Certainly, no one was surprised that Usain Bolt swept three gold medals in Rio, and reaffirmed his hold on the title of world’s fastest human. Nor the fact that all of the 100-meter finalists trace their ancestry to Africa—specifically West or South Africa. There were no European descended whites, Asians or East Africans, even from running-rich Kenya. It was a familiar phenomenon to anyone who even vaguely follows elite running.

Distance competitions? Athletes of East and North African ancestry dominate–most especially Kenyans from the Nandi Hills in the Rift Valley–while athletes of West African ancestry are no where to be seen.

Why are East Africans, including Kenyans, so dominant in distance running, but among the world’s slowest elite sprinters? And why are West African descended runners, including the sprinters from the tiny juggernaut of Jamaica, missing-in-action from elite distance running?

When it comes to opportunity, running is the most egalitarian of global sports, a natural laboratory. Unlike the props and costumes required for, say, professional football or ice hockey, or the intense coaching demanded of gymnastics, swimming or golf, anyone can just lace up and go for a run. Ethiopia’s Abebe Bikila proved this quite memorably in the 1960 Rome Olympics, when—shoeless, coachless and inexperienced—he won the marathon. Raw talent and the determination of a fierce competitor were on display. What shapes that talent?

The 100-meter race is one of the premier spectacles in sports. The gold sweep in the sprints by West African descended athletes that that we witnessed in Rio is commonplace at elite events now that many of the financial and cultural barriers to international competition have fallen. It has proven to be a dismal event for Asians, Caucasians or East Africans. There are exactly zero Asians, whites or East Africans among the top 200 all time 100 meter times.

Those who do not understand the power of genes might argue that the medal podium for runners should reflect a rainbow of diversity, as no country or region should have a lock on desire or opportunity. But just the opposite has happened in track and field: elite running has become almost entirely segregated by ancestry. African descended athletes dominate. And field events are dominated by athletes of Eurasian ancestry.

[These patterns hold true in both men’s and women’s events, though cultural taboos on female participation in sports and the history of their use of performance enhancing drugs makes the parallels inexact.]

The trends in running are eye opening: Among men, athletes of African ancestry hold every major running record, from the 100m to the marathon. Of the past eight Olympics men’s 100m races, 63 finalists have been of West African descent and one from South Africa, Akani Simbine, who finished in a pokey 9.94 in Rio this year. Only four non-African runners, France’s Christophe Lemaire, who is white, a Chinese runner, a Pole and Australia’s Irish-aboriginal Patrick Johnson, have run a sub 10-second 100 or cracked the top 500 100-meter times.

Population genetics and sports

What makes the running performance statistics so intriguing is how sharp the divide is in terms of which regions in Africa turn out elite athletes running in the sprints versus distance races. Although there is a great deal of genetic and cultural overlap, Africa represents a large number of different ‘populations’, as a geneticist would say, whose evolutionary history has been shaped by cultural tribal differences and geographic barriers such as the mountain ranges in East Africa that bifurcate the continent. [Let’s avoid using the word ‘race’, as scientists focus more on population groups, defined by ancestry.]

As journalist Steve Sailer noted in an article on Unz Review, the ranking of the fastest Jamaican men all time by distance shows the stark genetic/distance divide that separates the skill set of West and East African athletes in sprinting vs. long distance.

100m: #1 Usain Bolt

200m: #1 Usain Bolt

400m: #41 Rusheen McDonald

800m: #2887 Seymour Newman

1500m: No Jamaicans in top 7836 times ever

“Notice a pattern?” Sailer wrote. “It’s not like the Jamaican track teams don’t want to win gold medals at longer distances, they’re just better at the sprints.”

These differences were once again on display in Rio. Endurance races were dominated by North and East Africans—even on the female side despite huge restrictions still placed on women athletes in Africa. East and North African women dominated in almost all of the distance events. The near sweep was capped off by Kenya’s Jemima Sumgong victory over Eunice Kirwa of Bahrain and Mare Dibaba of Ethiopia to become that country’s first Olympic female marathon gold medalist and Kenya’s Eliud Kipchoge’s won the men’s marathon.

Kenya has tried to replicate its success in distance running by pouring millions of dollars and hiring top coaches in establishing a sprint development academy to no avail. The top Kenyan 100-meter time is 10.23-pathetic for an elite runner and not in the top 5000 of all time. US high schoolers do better than that–the record is 10.0.

Culture vs. genes?

The most frequently heard reason for this pattern by skeptics, including many scientists, is that Rift Valley Kenyans and Jamaican athletes just work harder at running. They had to run back and forth to school. It’s a way out of often deplorable poverty. There are in fact some cultural factors in play, just as there are cultural reasons for why Dominicans excel in baseball. In the case of Jamaica, the British introduced organized and informal athletics, and interscholastic competition, to the island and other colonies in the late 19th century, and running was the central sport–sprints and long distance. Jamaicans began excelling at what they were natural good at–the shorter distances. But culture alone can only take one so far; Jamaicans are terrible at distance running.

Culture clearly plays a huge role in most parts, though more so historically when fewer people had access to sports. Consider the Jewish over-representation in semi-pro basketball in the United States in the 1920s and 30s, when the famed Philadelphia South Philadelphia Hebrew Association (SPHAS) team, nicknamed the Hebrews, won 13 semi-pro titles over 22 years. Basketball was one of their few outlets, the story goes, to escape the trap of limited opportunity. Some commentators speculated genetics was at play.

“The reason, I suspect, that basketball appeals to the Hebrew, with his Oriental background,” wrote Paul Gallico, sports editor of the New York Daily News and one of the premier sportswriters of the 1930s, “is that the game places a premium on an alert, scheming mind, flashy trickiness, artful dodging and general smart-aleckness.”

But in this case, Jewish domination was overwhelmingly cultural. Following World War II, the new generation of Jews began moving on to other pursuits—not to mention the suburbs. By the late 1940s, dominion over the city basketball courts had passed to the fastest-growing group of urban dwellers, mostly blacks who had migrated north from dying Southern plantations. In a few short years, the SPHAs were reduced to a supporting role as the slow-footed white boys in one-night stands against the Harlem Globetrotters, who had evolved into a vaudeville act after the NBA began stealing its best players. Before long, the stereotype of the “scheming” and “trickiness” of Jews was replaced by that of the “natural athleticism” of “Negroes”.

We know now, of course, that Jews are not innately suited (or ill suited) for basketball. The SPHAs and the heyday of Jewish basketball resulted from a combination of environmental factors, hard work, and a dearth of opportunities elsewhere. For much of the past century, socioeconomic barriers determined who would compete in what sports—and therefore who could succeed. However, the situation in running is different. West African descended runners have dominated sprinting for most of the century, and that domination has intensified in recent years. As the barriers to African participation in global sports has fallen, and the playing field has leveled, East and North Africans have come to dominate distance running.

Blacks still find it difficult to break into many specialized or country-club sports, like bobsledding or golf, in which equipment costs are steep. But sports that excluded or segregated blacks until the 1960s—baseball, basketball, football and track-and-field—are now open to all races. Where cultural channeling once shaped the rosters of in sports, now the cream has fewer structural obstacles to rise to the top.

Still journalists and even many scientists attempt to persuade us that environment and culture are what matters most at the elite levels of competitive sports and genes play a limited or no role. The latest example: Harvard sociologist Orlando Patterson’s recent New York Times opinion piece on Jamaican running, which correctly points to the cultural factors that have contributed to sprinting success.

How do Jamaicans do it? It’s not because of genetics, as some claim. A vast majority of Jamaicans’ ancestors are from West Africa, which has relatively few outstanding sprinters.

According to this narrative, there’s a tradition of running in Africa, and among African descendants worldwide, that young athletes emulate; they’ve been running to school since kindergarten; they train harder for a chance at the golden ring that athletic success offers; athletes from other parts of the world have developed a toxic inferiority complex to Kenyan distance runners and West African descended sprinters and switch to other sports; blah, blah, blah.

As I noted in my book Taboo: Why Black Athletes Dominate Sports and Why We Are Afraid to Talk About It (Jon Entine, 2000, Public Affairs), West Africa, which has no strong cultural or state support for running, has dozens of outstanding sprinters relative to any group in the world other than sprinters in the West African diaspora. Each of these countries alone–Congo, Botswana, Ivory Coast, Nigeria, Ivory Coast, Liberia, Senegal–has more elite sprinters than all of Asia combined. Caribbean countries, North America and Europe–home to many people of West African descent—have faired particularly well in international sprinting. (As Sailer has noted, “West Africa has nature on its side in sprinting, but the West African diaspora has nature and nurture”).

Yes, it can be precarious to engage this topic…hence why I titled my book Taboo. And focusing on differences at the far end of the distribution curve, at the very pinnacle of human capacities, can create a misleading impression of group biases; human variability can get confused with average differences. But as Malcolm Gladwell, who is of European and African-descended parentage, has written, “There is a point at which it becomes foolish to banish speculation on the topic. Clearly, something is going on.”

The Rift Valley Kenyan story

Kenya, with but 44 million people, is home to athletes holding almost one half of the top times in distance races, and most of them trace their ancestry to a few tribes in and around the Nandi Hills. The Kalenjin tribe alone has less than 0.1% of the world’s population, yet members of this tribe have together won more than 60 Olympic medals in middle- and long-distance events. It’s often claimed that Kenyans dominate distance races because they “naturally trained” as children—by running back and forth to school, for example.

“That’s just silly,” Kenyan-born Wilson Kipketer told me. Kipketer the second fastest 800-meter runner of all time and holder of six of the top 20 all-time fastest 800m times. “I lived right next door to school,” he laughed, dismissing cookie-cutter explanations. “I walked, nice and slow.”

No study has been done that suggests that elite runners as a group were more likely to run great distances to school. That’s a particularly absurd claim in Kenya today where even small rural villages, particularly in the Rift Valley, have inexpensive public bus service. Yet, the blank slate theory is advanced all the time, even by some scientist who should know better than to rely on anecdotes, as if it’s beyond question

Like most young Kenyans, while growing up he hoped that he might catch the eye of a coach who combed the countryside to find the next generation of budding stars. He had dreams of being cheered as he entered the National Stadium in Nairobi. But his childhood fantasy was to be welcomed as a soccer player.

The national sport, the hero worship, the adoring fans, the social incentives that supposedly channel a kid into sports—that all speaks to Kenya’s enduring love affair with soccer, not running. Soccer was and is the national sports obsession of Kenyans. But Kipketer, like many Kenyans and East Africans, are rarely found among the elite of that sport; despite their zeal for soccer and all the social incentives to push them into playing high level soccer, most East Africans simply don’t appear to have the genetic package to make them the world-class quick burst runners that thrive in that sport. Social and cultural conditioning alone cannot turn athletic coal into diamonds.

But Kenyans from the Rift Valley mountains are more like to be diamonds at longer distance running. Some suggest that’s due to the outsized natural lung capacity and a preponderance of slow-twitch muscles. That’s a perfect biomechanical package for long-distance running–for example, 8 of the top 10 all time marathoners are Kenyan; the other two are Ethiopian. Out of the top 125 marathon times, 124 were run by East and North Africans. Sprinting is the mirror opposite, though: Kenya’s fastest 100m time, 10.26, is almost three-quarters of a second slower than Bolt’s world record. More than 5,000 times ranked higher than Kenya’s best. There are no elite East African sprinters.

Bengt Saltin, who recently passed away as the director of the Copenhagen Muscle Research Institute, and is considered one of the world’s expert in this area, has said his research estimated that an athlete’s “environment” accounts for no more than 25 percent of athletic ability. The rest comes down to the roll of the genetic dice, he said.

Evolution has shaped significant physical and physiological differences. Genetics circumscribes possibility; individual success is about coaching, training and dedication. Individuals win sporting events, not ancestral population groups. In other words, running success is both “in the genes” and linked to training and coaching, which can lead to a genetic and cultural feedback loop. That’s what’s happened in Jamaica in sprinting and Kenya in distance running (and the Dominican Republic in baseball).

Environmental factors undoubtedly influence how genes are expressed. Mathematics dictates that in a region of tens of millions of inhabitants, small differences that are not always apparent across much of the population can show up in an elite few. Robert Malina, a retired Michigan State University anthropologist and former editor of the Journal of Human Genetics:

Differences among athletes of elite caliber are so small that if you have a physique or the ability to fire muscle fibers more efficiently that might be genetically based … it might be very, very significant. The fraction of a second is the difference between the gold medal and fourth place.

Empirical evidence makes hash of the myth that culture and hard work alone makes the athlete. It’s nature and nurture in a bio-cultural loop.

Body types, ancestry and sports

Although people in every population come in all shapes and sizes, body types and physiological characteristics follow a distribution curve as a result of evolutionary adaptations by our ancestors to extremely varied environmental challenges. This adaptation continues to happen–the average height in some countries has changed by up to 5 centimeters over just the last 100 years. Every population group has some elite athletes in every sport. But the tails of the distribution curve and their thickness differs from population to population. Elite sports showcase these differences.

Here are the facts. Dozens of studies have confirmed that genetically linked, highly heritable characteristics such as skeletal structure, the distribution of muscle fiber types (for example, sprinters have more natural fast-twitch fibers, while distance runners are naturally endowed with more of the slow-twitch variety), reflex capabilities, metabolic efficiency, and lung capacity are not evenly distributed among human populations.

Comparing West Africans and East Africans, West Africans tend to have more efficient but smaller lung capacity. Shaped by many centuries of evolution in Africa, they generally have bigger, more developed overall musculature; narrower hips, lighter calves; higher levels of plasma testosterone; faster patellar tendon reflex in the knee; and a higher percentage of fast-twitch muscles and more anaerobic enzymes, which can translate into more explosive energy.

It’s been suggested that East African athletes from hill tribes, such as the Nandi, assimilate oxygen in regions of low oxygen pressure, and have increased levels of haemoglobin. It’s controversial stuff, although not to most sports scientists.

Although there is huge diversity in body types in every population, West Africans tend to be more mesomorphic–more heavily muscular. East Africans are more ectomorphic–leaner, particularly in the upper body. While West and some South Africans dominate in the sprints, they are terrible at distance races. East and North Africans are equally inept at sprinting. It’s certainly not because those athletes don’t try in those particular events—they just don’t have the body type or physiology to compete at the highest levels in these events.

Scientists dismiss the notion, voiced controversially by CBS sports analyst Jimmy the Greek Snyder in 1988, that African slaves brought to the Americas were “bred during slavery,” explaining in part their success in certain sports. However, the differences we are talking about are hard wired over thousands of years, and there is no real evidence that planned “slave breeding” actually occurred over successive generations on the level that could have shaped body types.

The evolutionary advantages that some African-descended populations have in running can be drags in some sports. People of African descent in general have longer arms and legs than other populations, an adaption to dissipate body heat in hot climates. They also, on average have heavier skeletons and less body fat—a possible genetic hindrances when it comes to competitive swimming.

On the other hand, populations that have evolved in colder climates are characterized by shorter extremities and longer torsos—evolutionary adaptations to harsh climes encountered by Homo sapiens who migrated to Eastern Europe and Northeast Asia 40,000 years ago. China, for example, excels in many Olympics sports, for a variety of reasons. One of those reasons could be that they are more flexible on average—it’s speculated that this may give them a potential advantage in diving, gymnastics (hence the term “Chinese splits”) and figure skating, although environmental and cultural factors could swamp genetic differences. Note, however, that China is a huge country subdivided by many different ethnicities and populations– so we must be careful about making definitive conclusions.

At the Olympics, with the exception of some competitive Koreans, the winners of the top strength events–weightlifting and most field events, such as the shot put, javelin and hammer–come from a band of Eurasian countries: across Eastern Europe and through Russia, Ukraine, China, Kazakhstan, Iran and Turkey. They have relatively muscular bodies with comparatively short limbs and thick torsos. No prototypical sprinter or marathoner here. Despite the image of the sculpted African body, no African nation has won an Olympic lifting gold medal.

Speed genes?

Michael Johnson, the previous 400-meter world-record holder, the only male athlete in history to win both the 200-meter dash and 400-meter dash events at the same Olympic (1996), has suggested that black sprinters benefit from the outsized presence of ACTN3. His 400-meter world record was broken by South African Wayde van Niekerk, who became the first ever athlete to have run the 100m under 10 seconds, the 200m under 20 seconds, and 400m under 44 seconds.

It’s a controversial idea among geneticists. ACTN3, dubbed the speed gene, does not seem to have any harmful health effects but its activity has correlated with running ability in some studies, though the research is somewhat contradictory. The ACTN3 is more common in those of West African descent than in Europeans, according to a study published in the American Journal of Human Genetics. Another gene, known as ACE, has been linked to endurance running. A 2013 meta-study of more than 450 articles on both the ACE and ACTN3 genes published in PLoS ONE concluded that the scientific literature correlates the effect of these genes with sports performance.

Our results provide more solid evidence for the associations between ACE II genotype and endurance events and between ACTN3 R allele and power events. The findings suggest that the genetic profiles might influence human physical performance.

A team of researchers at the Exercise Science and Sports Medicine, Department of Human Biology, at the University of Cape Town in South Africa reviewed the literature on these two genes in a 2013 study. They challenged critics who claim there is no genetic basis for the obvious patterns in elite running competitions and other some other sports:

We argue that research has in fact established a role for genes in performance, and that the frequency, rather than the prevalence, of favourable genetic variants within certain populations may account for the performance dominance in these populations.

Are these genes running’s “smoking gun” genes? No. The complexity of sports performance cannot be reduced to or explained by one or several genes. There are literally dozens of studies on this topic, with many contradictory findings. Some researchers sharply question a genetic explanation. Two respected sports scientists concluded in a 2012 journal article that “no genetic traits … or hematological advantages … have been identified that can conclusively explain the unique success of the East African runners.”

But their conclusion is viewed skeptically by top sports scientists and biological anthropologists who have researched the subject. In one of many responses, a team of scientists from various Brazilian universities documented the rise to dominance of East Africans, Rift Valley Kenyans in particular, in a 2014 review of the research, and hypothesized a number of genetic aspects that could be contributing to this phenomenon:

Many factors in the interesting interaction between genotype and phenotype (eg, high cardiorespiratory fitness, higher hemoglobin concentration, good metabolic efficiency, muscle fiber composition, enzyme profile, diet, altitude training, and psychological aspects) have been proposed in the attempt to explain the extraordinary success of these runners. Increasing evidence shows that genetics may be a determining factor in physical and athletic performance. But, could this also be true for African long-distance runners? Based on this question, this brief review proposed the role of genetic factors (mitochondrial deoxyribonucleic acid, the Y chromosome, and the angiotensin-converting enzyme and the alpha-actinin-3 genes) in the amazing athletic performance observed in African runners, especially the Kenyans and Ethiopians, despite their environmental constraints.

Sports ability is shaped by coordinated levels of expression of thousands of genes; genetic and non-genetic biological factors determine which genes switch on and when, and how strongly. We know that genes matter; what we don’t know, and many not know for years if ever, is which specific genes or gene combinations matter most and how much genes rather than training and culture drive outcomes. It is well established in the scientific community that both genetic and training factors determine sport performance but the relative ratio of these factors are different for each individual.

“Evolution has shaped body types and in part athletic possibilities,” Joseph Graves, Jr. told me. Graves, an African American, is an Associate Dean for Research and Professor at the Joint School of Nanoscience and Nanoengineering at the University of North Carolina, Greensboro, and an expert in the field of human differences. “Don’t expect an [Inuit] to show up on an NBA court or a Watusi to win the world weightlifting championship. Differences don’t necessarily correlate with skin color, but rather with geography and climate. Endurance runners are more likely to come from East Africa and sprinters from West Africa. That’s a fact. Genes play a major role in this.”

Why do we readily accept that evolution has turned out Jews with a genetic predisposition to Tay-Sachs, Southeast Asians with a higher proclivity for beta-thalassemia, and blacks who are susceptible to colorectal cancer and sickle-cell disease, yet find it inappropriate to suggest that Usain Bolt can thank his West African ancestry for a critical part of his success? Just last week, in a study published in Nature in which researchers analyzed the genetic codes of more than 60,000 people from five continents, concluded, “Finns Found to be Genetically Unique, Genes Vary Significantly From Europeans” (headline in Nature World News). Are these findings genomic trivia or consequential? “We demonstrate that these data can be used for the efficient filtering of candidate disease-causing variants, and for the discovery of human ‘knockout’ variants in protein-coding genes,” the researchers concluded.

Linking population circumscribed body type to elite sports performance is no different from acknowledging that Tibetan people are able to survive better at higher altitudes through higher no2 blood levels and a higher density of arteries in their limbs to transport what little oxygen is available. Genetics is color blind, and acknowledging human differences, is not racist.

Human “populations”—cohorts of people with shared genes—exist. But the degree to which differences based on ancestry and evolution remain a controversial subject. The difficulty is sorting out how much of a trait is genetically inbred, how much may be shaped by environmental factors, and what is just plain supposition, sometimes sprinkled with biases.

As UCLA professor Jared Diamond has noted, “Even today, few scientists dare to study “racial” origins, lest they be branded racists just for being interested in the subject.” But we have no choice but to face this third rail of genetics. Over the past decade, human genome research has moved from a study of human similarities to a focus on patterned based differences. Such research offers clues to solving the mystery of diseases, the Holy Grail of genetics.

Jon Entine, executive director of the Genetic Literacy Project, is author of the best selling book Taboo: Why Black Athletes Dominate Sports and Why We Are Afraid to Talk About It. Follow @JonEntine on Twitter

Note: This article was updated on August 14, 2016 to include Olympic gold medal results in the men’s 100 meters and the women’s marathon and again on August 21 and 22.