT). For humans, the IAT is a computer-based task that measures unconscious biases by determining how quickly we associate different words (e.g. &ldquogood&rdquo and &ldquobad&rdquo) with specific groups (e.g. faces of either African-Americans or European-Americans). If a person is quicker to associate &ldquobad&rdquo with African-American faces compared to European-American faces, this suggests that he or she harbors an implicit bias against African-Americans.
For the rhesus monkeys, the researchers paired the photos of insider andoutsider monkeys with either good things, such as fruits, or bad things, such as spiders. When an insider face was paired with fruit, or an outsider face was paired with a spider, the monkeys quickly lost interest. But when an insider face was paired with a spider, the monkeys looked longer at the photographs. Presumably, the monkeys found it confusing when something good was paired with something bad. This suggests that monkeys not only distinguish between insiders and outsiders, they associate insiders with good things and outsiders with bad things.
Overall, the results support an evolutionary basis for prejudice. Some researchers believe prejudice is unique to humans, since it seems to depend on complex thought processes. For example, past studies have found that people are likely to display prejudice after being reminded of their mortality, or after receiving a blow to their self-esteem. Since only humans are capable of contemplating their deaths or their self-image, these studies reinforce the view that only humans are capable of prejudice. But the behavior of the rhesus monkeys implies that our basic tendency to see the world in terms of &ldquous&rdquo and &ldquothem&rdquo has ancient origins.
Psychologist Catherine Cottrell at the University of Florida and her colleague Steven Neuberg at Arizona State University, argue that human prejudice evolved as a function of group living. Joining together in groups allowed humans to gain access to resources necessary for survival including food, water, and shelter. Groups also offered numerous advantages, such as making it easier to find a mate, care for children, and receive protection from others. However, group living also made us more wary of outsiders who could potentially harm the group by spreading disease, killing or hurting individuals, or stealing precious resources. To protect ourselves, we developed ways of identifying who belongs to our group and who doesn&rsquot. Over time, this process of quickly evaluating others might have become so streamlined that it became unconscious.
Psychologists have long known that many of our prejudices operate automatically, without us even being aware of them. Most people, even those who care deeply about equality, show some level of prejudice towards other groups when tested using the IAT. Despite this overwhelming evidence that our brains are wired for bias, our society continues to think about prejudice as premeditated behavior. Our current laws against discrimination, as well as the majority of diversity training programs, assume that prejudice is overt and intentional. Rarely do we teach people about how automatic prejudices might taint their behavior towards others.
The fact that prejudice often occurs automatically doesn&rsquot mean we can&rsquot find ways of overcoming its negative effects. For example, there is evidence that when people are made aware of their automatic prejudices, they can self-correct. And when we are encouraged to take the perspective of an outsider, it reduces our automatic prejudice towards that person&rsquos group.
Given that most of the difficult conflicts we face in the world today originate from clashes between social groups, it makes sense to devote time to understanding how to reduce our biases. But our evolutionary past suggests that in order to be effective, we may need to adopt a new approach. Often we focus more on political, historical, and cultural factors rather than the underlying patterns of thinking that fuel all conflicts. By taking into account the extent to which prejudice is deeply rooted in our brains, we have a better chance of coming up with long-term solutions that work with, rather than against, our natural tendencies.
Evolution has given humans a huge advantage over most other animals: middle age
Orcas, like humans, undergo menopause. Also like humans, they are long-lived, slow to develop, intelligent and vocally communicative. (NOAA FISHERIES VIA ASSOCIATED PRESS)
As a 42-year-old man born in England, I can expect to live for about another 38 years. In other words, I can no longer claim to be young. I am, without doubt, middle-aged.
To some people that is a depressing realization. We are used to dismissing our fifth and sixth decades as a negative chapter in our lives, perhaps even a cause for crisis. But recent scientific findings have shown just how important middle age is for every one of us, and how crucial it has been to the success of our species. Middle age is not just about wrinkles and worry. It is not about getting old. It is an ancient, pivotal episode in the human life span, preprogrammed into us by natural selection, an exceptional characteristic of an exceptional species.
Compared with other animals, humans have a very unusual pattern to our lives. We take a very long time to grow up, we are long-lived, and most of us stop reproducing halfway through our life span. A few other species have some elements of this pattern, but only humans have distorted the course of their lives in such a dramatic way. Most of that distortion is caused by the evolution of middle age, which adds two decades that most other animals simply do not get.
An important clue that middle age isn’t just the start of a downward spiral is that it does not bear the hallmarks of general, pa ssive decline. Most body systems deteriorate very little during this stage of life. Those that do, deteriorate in ways that are very distinctive, are rarely seen in other species and are often abrupt.
For example, our ability to focus on nearby objects declines in a predictable way: Farsightedness is rare at 35 but universal at 50. Skin elasticity also decreases reliably and often surprisingly abruptly in early middle age. Patterns of fat deposition change in predictable, stereotyped ways. Other systems, notably cognition, barely change.In Namibia, Eastern Bushmanland, Tsumkwe, an old !Kung man. The !Kung are San hunter-gatherers. Many modern hunter-gatherers live well beyond age 40. Skeletal remains suggest that our ancestors frequently did the same. (Nigel Pavitt/GETTY IMAGES/AWL IMAGES RM)
Each of these changes can be explained in evolutionary terms. In general, it makes sense to invest in the repair and maintenance only of body systems that deliver an immediate fitness benefit — that is, those that help to propagate your genes. As people get older, they no longer need spectacular visual acuity or mate-attracting, unblemished skin. Yet they do need their brains, and that is why we still invest heavily in them during middle age.
As for fat — that wonderfully efficient energy store that saved the lives of many of our hard-pressed ancestors — its role changes when we are no longer gearing up to produce offspring, especially in women. As the years pass, less fat is stored in depots ready to meet the demands of reproduction — the breasts, hips and thighs — or under the skin, where it gives a smooth, youthful appearance. Once our babymaking days are over, fat is stored in larger quantities and also stored more centrally, where it is easiest to carry about. That way, if times get tough we can use it for our own survival, thus freeing up food for our younger relatives.
These changes strongly suggest that middle age is a controlled and preprogrammed process not of decline but of development.
When we think of human development, we usually think of the growth of a fetus or the maturation of a child into an adult. Yet the tightly choreographed transition into middle age is a later but equally important stage in which we are each recast into yet another novel form.
That form is one of the most remarkable of all: a resilient, healthy, energy-efficient and productive phase of life that has laid the foundations for our species’s success. Indeed, the multiple roles of middle-aged people in human societies are so complex and intertwined, it could be argued that they are the most impressive living things yet produced by natural selection.
The claim that middle age evolved faces one obvious objection. For any trait to evolve, natural selection has to act on it generation after generation. Yet we often think of prehistoric life as nasty, brutish and short. Surely too few of our ancestors lived beyond age 40 to allow features of modern-day middle age, such as the deposition of a spare tire around the middle, to have been selected for.
This is a misconception. Although average life expectancy may sometimes have been very low, this does not mean that humans rarely reached the age of 40 during the past 100,000 years. Average life expectancy at birth can be a misleading measure if infant mortality is high, then the average is skewed dramatically downward, even if people who survive to adulthood have a good chance of living a long, healthy life.
The evidence from skeletal remains suggests that our ancestors frequently lived well into middle age and beyond. Certainly many modern hunter-gatherers live well beyond 40.
The probable existence of lots of prehistoric middle-aged people means that natural selection had plenty to work on. Those with beneficial traits would have been more successful at nurturing their children to reproductive age and helping provide for their grandchildren, and hence would have passed on those traits to their descendants. As a result, modern middle age is the result of millennia of natural selection.
But why did it evolve as it did? In prehistory, and still today, human survival is entirely dependent on skilled gathering of rare, valuable resources. Humans cooperate, plan and innovate so they can extract what they need from their environment, be that roots to eat, hides to wear or rare metals to coat smartphone touch screens. We lead an energy-intensive, communication-driven, information-rich way of life, and it was the evolution of middle age that supported this.
For example, hunter-gatherer societies often have complex and difficult techniques for finding and processing food that take a long time to learn. There is evidence that many hunter-gatherers take decades to learn their craft and that their resource-acquiring abilities may not peak until they are older than 40.
Gathering sufficient calories is crucial for the success of a human community, especially since young humans take so long to grow up. Indeed, for the early years of life they devour calories without contributing many to the group themselves. Research suggests that a human child requires resources to be provided by multiple adults, almost certainly more than two young parents. For example, a recent study of two groups of South American hunter-gatherers suggested that each couple required the help of an additional 1.3 non-reproducing adults to provide for their children. Thus, middle-aged people may be seen as an essential human innovation, an elite caste of skilled, experienced super-providers on which the rest of us depend.
The other key role of middle age is the propagation of information. All animals inherit a great deal of information in their genes some also learn more as they grow up. Humans have taken this second form of information transfer to a new level. We are born knowing and being able to do almost nothing. Each of us depends on a continuous infusion of skills, knowledge and customs, collectively known as culture, if we are to survive. And the main route by which culture is transferred is by middle-aged people showing and telling their children — as well as the young adults with whom they hunt and gather — what to do.
These two roles of middle-aged humans — as super-providers and master culture-conveyers — continue today. In offices, on construction sites and on sports fields around the world, we see middle-aged people advising and guiding younger adults and sometimes even ordering them about. Middle-aged people can do more, they earn more and, in short, they run the world.
This has left its mark on the human brain. As might be expected of people propagating complex skills, middle-aged people exhibit no dramatic cognitive deterioration. Changes do occur in our thinking abilities, but they are subtle. For example, response speeds slow down over the course of adulthood. However, speed isn’t everything, and it is still debated whether other abilities deteriorate at all.
To carry out their roles in society, middle-aged people need not necessarily think better than younger adults, but they may have to think differently. Indeed, functional brain imaging studies suggest that they sometimes use different brain regions than young people when performing the same tasks, raising the possibility that the nature of thought itself changes as we get older.
A central and related feature of middle age is the many healthy years we enjoy after we have stopped reproducing. Female humans are especially unusual animals because they become infertile halfway through their lives, but male humans often also effectively “self-sterilize” by remaining with their post-menopausal partners. Almost no other species does this.
The possible benefits of menopause are not immediately obvious: After all, natural selection favors individuals who rear the most offspring. Yet there are other, rare examples of reproductive cessation in the animal kingdom that may provide some clues. Orcas also undergo menopause, and it is striking how much their lives mirror ours. They are long-lived, slow to develop, intelligent and vocally communicative. They invent and apply a complex array of techniques for communal food acquisition, and they are extremely widespread.
Thus, humans can be seen as members of an elite club of species in which adulthood has become so long and complicated that it can no longer all be given over to breeding. Just like farsightedness and inelastic skin, menopause now appears to be a coordinated, controlled process. It liberates women and their partners from the unremitting demands of producing children and gives them time to do what middle-aged people do best: live long and p amper.
Bainbridge is a lecturer at the University of Cambridge and author of “Middle Age: A Natural History” (Portobello). This article, based on that book, was written for New Scientist magazine, from which it is reprinted.