Whither Homo Sapiens? Part II: The Future

April 21, 2022

IV. humans will evolve

In this section we discuss some educated guesses of how various driving trends of modern human life will influence the future natural evolution of homo sapiens. We organize the discussion around those driving trends, and postpone consideration of the more speculative impacts of human selection guided by cutting-edge technology until section V.

Changing fertility:

Both natural and sexual selection tend to increase the fertility rate of species. But among humans over the last half-century, demographic, social, cultural, and medical trends have outweighed evolution to produce a rapidly decreasing fertility rate. If current trends persist, the rate is projected to fall further, as shown in Fig. IV.1, and to lead to a stabilization of the global human population by the end of the century.

Figure IV.1. The number of live births per woman, averaged over the global population, as a function of year, including projections of current trends out to 2100.

A major contributor to the decreasing fertility rate is a demographic transition that most developed countries have by now gone through, under the influence of medical, economic and educational development and resource management. The features of the demographic transition are illustrated in Fig. IV.2, along with indications of which countries are at which stage of the transition. In undeveloped countries and communities, birth rates are initially high but population growth is slow as a result of abundant infant mortality, due to disease, poor diet, famine and poor medical care. Thus, a relatively small fraction of children survive long enough to produce their own children. In the second stage of the transition, improvements to medical care, water quality and sanitation lead to a rapid drop in infant mortality, but birth rates remain high, leading to exponential population growth. In the third stage, birth rates begin to fall in lagging recognition of increased childhood survival, but the death rates are still falling faster from medical improvements. In the fourth and fifth stages, both birth and death rates are low, leading to aging, and eventually dwindling, populations.

Figure IV.2. Stages of the demographic transition model described in the text, indicating typical birth rates, death rates, and population growth in each stage. The table at the bottom lists reasons for the transitions among stages and examples of countries currently at each stage.

Social trends in the developed world have also contributed to the rapid fertility decline. The wide availability of birth control, the expansion in educational and career opportunities for women, and the increasing cost of raising children have led women in industrialized countries to have their first children later in life. According to Scott Solomon, “In the thirty-four nations that constitute the Organization for Economic Cooperation and Development, the average age at first birth went up from twenty-four to twenty-eight between 1970 and 2008.” Men are also having children at later ages than they did historically. “In Iceland, for example, the average age of fathers went up from twenty-eight to thirty-three between 1980 and 2011.” The older the father, the more mutations the father tends to pass on to his children. “This is because, unlike eggs, sperm are continuously created throughout a man’s life, meaning that sperm created by older men are made from cells that have divided many times, creating lots of opportunities for mutations to occur.” Hence, older fathers increase the mutation pool upon which natural selection may act to influence subsequent evolution.

There has also been a rapid recent decrease in male sperm counts, as indicated in Fig. IV.3. The causes of this decrease are not yet completely understood. It may be related to changes in diet, exposure to environmental toxins and new chemicals, or the increasing prevalence of obesity. But it’s not out of the question that it stems from Y chromosome mutations, which are less subject to efficient DNA repair than genes on other chromosomes, as we have described in our post on Sex, Gender, Genome and Hormones. The contribution of the decreasing sperm counts to the drop in worldwide fertility rate is not yet clear.

Figure IV.3. The percentage decline in 35-year old Western male sperm counts from 1989 to 2005.

The decrease in worldwide fertility to date has been largely welcomed to temper exponential population growth among humans competing for Earth resources that may eventually be exhausted. But it is likely to influence future human evolution. First of all, the decrease is hardly uniform across all regions of the globe, as illustrated in the fertility map of Fig. IV.4. Ethnic groups that tend to produce more than the global average number of children who reach adulthood – e.g., those in stage 2 of the demographic transition – will naturally contribute more to future worldwide allele frequencies. And with increasing worldwide mobility, migration, and intermarriage those allele preferences may well flow into other ethnic groups.

Figure IV.4. A map showing fertility rate ranges in the countries of the world. Note the low rates throughout Europe.

Even without that gene flow, it is the most fertile couples within a given population who pass their genes along most successfully to future generations. A multi-generational six-decade study of life history and health data from people living in the town of Framingham, Massachusetts has revealed that “the women who have the most children were those who were, compared to the population averages, somewhat shorter and heavier, with lower total cholesterol and systolic blood pressures, and who began reproducing earlier and experienced menopause later.”

In the developed world, cultural pressures may continue to counteract evolutionary pressures to push women to begin having children earlier in life. In compensation, however, natural selection is likely to favor a gradual delay, or even elimination, of female menopause, so that women can produce babies later in life. Scott Solomon speculates that the increased use of birth control may lead sperm and egg production and capabilities to develop naturally selected countermeasures to increase fertility. Alternatively, technology might develop to allow the cryogenic storage of eggs and sperm to permit in vitro fertilization at any time during human lives, and perhaps, as in Brave New World, even laboratory environments for fetal development. The babies born that way, however, would be quite different from ones that develop in the environment of the womb.

If sperm counts continue to decrease, there may be increasing use of sperm donors from among the men with allelic advantages in sperm production and quality, and they may gain additional influence on future allele frequencies. Thomas Mailund, while guessing that the primary cause of the ongoing sperm count decrease is probably environmental pollution, points out that: “Different countries have laws that limit the number of children that a single donor can be the father of, but international trade of sperm bypasses these laws, and a single donor can be the father to vastly more children than he would normally be able to without sperm donation. Sperm donation and variation in fertility give us such a steep selective gradient that we can see an adaptive effect even if the exposure to pollution is temporary.”

Other demographic changes:

The demographic transitions in developed countries are leading to an aging population. According to Mailund: “At the global level, the estimate for 2050 is that 22% of the human population is older than 60. Already today, in seven countries, more than 20% of the population is older than 65; in order of percentages: Japan, Italy, Germany, Portugal, Finland, Bulgaria, and Greece…This is a dramatic change to our species, and it will affect our future evolution.”

If the increasing age of parents at childbirth is accompanied by continuing medical advances to increase the maximum age of humans, it will lead to further evolutionary or social changes. For example, it will lengthen the time interval between generations and thereby slow human evolution in the future. However, extending maximum age can also place greater pressure on the fight for resources. Mailund asks: “How would we get to a stable population with early [compared to life expectancy] reproduction and long lives if available resources limit our population size? If we have a constant population size and then add longer lifespans, the population will grow. Each couple can have two children, so the number of people per generation doesn’t increase, but there are now more generations alive.”

If population growth does threaten to exhaust Earth’s resources, the net result would be either various government-imposed limits on number of children or on resource rationing, or migration to another planet whose resources are yet to be exploited. We will deal with the latter possibility in section V. Government regulations have already been tried in China’s controversial One Child Policy, which did further reduce already decreasing population growth, as shown in Fig. IV.5. However, when all couples are constrained to produce only a single child, one no longer has the most fertile couples dominating future generations’ allele frequencies. So government-imposed limits stunt the evolutionary drive toward increased fertility.

Figure IV.5. The impact of China’s government-imposed One Child Policy (OCP), implemented from 1980 to 2016, on Chinese fertility and population growth rates.

Furthermore, in China the cultural preference for male firstborns, coupled with the One Child Policy and sex-selective abortion, has led to a rather severe gender imbalance in the population: for example, in 2005 there were about 32 million more males than females under the age of 20. Such a surplus has its own potential impacts on society; those excess males who are unable to find mates may lack a stake in the existing social order and form an outcast subculture.

If government regulations are used to limit overall population growth, Mailund worries also that that is “an ideal setup for eugenics, the improvement of the human gene pool based on desirable [by governments] properties…The ethical issue with eugenics is not, for most people, actively selecting traits we want in our children when we get to choose. It is preventing someone who wants children, from having them, based on characteristics and genes they possess…A side effect of living longer might be that we expose our species to intense artificial selection.”

Most of the evolution we have seen to date in homo sapiens was tailored to supporting social behavior in ancient modest-sized groups of families and friends in small communities with perhaps 100-200 inhabitants. Such groups provided added protection against predators and sharing of responsibilities and resources. And it led to selection pressure on human brain development to facilitate close and stable relationships with about 100-200 other humans, the so-called Dunbar number, after the anthropologist Robin Dunbar.

However, modern human life has led to rapid increases in urbanization. Mailund: “Fewer than a billion humans lived in urban areas in 1950, while more than 4 billion do so today…In the future, most of humanity will live in megacities, cities that are the result of smaller cities merging as they expand into each other, cities with tens or even hundreds of millions of inhabitants…The population density will increase, and we will have to interact with more people than we have evolved to do.” The increasing urbanization is driven in part by the wider availability of diverse foods, services and jobs in urban areas, and in part by advances in farming technology, which require many fewer farmhands and rural inhabitants. It is thus conceivable that human brains will evolve under natural selection pressure to accommodate larger numbers of close contacts and improved socializing.

It is also possible that high urban population densities will lead to natural or sexual selection pressure on human brains to sharpen bullshit detection. In the smaller communities in which human civilization began, it paid not to become a pariah through cheating or stealing, lest one couldn’t find a mate to reproduce with. But the benefits to cheating or manipulating others in order to gain wealth and power, to say nothing of a greater share of scarce resources, changes the calculation for some when it is easier to hide bad behavior in urban crowds. It is possible that the ability to see through deceptive schemes will aid both survival through child-bearing years and the ability to attract mates. Such improvements can aid fitness not only in urban settings, but also more generally in the age of viral internet and social media spread of misinformation.

Another evolutionary impact of increased urbanization is that there will be increased gene flow between previously distinct populations. In the long-term future, Mailund predicts that “Humanity will interact more, our genes will blend more, and ethnic differences will diminish…We will not lose our genetic diversity when we start mixing more, but phenotypic diversity will be reduced.” This trend will be reinforced by increases in mobility and globalization, international companies hiring more and more skilled foreigners. Climate change is also likely to lead to massive migration to different countries and populations. “In the past, the human species was highly structured, into different groups with different allelic compositions. In the future, with higher and higher mobility, this structure will erode. There will still be variation between people, of course, but as ethnic groups mix, the population differences will eventually disappear.”

Mating changes:

We are currently in the midst of a rapid transition in the ways humans find mates, and that transition is likely to have a strong effect on future human evolution. The primary cause of this transition is the rise of online dating (see Fig. IV.6). Mailund: “A study from 2019, based on a 2017 survey, shows that in the United States, finding dates online surpassed getting introduced through friends in 2013. By 2017, almost 40% of all couples met through some form of online dating. Finding a mate online is more popular than any other approach, and the fraction of couples that meet this way is growing steeply.” Online dating increases the probability of mating with partners from a different ethnic, cultural or religious group, or from a different country, and thereby reinforces the enhanced mixing of different allelic compositions that will accompany greater mobility, migration, and urbanization.

Figure IV.6. The rapid rise in the number (in millions) of U.S. users of online dating services between 2017 and the present (including projections to 2024).

Furthermore, online dating is likely changing the criteria people apply in choosing a mate, so that there will undoubtedly be impacts on sexual selection going forward. A major aspect of sexual attractiveness in the future will be the ability to present an appealing online profile, which at least gets one past the initial step of finding a mate. Thus, for example, there may be sexual selection pressure for males to develop better written communication skills.

Pheromonal odor has traditionally been an important aspect of sexual attraction among humans, as among most animal species. But clearly, online dating removes the sense of smell from an early signal of attraction. Some studies have suggested that women’s preferences for male odor also change when they are on hormonal birth control pills. So widespread access to birth control pills, along with widespread use of deodorants and perfumes, are further complicating the use of smell in choosing mates. Dishonest online dating presentations and widespread use of cosmetics are affecting perceptions of appearance as a sexual attractor, as well.

The demographic trend for women to have their first children at later ages is also likely to alter sexually selected traits. Some women may still choose to have early safe sex with tall, dark and handsome, risk-taking rogues. But for mates to have children with, they may prefer characteristics that signal responsible future fatherhood. Thus, future sexual selection may favor genes that lend themselves to calm, gentle, stable, prosperous behaviors. Perhaps male aggressiveness will be gradually deselected in aging populations, with aggressive individual males being driven increasingly into violent subcultures like the “incels” (involuntary celibates) seen recently in North America and Europe.

We have also seen a rapid recent increase in the prevalence of individuals who self-identify as non-binary or gender fluid. It is not yet clear to what extent this trend is a passing socially influenced change that will have little effect on future evolution, or rather reflective of changes in testosterone secretion levels in the womb during the critical stages of pregnancy for fetal brain development. If the latter explanation is the dominant effect, and non-binary gender identification and sexual preferences continue to grow in abundance, this will also likely have significant future impacts on birth rates and sexual selection.

Changes to environment, ecology and diet:

Since the Industrial Revolution, humans have had major impacts on the global environment and the ecology of other species, and both of these effects are likely to introduce significant changes in human diets and disease exposure in the future. Even aside from the environmental and ecological changes, humans have been recently changing their diets in harmful ways. As we saw in part I of this post, clear evolutionary changes in the past have resulted from selection pressures supporting adaptation to environmental, disease, and dietary changes.

For example, modern human technology has been producing new and more abundant forms of air pollution, e.g., of fine particulate matter and ground-level ozone.  Studies such as the Harvard Six Cities investigation and a survey of 545 U.S. counties have revealed a correlation between high levels of fine particulate matter and shortened life expectancy.  Other studies have suggested that the level of risk from pollution exposure is genetically influenced, so some alleles presumably offer better resistance than others. In urban areas of increasing population density, pollution is likely to be more of a problem, and there may thus be natural selection pressure to enhance the frequency of alleles that yield increased resistance to pollution-caused diseases, such as lung diseases, asthma and hypertension. On the other hand, new, cleaner technologies like electric vehicles and renewable energy sources may actually substantially reduce pollution in the not-too-distant future.

We have mentioned above, in association with Fig. IV.3, that some have suggested a link between human-caused pollution and the ongoing reductions in male sperm count. Mailund: “Studies indicate that air pollution might be linked to lower sperm quality and that plastic can release chemicals that act like the hormone estrogen. Estrogen affects sperm count, and sperm count is decreasing worldwide. There is a genetic component to sperm quality. If increasing pollution (for example, exposure to plastic) affects fertility, then pollution can undoubtedly affect our genetic future on a short time scale. Infertile men will not pass their genes, that are sensitive to chemical exposure, on to future generations.”

It is not just atmospheric pollution and exposure to plastic that pose human health hazards. We have been learning that new synthetic chemicals, such as the PFAS “forever chemicals” found in Teflon, fire-fighting foams, and many other commercial products, are exposing humans to severe diseases. So, again, any alleles that offer improved resistance to new sources of toxicity are likely to increase in frequency in future human generations. If these chemical exposures have a direct effect on fertility, that will again lead to strong natural selection pressure.

The burning of fossil fuels by humans is the dominant contributor to ongoing global warming and the consequent changes in Earth’s climate. The increasing frequency of severe storms, forest fires, droughts, and extreme weather episodes, coupled with the continuing rise of global sea levels, will likely lead to massive human migration in the coming centuries, adding eventually to the enhanced gene flow that will tend to reduce human phenotypic diversity. In addition, future changes in rainfall and growing season length will change agricultural emphases both regionally and worldwide. Those agricultural changes will necessarily lead to dietary changes for humans and resulting natural selection pressures to aid digestion of new dietary staples.

In Earth’s ancient history, well before the birth of homo sapiens, there were five mass extinction episodes, wiping out large fractions of the species then extant. Human activities are now causing a 6th mass extinction (see Fig. IV.7) through our impacts on climate and our destruction of animal habitats. As human populations continue to grow, food production may require cultivation of much more land, further encroaching on animal habitats. Though we may be at the top of the food chain, the food chain below us may change substantially as more species go extinct, and that can end up also altering human diets and the sources of needed nutrients. An example of a needed nutrient is the omega-3 fatty acid DHA important for brain development, which humans now get primarily from eating fish. But many fish species are endangered by worldwide coral reef bleaching, resulting from global warming, and overfishing.

Figure IV.7. A chart indicating the rapid post-industrial rise in animal species extinctions that has accompanied rapid growth in the global human population, suggesting that we have entered a 6th mass extinction period in Earth’s history.

Domesticated cattle contribute significantly to global methane emissions and, hence, to global warming. That is just one factor contributing to an ongoing trend toward a more plant-based human diet. But a plant-only diet would complicate humans’ need for iron. Mailund again: “[Iron] is a crucial complement in hemoglobin that transports oxygen around your body – no iron, no hemoglobin, no oxygen transportation, no you…There are two kinds of iron that we can get from our food. One, heme iron, you can only get from meat and the other, non-heme, from meat and plants. Meat contains 40% heme and 60% non-heme iron, while plants only have non-heme iron. The problem with a plant-only diet is that non-heme iron is a lot harder for us to absorb…If we go vegetarian, there might be a selection for something like getting better at absorbing non-heme iron.”

As is the case for a number of changes caused by humans, it is conceivable that future impacts on humans may be ameliorated by new technologies as much as by evolution. In the case of diet, lost nutrients can be compensated by the use of dietary supplements, which can also be added by genetic engineering to food staples. “Golden rice, for example, is an existing GMO that produces vitamin A. Vitamin A deficiency kills an estimated 1—2 million each year and causes blindness in 250,000 to 500,000 children.” However, dietary supplements, and GMOs in particular, may expose humans to new sources of toxicity. We have previously described how the majority of GMO crops now in use have been engineered to be resistant to the weed killer Roundup, but exposure to Roundup and the chemical glyphosate, which is its main ingredient, has been found to cause increased incidence of cancer in humans. Even aside from issues of toxicity, it is possible that humans will have to enhance genetic adaptations to digesting GMO foods.

Some animal species have responded to humans’ expanding encroachment on their habitats by adapting to life in urban environments. This has already happened in raccoons, rats, doves, crows, foxes, and to some extent deer. Frequent exposure to such animals may increase the jump of new infectious diseases to humans, as happened with rats in the Black Death of the Middle Ages. But our encroachment also increases the likelihood that potential human predators, such as bears and mountain lions, which themselves evolve to adapt to their new environment, may show up in urban settings in increasing numbers.

Of course, not all environmental and ecological changes are caused by humans. Eventually, perhaps several tens of thousands of years from now, we expect the Earth to enter into the next Ice Age, with such drastic changes to Earth’s climate that humans, if they survive that transition, will have to adapt. Some of that adaptation may occur genetically, allowing humans who have thrived in warm climates to withstand much colder temperatures. But much of the adaptation would have to be technologically driven, including new methods of growing and gathering food. It should be noted, however, that among several hominin species homo sapiens appear to be the only one that survived the last Ice Age, perhaps because of their greater skill in devising tools to aid survival.

One modern trend in human diets is also likely to produce new evolutionary pressures. As we noted in part I, humans evolved to crave high-fat foods and store fat, to tide them over between occasional animal consumption in early hunter-gatherer communities. But today for most of the developed world, food is not scarce and the increased consumption of high-fat foods, processed foods and high-fructose corn syrup has fueled a worrisome increase in obesity (see Fig. IV.8). And that obesity, in turn, increases susceptibility to other diseases, like diabetes and heart disease. Both natural selection and sexual selection (attractiveness to potential mates) in the future, then, may select against those fat-loving genes favored in our ancestors, or in favor of genetic changes that increase human metabolisms to burn calories faster. This will be important if we are to avoid a long-term future like that envisioned in the Pixar movie Wall-E, where chubby humans reside in space and lose the ability and desire for exercise or physical labor.

Figure IV.8. Maps showing the rapid increase in the percentage of obese individuals throughout the United States from 1990 to 2010.

The combination of urban living, modern dietary choices, and the widespread use of antibiotics has caused dramatic changes in humans’ microbiome, in which gut bacteria, in particular, have historically been important in aiding digestion. Future human evolution may have to take over some digestive functions previously left to the microbiome. Scott Solomon describes some of the specific microbiome losses as follows. “One type of bacteria, known as Treponema, is consistently found in the guts of people living traditional lifestyles, both in South America and in Africa, but absent from Westerners. Treponema had also been found in the guts of wild apes, suggesting they may have been present in our ancestors…Another type of bacteria, Prevotella, which are also thought to facilitate digestion of a diet high in fiber and complex carbohydrates, are common in the guts of traditional people but rare in urban Westerners. In contrast, urban Westerners tend to have a lot more bacteria in the genus Bacteroides—consisting of as much as a quarter of the microbial species in the intestine—than do people living traditional lifestyles.”

A major casualty of antibiotics has been the bacteria H. (Helicobacter) Pylori, which lives in the inner lining of the stomach and cannot live outside the human body. H. Pylori appears to have co-evolved with humans over more than a hundred thousand years, to help regulate the production of stomach acid and communicate with the body’s immune system. But it is now rapidly disappearing in Westerners. For example, “A study led by American medical doctor and infectious disease researcher Martin Blaser found that among children born between 2002 and 2006 in the Dutch city of Rotterdam, 75 percent fewer of them had H. Pylori in their stomachs than did their mothers. It was an incredible decline in just one generation. Fewer than 6 percent of children born in the United States after 1995 had H. Pylori, and fewer than 2 percent of children in Japan born after 2010 did.” Food allergies, asthma and other plagues that have rapidly increased in frequency since the second half of the 20th century, while antibiotic usage helped to cure many bacterial infections, are not present at all among people living traditional lifestyles like the Yanomami, who all have H. Pylori in their stomachs. So, again, future human evolution may have to compensate for the microbiome losses.

Disease exposure:

The human immune system has evolved over millennia to increase resistance to a wide variety of infections. But now various features of modern human life are changing selection pressures on the immune system. On the one hand, improved hygiene, vaccines and medical treatments provide technological protection that weakens our dependence on our immune system. Then, as Mailund explains, “We will not have the selective pressure to keep the immune system strong, so random processes – mutations and genetic drift – could erode it. If there is selection on top of the random processes, because our immune system is a potential danger to ourselves, then the erosion can happen quickly.” The weakening pressure would come from the need to eliminate autoimmune disorders responding to non-threatening invaders in our bodies. The recent rises in autoimmune diseases and in allergies may be triggered, in part, by the vast improvements in human hygiene and childhood vaccines, which tend to eliminate much of the immune system training that otherwise comes from fighting off childhood infections.

Counteracting possible negative selection pressure from autoimmune disorders is the enhanced exposure to new types of infection, which may jump to humans from animals into whose habitats humans have expanded. As we have seen dramatically with COVID-19, the increased mobility of our species and increasing urban population densities can lead to rapid pandemic spreads of such infections among humans worldwide. The danger comes not only from pathogens we have never encountered before, but also from ones like influenza that have mutated quickly to evade our medical treatments or vaccines. The rapid rise in antibiotic-resistant bacteria is an example of the threat to humans from evolution of other species.

Humans might evolve resistance to such new or evolved pathogens in similar fashion to the way bacteria gain antibiotic resistance, but over greatly expanded time scales since our generations are so much further apart in time. Relatively rare alleles that offer some humans increased resistance to a pathogen could increase in frequency over one or two generations following a worldwide pandemic. However, it seems likely that modern medical science – especially in the age of CRISPR gene editing – can respond much more quickly than human evolution to the presence of new or resistant pathogens.

New medical treatments may switch some historically positive natural selection pressures to negative. As we saw back in part I, the sickle-cell allele has become prevalent in those regions of the globe where malaria-carrying mosquitoes are abundant. But various efforts are under way to control, or even eliminate, malaria. These include ongoing attempts to develop a vaccine or to wipe out some mosquito species, either through the use of insecticides or gene drives to induce sterility among those mosquitoes. If these efforts succeed, the positive natural selection pressure to increase sickle-cell allele frequencies will turn rapidly to negative pressure to reduce the occurrence of sickle-cell anemia, which affects individuals who carry sickle-cell alleles from both parents.

Some human diseases, even if they are successfully managed through medical treatments, may still influence sexual selection in future humans. Mailund discusses the example of HIV: “Today, HIV-infected people on proper medication have the same life expectancy as non-infected, and they will not infect sexual partners…Still, I think you will agree that many will hesitate with being in a relationship with an infected [individual], all else being equal.” Hence, even in the presence of treatment for HIV, there may still be a positive sexual selection for genes that favor resistance to the infection.

As medical science finds cures for other diseases, more people will likely end up dying of cancer. However, cancer resistance is unlikely to be subject to significant natural selection pressure, in part because the elderly do not play crucial evolutionary roles. More importantly, “we know many genes linked to cancer, but the alleles we know about, with few exceptions, only influence cancer risk by tiny amounts. Combined, they can have a substantial effect, but individually they do not, and selection works on individual alleles.” So here, too, medical treatments are much more likely than evolutionary developments. When such future medical advances extend human life spans even further, it will increase human population and the competition for increasingly sparse resources. That competition may spur some of the radical “solutions” discussed later in this post.

Disaster impacts:

99.9% of all species that have ever existed on Earth have gone extinct. It seems likely that human intelligence and technology will keep homo sapiens from being completely extinguished by anything short of Earth destruction. But natural and man-made disasters can cause major die-offs, terminating some populations, and thereby changing allele frequencies suddenly. Examples of natural disasters causing potentially widespread deaths are: asteroid or comet impacts with Earth, such as the one that triggered the extinction of the dinosaurs 66 million years ago; massive volcanic eruptions, like the ones correlated with several of Earth’s previous mass extinction episodes (see Fig. IV.9); and deadly pandemics, like the Black Death that killed off roughly half of Europe’s population in the 14th century.

Figure IV.9. The correlation of most known mass extinction events with massive volcanic eruptions in large igneous provinces (LIP). The extinction events lined up on the zero axis occurred in close time proximity to the LIP events.

But humanity’s greatest threat is probably humans themselves. Human intelligence is sufficiently great to extract the energy from fossil fuels, but not necessarily great enough to avoid runaway consequent change in Earth’s climate; great enough to design nuclear weapons, but not necessarily great enough to avoid nuclear war; great enough to discover CRISPR gene editing, but not great enough to prevent its misuse to produce deadly new bioweapons. Various conceivable human-caused catastrophes could have similar effects on the surviving gene pool as a deadly pandemic.

Nuclear war, ecosystem collapse, deadly new bioweapons, rapidly dwindling food supplies are some examples that might lead to dramatic reductions in worldwide human population. As in the case of a deadly pandemic, survivors’ alleles may come to dominate and furthermore be strongly selected by continuing survival and reproduction of the species. The wealthy might survive by preparing amply stocked luxury doomsday bunkers underground, as many have already done. But survivors on the surface may have alleles that enhance their resistance to radiation damage or bioweapons, and those alleles would prosper in the aftermath. One can even imagine a consequent long-term bifurcation of the human species, akin to that envisioned for the very far future in H.G. Wells’ 1895 novel The Time Machine.

V. impacts of modern human technology

Over recent decades, humans have developed cutting-edge technologies that will allow a human imprint on the species’ future evolution. The technologies we consider in this section include CRISPR gene editing, in vitro fertilization, medical implants and human brain augmentation, artificial intelligence, space travel, robotics, and even the ability to resurrect extinct species. One can only speculate on how humans will choose to apply these technologies in the future, but we offer some evolution-relevant speculations here.

CRISPR gene editing:

As we pointed out in part I and in a previous post, the development of CRISPR-Cas9 gene editing opens up, in principle, an expansive range of possible human selection of desired evolutionary traits. Heritable edits require engineering on germline cells involved in sexual reproduction. Despite the clumsy first attempt in China to produce the world’s first CRISPR babies, and some apparent research in China on germline editing of non-viable human embryos, humanity is not yet ready to go down the path of heritable edits. The use of CRISPR on humans to date focuses on editing DNA in somatic cells, which are not heritable. But as the precision of CRISPR editing advances and our understanding of the roles played by each human gene improves, the pull of using CRISPR germline editing at the very least for eliminating some genetic diseases from a line of descendants may become irresistible.

The simplest edits to make at first will be ones correcting a disease-causing single-point mutation in a well-understood gene that otherwise may be passed on to children. Sickle-cell anemia and cystic fibrosis are two specific examples. In the case of sickle-cell anemia, we have pointed out above that natural selection may well lead to reducing the frequency of this allele in regions where malaria is rare, or once medical science has produced a reliable way of preventing malaria. Correcting this harmful allele by germline editing can thus be viewed as a way of accelerating natural selection.

Eventually, at least some humans may contemplate using germline editing not only for removing genetic diseases, but for enhancing “desirable” human features. As we pointed out in our post on the CRISPR Arms Race, just such enhancements of soldiers are currently under consideration by military and defense research establishments in several countries. Some contemplated enhancements include increased strength, resistance to radiation poisoning or biological weapons, improved night vision, improved genetic repair mechanisms, possible regeneration of lost or damaged limbs. Some of these changes might be realized by somatic cell edits on individual soldiers, or CRISPR on-off switches to activate or deactivate expression of certain genes. The only reason for attempting such edits to the germline would be to grow “super soldiers” from the embryo, or in the words of military history author Joseph Micallef, to create “a permanent, genetically enhanced military caste, a sort of super Praetorian Guard.”

If the military in advanced countries adopt germline editing of humans, it seems likely that some wealthy individuals would eventually seek out CRISPR treatments to enhance the survival, intelligence and procreativity of their descendants, in attempts to bypass the genetic lottery. Indeed, as we will discuss further below, some already use in vitro fertilization to impose a crude form of human selection. If governments end up controlling access to germline editing, it is not that much of a stretch to imagine its use to establish a Brave New World-like caste system, in which some classes get genetically enhanced and others genetically dulled.

The complexity of the human genome, in which the majority of genes affect multiple bodily functions, introduces high risk of unintended consequences for most conceivable germline editing. And while we are discussing risks, we must mention the possibility of humans using CRISPR not on humans, but to resurrect extinct species. In principle, it is possible to bring back a temporarily extinct species by capturing DNA from remains of the extinct species and using CRISPR editing to modify DNA extracted from closely related, still extant species so that it more closely resembles that of the extinct species. Females of the living species would then be used to bring those edited, fertilized eggs to the birth stage. Mailund informs us that: “There are currently active projects for bringing seven species back to life: Quaggas, a relative of zebras from South Africa; aurochs, a wild ox from Europe; Floreana Island giant tortoise; passenger pigeons; woolly mammoths; hearth hens; and little bush moa, despite the name a large (1.3 meters tall) bird from New Zealand. If any of these projects are successful, more will follow.”

Restoring an extinct species only makes sense if humans simultaneously restore their lost habitat. And even then, we don’t know whether implanting those modified eggs in a distinct species, even if closely related, might introduce birth defects or new diseases, some of which might subsequently jump to humans. Shades of Jurassic Park!

In vitro fertilization:

In vitro fertilization (IVF) is an assisted reproductive technology in which an egg is extracted from the ovaries of a female, fertilized by sperm donated by a male in a culture medium, and the resulting embryo implanted in a woman’s (possibly a surrogate’s) uterus for what is hoped to be a normal pregnancy (see Fig. V.1). Louise Brown was the first child born through IVF in July, 1978. In 2016 Scott Solomon reported that: “More than 5 million babies worldwide have now been born using in vitro fertilization…In 2011, about half of all in vitro pregnancies in the United States involved twins or triplets, compared to only 3 percent in the general population. Indeed, babies conceived in vitro are now estimated to comprise between 1 percent and 5 percent of all births in developed nations, with the highest rates occurring in western Europe. If both trends continue, the total proportion of people in these populations born through assisted reproductive technology will continue to climb.”

Figure V.1. Illustration of the basic procedures of in vitro fertilization.

IVF, especially in association with surrogate pregnancies, can be used to extend the fertility window of modern women, thus providing, in principle, one avenue to combat the dwindling human fertility rates seen in Fig. IV.1 and alleviating natural selection pressure to delay menopause in human women. In his book Sex in the Future from the year 2000, Robin Baker predicted that birth control and modern technology will lead to a future in which sex becomes purely recreational and all reproduction is done in vitro. Of course, Aldous Huxley beat Baker to this prediction by the better part of a century in his 1932 novel Brave New World. Huxley went further to envision a future in which human pregnancies are replaced by laboratory development of fetuses. Huxley even foresaw something akin to CRISPR germline editing, when he had the Director of Hatcheries and Conditioning in his novel anticipate “a germinal mutation” to speed up the physical maturation of humans.

In Brave New World, all of this envisioned medical technology was exploited in the service of a government-imposed strict caste system, in which each caste is developed and conditioned to be content with their place in a hierarchical society. One of the motivations for dystopian novels such as Huxley’s is to try to dissuade humanity from following their envisioned path. It is worth noting that while Huxley was writing Brave New World, most U.S. states were busy adopting sterilization laws based on the flawed “theory” of eugenics and the intention of improving human genetics via selective breeding. IVF in combination with CRISPR germline editing provides the technology that could be used in the future to facilitate a high-tech eugenics or a caste system. We can only hope that humans avoid that future.

At the moment, IVF is an expensive procedure, mostly available to the well-off. In recent years, it has been “enhanced” by the technique of preimplantation genetic diagnosis (PGD). In this technique, couples who contribute multiple eggs fertilized in vitro can choose which one(s) to implant in the womb, discarding the rest, based on their preferences among genetic characteristics determined from DNA sequencing of the various embryos. This is already the first technical step in imposing some human selection on future evolution. Although the technique was originally designed to screen for embryos carrying hereditary genetic diseases, it has already been applied to select desirable features unrelated to diseases. Germline editing, whether for therapy or enhancement, is a considerable technical step, but only an incremental ethical step, beyond PGD. The future of human selection will depend on how humans weigh ethics, technology, the desire to improve their descendants and society as a whole, and their own wisdom in choosing paths for such improvement.

Medical implants:

Human performance has already been improved in many areas without gene editing, by the use of medical implants. Although these are not heritable, they have been used to replace or support damaged biological systems. Examples include artificial joints and hips, pacemakers to maintain cardiac rhythm, cochlear implants to mitigate deafness, and intraocular lenses to treat cataracts or severe myopia. In the future, we can expect implants to be used not only to correct disorders, but simply to enhance human capabilities. Such enhancements are being considered, for example, by defense and military agencies also contemplating genetic enhancements to soldiers. For example, French funding currently supports research on neural implants “that could ‘improve cerebral capacity’ or help soldiers tell enemy from ally. These could also allow commanders to locate them or read their vital signs from a distance.”

A BBC article on future humans speculates further: “As well as brain implants, we might have more visible parts of technology as an element of our appearance, such as an artificial eye with a camera that can read different frequencies of colour and visuals.” Brain implants, for example, might be used to enhance human memories and allow us to make more efficient use of information. So implant technology may play as important a role as gene editing in implementing human selection on the future of humanity.

Peter Diamandis goes further, imagining a not-distant future in which human brain implants will allow our brains to be interconnected with artificially intelligent computers via the cloud: “Enabled with BCI [brain-computer interface] and AI, humans will become massively connected with each other and billions of AIs (computers) via the cloud, analogous to the first multicellular lifeforms 1.5 billion years ago. Such a massive interconnection will lead to the emergence of a new global consciousness and a new organism I call the ‘meta-intelligence.’” He goes on: “Within a decade, every single human on the planet will have access to multimegabit connectivity, the world’s information, and massive computational power on the cloud. A multitude of labs and entrepreneurs are working to create lasting, high-bandwidth connections between the digital world and the human neocortex.”

Diamandis’ ecstatic vision is not quite so pie-in-the-sky as it might sound at first. Technological advances in brain-machine interfaces were highlighted in a 2019 documentary film I Am Human. The American entrepreneur Bryan Johnson, pictured in Fig. V.2, has already sought to revolutionize the monitoring and recording of brain activity with his new company Kernel. Kernel is now marketing a helmet called Kernel Flow (modeled by Johnson in Fig. V.2), which it describes as a “non-invasive, full-coverage optical headset that can be used in nearly any environment for recording real-time cortical hemodynamics to establish precise patterns of brain activity.”

Figure V.2. Bryan Johnson modeling the Kernel Flow headset his company has developed to monitor neural activity in the brain in real time.

The Flow uses a number of low-power infrared lasers to measure changes in the oxygenation of blood in the brain, which is a proxy for neural activity, with a spatial resolution of one centimeter. The futuristic device is designed to “facilitate communication between brain cells by hacking the “neural code” that enables our brain to store and recall key information. With proper implementation, such a device could correct faulty signals to mend a cognitive impairment.” Early reports on tests of the Kernel Flow by knowledgeable users are encouraging.

It is a substantial, but hardly unimaginable, next step to connect such a helmet, or later-generation sensors implanted in the brain, to an external network. Diamandis expects the developments to be enormously aided by AI: “Whatever challenges we might have in creating a vibrant brain-computer interface (e.g. designing long-term biocompatible sensors or nanobots that interface with your neocortex), those challenges will fall quickly over the next couple of decades as AI power tools give us ever increasing problem-solving capability.” He furthermore considers this development to be just a part of a transformational change in evolution – including CRISPR gene editing — on the cusp of which we now sit: “The rate of human evolution is accelerating as we transition from the slow and random process of ‘Darwinian natural selection’ to a hyper-accelerated and precisely directed period of ‘evolution by intelligent direction.’”

In this view, we need not worry about the frequent science fiction theme of being “overtaken” by robots humans might design with AI built in (to be discussed further below), because humans with BCI could keep pace with robot intelligence and maintain command. The chances are that the technology enabling this development will be ready for transformational change before human attitudes, but the human attitudes will most likely follow after some delay, if initial results are promising and do not lead to unintended disasters, as we must be prepared for with any radical change in technology.

Colonization of other planets:

The natural selection pressure to increase human fecundity eventually runs up against a brick wall, when the growing human population begins to exhaust many of the resources on Earth that they need to survive. That will likely usher in an era of warfare to control scarce resources. In anticipation of that era, more and more scientists and entrepreneurs have been proposing human colonization of other planets, most especially Mars. Before he died, Stephen Hawking famously predicted that human overcrowding and energy consumption would make Earth uninhabitable within 600 years. Elon Musk agrees and says that he launched his company SpaceX with the ultimate goal of making humanity a “multi-planetary species,” starting with colonization of Mars. NASA has clearly already flown unmanned flights to Mars, so the technology for transporting humans is almost available. The big remaining challenge is how to shield humans on deep space flights from the abundant radiation they will encounter along the way.

Travel for such colonization would, in the long run, likely be one-way, because the reduced gravity and much higher radiation environment on Mars (outside human-developed enclosures) would gradually reduce bone strength in the colonizers, making it difficult for them to survive the extreme decelerating forces of a re-entry to Earth. That consideration suggests that colonies on Mars would represent an isolated outpost of people initially drawn from the present population of homo sapiens on Earth. That is the condition for a branching-off into a new species, as was last seen among the multiple species of human precursors who migrated away from the African lowlands of their origin to form isolated outposts.

Humans on Mars would be subjected to an environment radically different from that on Earth: the gravity is one-third that of Earth; the absence of a magnetosphere and near-absence of an atmosphere yields a much higher radiation environment from cosmic rays, UV and particles from the sun; what water may exist on Mars is frozen in soil in its polar regions and much less plentiful than on Earth; animal protein for the humans in a Mars colony is more likely to depend on insects than on cattle or chicken; the high radiation environment makes cancers much more probable; childbirth and child growth in the low-gravity environment would pose presently unknown challenges. To quantify some of these differences: “Even the journey from Earth to Mars, estimated to take about six months, would expose colonists to a dose of radiation equivalent to sixteen times the maximum annual amount allowed by the U.S. Department of Energy for people working with radiation. They would be exposed to a similar amount every 500 days spent on the Martian surface.” In addition, “microgravity and radiation both lead to bone loss, at a rate of about 1 to 2 percent bone density lost per month in space.”

These radical environmental differences make it highly uncertain that a human colony on Mars is feasible. But if it is, those differences would radically alter the evolutionary fitness landscape of the colonists. They would likely have to live in underground shelters, like that envisioned in Fig. V.3. This would require changes in how humans there would produce vitamin D and other nutrients needed for human survival. There would likely be evolutionary changes to handle the different temperature ranges from those on Earth. The low gravity would probably lead to a human height expansion. Other evolutionary adaptations are indicated in Fig. V.4. One aspect of human bodies that would not need significant adaptation is our circadian rhythm biological clock, since the Martian day is only about 40 minutes longer than an Earth day.

Figure V.3. Artist’s conception of a possible human colony on Mars, complete with an underground vegetable garden shown in the cutaway and a concentrating solar plant in the distance to provide power.
Figure V.4. Some of the evolutionary adaptations one might expect for human colonizers on Mars or other planets.

Both gene drift among the small, isolated initial colonies and natural selection to adapt to the radically new environment of Mars could well lead over multiple generations to a new human species, distinct from homo sapiens on Earth. On the other hand, if colonists were continually replenished with new colonists from Earth, who mated with later-generation colonists already there, gene flow might keep the two isolated populations capable of interbreeding, and hence not truly distinct human species.

There are groups on Earth who are currently preparing for eventual colonization of Mars. The Mars Desert Research Station (see Fig. V.5), owned and operated by the non-profit Mars Society, “is a space analog facility in Utah that supports Earth-based research in pursuit of the technology, operations, and science required for human space exploration. We host an eight month field season for professional scientists and engineers as well as college students of all levels, in training for human operations specifically on Mars. The relative isolation of the facility allows for rigorous field studies as well as human factors research. Most crews carry out their mission under the constraints of a simulated Mars mission. Most missions are 2-3 weeks in duration, although we have supported longer missions as well. The advantage of MDRS over most facilities for simulated space missions is that the campus is surrounded by a landscape that is an actual geologic Mars analog, which offers opportunities for rigorous field studies as they would be conducted during an actual space mission.”

Figure V.5. The Mars Desert Research Station in Utah.

Furthermore, as Scott Solomon describes: “In 2011, Dutch entrepreneur Bas Lansdorp established the non-profit organization Mars One to follow through on [Robert] Zubrin’s vision. Mars One’s stated goal is to establish a human colony on the Red Planet beginning in 2027, sending colonists there on a one-way trip. Despite the certainty that they would never return to Earth, more than four thousand people applied to be among the first colonists.” Mars One has since (2016) run out of funds, but it established the fact that there is a potential pool of willing explorers. By its final stages, according to the Mars One website, “More than 10,000 potentials finished the job application process, and Mars One narrowed down the applicants, first to 1058 round 2 candidates and then to 100 round three candidates.” For now, a Mars colony should still be considered science fiction, but it seems increasingly likely that there will at least be attempts to make it happen during this century.

Evolving robots:

As long as we’re discussing science-fiction outcomes, we might as well also consider the widely discussed possibility that humans will eventually be subservient to or defeated by the advanced robots with artificial intelligence that we design, if we allow them to reproduce themselves with AI-evolved software. It is inevitable that robots and AI will soon replace humans in a variety of industrial jobs, and thereby change human employment and free time. But the technology to produce robots that are capable of reproducing and evolving is essentially at hand today. In fact, some of the ongoing R&D is driven by the prospect of carrying out tasks on, or preparations for establishing, eventual human colonies on other planets.

It would be advantageous for a future Mars colony to first send a community of robots that are capable of reproducing and evolving to adapt to the remote environment, and to build structures and infrastructure to house subsequent human colonists. Alternatively, such robots could be used to do clean-up on or decommission nuclear power plants, perform sea-floor mining, or work in other environments where humans would be at enormous risk. These are, in fact, the motivations for the Autonomous Robot Evolution (ARE) project currently being led by the University of York in the U.K., and involving partnerships with Edinburgh Napier University, Bristol Robotics Laboratory, and Vrije Universiteit Amsterdam.

Robots capable of designing and constructing other, so far primitive, robots and then iterating on the design based on the success of previous generations in accomplishing designated tasks – those have already been designed and tested at the University of Cambridge. In other words, evolutionary computer algorithms informing the robot “mother” allowed for robot evolution. However, in that case the “mother” robot was not yet capable of producing other mothers, and so was not truly capable of reproduction. The ARE project aims for full reproduction capabilities with software- and hardware-generated evolution, and with the enormous advantage over human evolution that reproduction and multiple evolutionary generations can be accomplished very quickly.

Professor Emma Hart of the ARE project describes the aim this way: “Although biological evolution takes millions of years, artificial evolution – modelling evolutionary processes inside a computer – can take place in hours, or even minutes… If artificial evolution is to design a useful robot for exoplanetary exploration, we’ll need to remove the human from the loop. In essence, evolved robot designs must manufacture, assemble and test themselves autonomously – untethered from human oversight… robots will be “born” through the use of 3D manufacturing. We use a new kind of hybrid hardware-software evolutionary architecture for design. That means that every physical robot has a digital clone. Physical robots are performance-tested in real-world environments, while their digital clones enter a software programme, where they undergo rapid simulated evolution. This hybrid system introduces a novel type of evolution: new generations can be produced from a union of the most successful traits from a virtual ‘mother’ and a physical ‘father’”.

Note that this mating of virtual mother and physical father allows an algorithmic version of gene flow, where improvements suggested by computer simulations can be combined in offspring with improvements determined by learning from performance testing on physical robots in the real outside environment. An intelligent robotic ecosystem manager will have to decide on the optimal ways to combine the virtual and physical evolutionary gains in producing the next generation of worker robots.

Hart goes on: “As well as being rendered in our simulator, ‘child’ robots produced via our hybrid evolution are also 3D-printed [and robot-assembled] and introduced into a real-world, creche-like environment. The most successful individuals within this physical training centre make their ‘genetic code’ available for reproduction and for the improvement of future generations, while less ‘fit’ robots can simply be hoisted away and [their parts] recycled into new ones as part of an ongoing evolutionary cycle… Looking forward, the long-term vision is to develop the technology sufficiently to enable the evolution of entire autonomous robotic ecosystems that live and work for long periods in challenging and dynamic environments without the need for direct human oversight.

In this radical new paradigm, robots are conceived and born, rather than designed and manufactured. Such robots will fundamentally change the concept of machines, showcasing a new breed that can change their form and behaviour over time – just like us.” See also Emma Hart’s lecture video on the research. Note that robots designed and evolved for operations on a different planet are unlikely to resemble the humanoid form favored by much science fiction.  They may more closely resemble NASA’s Perseverance Rover pictured in Fig. V.6, which is already in operation on Mars.

Figure V.6. The NASA Mars Perseverance Rover, a possible model for the starting design of future robots on other planets.

So, the prospect of a robotic “species” – one based on computer algorithms rather than DNA — is real and likely to be realized in the not-distant-future. However, it will still be true that humans select the initial criteria that guide future evolution of that species to be optimized for carrying out specified tasks in a new environment. So it is unlikely that the robotic species will “overtake” humans, although evolution – even, or perhaps especially, when it is guided by AI software that can evolve the selection criteria – can take unexpected turns.

VI. summary

We have presented numerous examples of how homo sapiens have evolved since their emergence from Africa 50,000 – 100,000 years ago, and especially since they developed agriculture and began to domesticate animals some 10,000 years ago. We reviewed the various processes by which that evolution was realized, including mutation and natural selection, sexual selection, gene drift and gene flow between different populations. Natural and sexual selection both work to increase a species’ ascent up a local peak in a multi-dimensional fitness landscape, where fitness is determined by a species’ success in producing descendants. The many peaks and valleys in the fitness landscape make it highly unlikely for natural evolution to produce radical phenotypic changes, which would require a species to pass through many generations of decreased fitness. But the fitness landscape shifts as conditions of life and environment change.

Major drivers of past homo sapiens evolution were adaptation to different Earth environments, to changes in diet, to exposure to diseases, to the long child-rearing times for humans, and to disasters such as the Black Death of the Middle Ages, which killed roughly half of the European population.

With the human population as large as it is today, and given human DNA mutation probabilities, essentially all alleles that can influence future evolution of humans on Earth are already present in the global population at some frequency. Future evolution of humans will be determined by changes in allele frequencies needed to ascend toward a local peak in the now altered fitness landscape.

The drivers of change in the fitness landscape can be deduced from the vast differences in the conditions of modern and near-future vs. ancient human life and civilization. The drivers we have considered are: demographic changes, prominently including currently dwindling human fertility; changes in the way humans select partners for mating; changes in Earth environment and ecology and human diet, many of them resulting from human technology; changes in disease exposure; and the impacts of possible modern disasters, some of them posed by humans themselves. For each of these drivers, we have offered several educated guesses regarding how they may influence future human evolution.

The biggest uncertainties surrounding the future of homo sapiens are introduced by modern human technologies that open the possibility for humans themselves to select their future evolution. We have considered five of these technological developments: CRISPR gene editing within human germline cells; in vitro fertilization and genetic preimplantation diagnosis; medical implants and conceivable connections of human brains to AI-enhanced computer networks and cloud computers; space travel and the possibility of human colonies on Mars; and the realization of robots that can reproduce and evolve on their own, without humans in the loop. These technologies are developing, but we can only speculate on how humans will choose to use them to intervene in human evolution.

The human impact on Earth’s environment and ecology, the human propensity for war with modern weapons, and the open season on the use of cutting-edge technologies to alter human evolution make the oft-quoted blessing/curse, “May you live in interesting times,” more relevant today than ever before in human development.


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