Where are we headed - and the evolution still continues (after so many years) / John Hawkes

For 30,000 years, the human race has changed at a surprising speed, and the end is not in sight

Humans are tough creatures. No other species on Earth can direct its own destiny like we do. We neutralized countless threats that in the past killed millions of us: we learned to protect ourselves from the forces of nature and beasts of prey; We have developed drugs and treatments for many deadly diseases; We have turned the small farms of our farming ancestors into vast fields of industrial agriculture and have significantly increased the chance of having healthy children in spite of all the usual difficulties.

Many claim that technological progress, meaning our ability to defy nature and control it, exempts humans from natural selection, and that human evolution has effectively stopped. The argument is that "survival of the fittest" no longer exists if we all survive to old age. This is not a passing thought in the public mind, but a widespread claim by professional scientists such as Stephen Jones from University College London, or researchers and nature broadcasters such as David Attenborough who have declared that human evolution is over.

But it is not so. We have undergone evolutionary changes in our recent past, and will continue to undergo such changes as long as we exist. If we look at the more than seven million years that have passed since humans and chimpanzees split from their last common ancestor as one 24-hour day, then the last 30,000 years are equivalent to only six minutes. However, in this last chapter of our evolution there were many events: massive migrations of peoples into new habitats, dramatic changes in diet and a more than 1,000-fold increase in the world's population. All these new individuals added many unique mutations to the general pool. The result is a pulse of rapid natural selection. Human evolution does not stop at all, if anything, it accelerates.

Anthropological heritage

It has long been known that ancient human skeletons indicate that people developed certain traits quickly and in the recent past. About 11,000 years ago, when humans began to shift from hunting and gathering to farming and cooking, human anatomy changed. 10,000 years ago, for example, people in Europe, Asia and North Africa had teeth 10% larger, on average, than our teeth today. As our ancestors began to eat softer, cooked food that required less chewing, their jaws gradually shrank, generation after generation.
Although anthropologists have known for many decades about the existence of such features, only in the last decade has it become clear how new they really are. Studies of human genomes have shed light on the genes that have recently undergone changes in human natural selection. It turns out, for example, that descendants of farmers tend to produce more salivary amylase, an important enzyme that breaks down the starch in food. Most people alive today have several copies of the AMY1 gene, which contains the code for amylase. In contrast, modern hunter-gatherers, such as the Datoga tribe in Tanzania, have far fewer copies of this gene compared to people of agricultural descent, and one is whether they originated in Africa, Asia, or the Americas. Increased processing of starch already at its point of entry into the body probably gave ancient farmers an advantage wherever they switched to a diet that included starchy grains.

Another nutritional adaptation is one of the most studied examples of recent human evolution: the ability to digest lactose. Almost all are born with the ability to produce the enzyme lactase, which breaks down the milk sugar, lactose, and facilitates the production of energy from milk, an essential ability for breastfed babies. Most people lose this ability as adults. At least five different times in the recent evolutionary past, when people began to discover dairy products, a genetic mutation emerged that extends the period of activity of the lactase gene. Three of these mutations originated in different parts of sub-Saharan Africa, where there is a long history of cattle grazing. Another mutation among the five is widespread in the Arabian Peninsula and probably originated in ancient populations of camel and goat herders.

The fifth and most common version of the mutation for which the lactase gene is active even in adulthood is currently found in human populations from Ireland to India, and its highest frequency is in Northern Europe. The origin of the mutation is in a single person who lived 7,500 years ago (with the possibility of an error of a few thousand years). In 2011, scientists analyzed DNA from Uzzi the Iceman, a body that was naturally embalmed about 5,500 years ago in northern Italy. Uzzi did not have a mutation that allowed him to digest lactose, a hint that it was still not common in this area thousands of years after it was first created. In recent years, researchers have sequenced the DNA of skeletons of farmers who lived in Europe more than 5,000 years ago. None of them had a mutation in the lactase gene. However, in the same region today, lactase mutations are common in hundreds of millions of people, which is more than 75% of the gene pool. This is not a paradox, but a consequence of the mathematical properties of natural selection. The distribution of a new mutation under selection increases exponentially, and many generations pass before it can be detected in the population. But once it is widespread enough, it spreads at an accelerated rate and eventually quickly takes over the population.

The shallowness of the race

Perhaps one of the most amazing things about recent evolution is the amount of completely new common physical features in human anatomy. The thick, straight, black hair that characterizes most people in East Asia, for example, appeared only in the last 30,000 years, due to a mutation in a gene called EDAR, which is essential for organizing the early development of skin, hair, teeth and nails. This genetic variant came to North and South America through the first settlers, all of whom had an East Asian evolutionary past.

In fact, the entire evolutionary history of human skin, hair, and eye color is incredibly shallow. In the earliest stages of our evolution, all our ancestors had dark eyes, skin and hair. Since then, several dozen genetic changes have lightened the shade to some extent. Some of these modifications are ancient versions found in Africa, but more common in other parts of the world. But most of them are new mutations that appeared in some population: a change in a gene called TYRP1, for example, makes some Solomon Islanders blond; A mutation in HERC2 causes blue eyes; Changes in MC1R cause red hair instead of black; And a mutation in the SLC24A5 gene lightens the skin color and is currently found in more than 95% of Europeans. As in the case of lactase, ancient DNA provides clear information about the age of such mutations. Blue eyes probably appeared in people who lived more than 9,000 years ago, but the SLC24A5 mass change has not yet been detected in the DNA of ancient skeletons from that time. Skin color, hair and eyes developed at breakneck speed.

Changes in pigmentation are among the most obvious differences between races, and to some extent the easiest to study. Scientists have also studied far stranger and less obvious features of the human anatomy, such as earwax. Most people in the world today have sticky earwax. But many East Asians have dry, flake-like earwax that doesn't stick together. Anthropologists have known about this trait for 100 years, but geneticists have only recently discovered the reason for it. Dry earwax results from a relatively new mutation in a gene known as ABCC11. The mutation, which is only 30,000 to 20,000 years old, also affects the sweat glands. If you have dry earwax and hardly ever need deodorant, you most likely carry the new mutation.

A few thousand years before dry earwax first appeared in East Asians, another seemingly simple mutation began to save the lives of millions of Africans from a deadly disease. A gene known as DARC produces a starchy molecule on the surface of red blood cells, which absorbs excess immune system molecules called chemokines from the blood. 45,000 years ago, a mutation in DARC conferred exceptional resistance to one of the two most common malaria parasites in humans today, Plasmodium vivax. These parasites enter the red blood cells through the DARC molecule that this gene produces. A mutation that impairs the expression of the gene suppresses the pathogens. The absence of DARC molecules in the blood also increased the amount of inflammatory chemokines in the blood, which in turn is linked to an increase in prostate cancer in African-American men. However, in the end, the mutation was so successful that today it is found in 95% of the population south of the Sahara, and only in 5% of Europeans and Asians.

The power of randomness

We are used to thinking of evolution as a process where "good" genes replace "bad" genes, but recent human evolution is a testament to the power of randomness in evolution. Beneficial mutations do not necessarily survive: it all depends on timing and population size.

I first learned this lesson from the late anthropologist Frank Livingston. The beginning of my training coincided with the end of his long career, during which he studied the genetic basis of resistance to malaria. More than 3,000 years ago, a mutation emerged in Africa and India in the gene encoding hemoglobin, the molecule responsible for transporting oxygen in the blood. When people inherited two copies of the mutation, known as hemoglobin S, they developed sickle cell disease, a disease in which misshapen blood cells clog blood vessels. Normally, red blood cells are soft and flexible enough to pass through tiny capillaries, but the mutant blood cells are rigid and curved in a characteristic "sickle" shape. It turns out that the change in the shape of the blood cells also impairs the malaria parasite's ability to infect these cells.

Another mutation that interested Livingstone is hemoglobin E, which is common today in Southeast Asia. The mutation confers considerable resistance to malaria without the severe side effects of hemoglobin S. "Hemoglobin E seems like a much better option than hemoglobin S," I said in class one day. "Why didn't they get the E mutation in Africa?"
"Because it didn't happen there," Livingstone replied.

His answer stunned me. I assumed that natural selection was the most powerful force in evolution's armory. People have lived in Africa with deadly malaria transmitted by the plasmophorum parasite for thousands of years. It goes without saying that natural selection will weed out the less effective mutations and leave the successful ones.

But Livingstone showed us why the early presence in the population of the mutation to hemoglobin S actually made it difficult for the mutation to hemoglobin E to penetrate. Malaria makes its name in a population consisting only of carriers of normal hemoglobin, and a new mutation that gives even a slight advantage quickly becomes widespread. However, in the population already equipped with a protective mutation, such as hemoglobin S, the risk of dying decreases. Carriers of sickle cell anemia still face considerable risks, but hemoglobin E confers a smaller relative advantage in a population that already has partial resistance to malaria. It is not enough that luck gets lucky and an effective mutation emerges, it also needs to appear at the right time. Partial adaptation, even if it has negative side effects, can win, at least in the time period of the last thousands of years when humans are adapting to malaria.

Since humans first started getting malaria, dozens of different mutations have emerged that increase immunity to the disease, each mutation in a different area. Each one started as a successful random event that managed to take root in the local population even though it was very rare at first. It is likely that each of these mutations would not have survived long enough to become established, but the large population of our ancestors, who reproduced at a rapid rate, allowed many opportunities for mutations. When humans began to reproduce and spread to new areas of the world, they quickly adapted to their new homes precisely because the populations were so large.
Our evolutionary future

Human populations continue to undergo evolutionary changes even today. Unlike in the distant past, where we must infer the existence of natural selection based on its long-term effects on genes, today scientists can observe evolution in action, often by studying trends in health and fertility. Although medical technology, the level of cleanliness and vaccinations have considerably extended life expectancy, the birth rate in many populations still fluctuates.

In sub-Saharan Africa, a particular version of the FLT1 gene makes it less likely that malaria parasites will infect the placenta during pregnancy. The birth chances of women who carry this version and become pregnant during the malaria season are slightly higher than other women. We still don't understand how this gene reduces the risk of placental malaria, but the effect is definite and measurable.

Stephen Stearns of Yale University and his colleagues examined records of long-term public health studies to discover possible correlations between traits and current birth rates. During the last 60 years in the US, the average number of children of relatively short and obese women, who have low cholesterol values, was slightly lower than the number of children of women with the opposite characteristics. It is not clear why these traits are related to family size.

On the horizon are new studies in public health, such as the UK Biobank project, which will analyze the gene sequence of hundreds of thousands of people and compare it to their health status during life. Such studies are conducted because the interrelationships between genes are complicated, and we must test thousands of possibilities to understand which genetic changes underlie human health. Tracing genetic lineages of human mutations gives us enormous power to observe evolution over hundreds of generations. But such monitoring may mask the complicated interrelationships that developed in the past between the environment and between survival and fertility. We only see the genes that "won" over time, such as the activity of the lactase gene, but miss the short-term events. Human populations are about to become a long-term experiment that will attract more attention in the field of evolutionary biology than any experiment before it.
What will the future of human evolution look like? Over the past thousands of years, human evolution has followed different paths in different populations, but has also maintained a surprising uniformity. New mutations did infiltrate different human populations, but they did not push aside the old versions of the genes. Most of the "early" versions of the genes have remained with us. Meanwhile, millions of people move from country to country every year, causing gene exchange and mixing at an unprecedented rate.
With such a rate of genetic mixing, it is reasonable to expect that traits such as skin color, which is the sum of the independent effects of many genes, will become more uniform in the future. Will humans in the future look like a uniform soup instead of a colorful stew of diversity?
The answer is no. Many traits, which are expressed differently in different populations, cannot be connected. Even skin color is not that simple, as you can easily see in the mixed populations in the USA, Mexico and Brazil. Instead of an unremarkable herd of mocha-colored cloned people, we're already beginning to see a vibrant riot of variants: freckled blonds with dark skin and spectacular combinations like green eyes and olive skin. Each of our descendants will be a living mosaic of human history.

in brief
There are scientists and science writers who claim that humans are no longer subject to the influence of natural selection and that human evolution has ceased.
In fact, humans have undergone an amazingly rapid evolution in the last 30,000 years. Straight black hair, blue eyes, and the ability to digest lactose are examples of relatively new traits.
This rapid evolution was made possible for several reasons, including the transition from a hunter-gatherer society to an agricultural society, which enabled a considerable growth of the human population. The more people produce offspring in a given population, the greater the chance of new and beneficial mutations.
There is no doubt that humans will continue to evolve in the future. Although we seem to be marching towards a uniform and cosmopolitan mixing of human genes, future generations are likely to be a bewildering mosaic of our entire evolutionary past.
About the author
John Hawks is an anthropologist and expert on human evolution at the University of Wisconsin-Madison.
More on the subject
Are Human Beings Still Evolving? It Would Seem That Evolution Is Impossible Now That the Ability to Reproduce Is Essentially Universally Available. Are We Nevertheless Changing as a Species? Meredith F. Small; Ask the Experts, ScientificAmerican.com, October 21, 1999.
African Adaptation to Digesting Milk Is "Strongest Signal of Selection Ever." Nikhil Swaminathan; ScientificAmerican.com, December 11, 2006.
Did Lactose Tolerance First Evolve in Central, Rather Than Northern Europe? Lynne Peeples; ScientificAmerican.com, August 28, 2009.

The article was published with the permission of Scientific American Israel

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