The Diversity of Humankind
August 19, 2009
We’ve all heard the statistic that humans are genetically 99.9% the same. Yet we marvel at how diverse we look and act. A new study confirms that there is little genomic variation between most of us [“Among Many Peoples, Little Genomic Variety,” by David Brown, Washington Post, 22 June 2009].
“There is a simplicity and all-inclusiveness to the number three — the triangle, the Holy Trinity, three peas in a pod. So it’s perhaps not surprising that the Family of Man is divided that way, too. All of Earth’s people, according to a new analysis of the genomes of 53 populations, fall into just three genetic groups. They are the products of the first and most important journey our species made — the walk out of Africa about 70,000 years ago by a small fraction of ancestral Homo sapiens. One group is the African. It contains the descendants of the original humans who emerged in East Africa about 200,000 years ago. The second is the Eurasian, encompassing the natives of Europe, the Middle East and Southwest Asia (east to about Pakistan). The third is the East Asian, the inhabitants of Asia, Japan and Southeast Asia, and — thanks to the Bering Land Bridge and island-hopping in the South Pacific — of the Americas and Oceania as well. The existence of this ancient divergence has long been known.”
With over 6 billion of us on the planet, one would surmise that we have survived because of our diversity not in spite of it. In fact, that is exactly what the study concludes.
“People adapted to what they encountered the way all living organisms do: through natural selection. A small fraction of the mutations constantly creeping into our genes happened by chance to prove beneficial in the new circumstances outside the African homeland. Those included differences in climate, altitude, latitude, food availability, parasites, infectious diseases and lots of other things. A person who carried, by chance, a helpful mutation was more likely to survive and procreate than someone without it. The person’s offspring would then probably be endowed with the same beneficial mutation. Over thousands of generations, the new variant (what geneticists call the “derived allele”) could go from being rare to being common as its carriers fared better than their brethren and contributed more descendants to the population.”
The most obvious changes were to skin tone. I recall reading an earlier study that correlated varied skin tones with latitude. The reason skin tones changed was that lighter skin tones were needed in northern latitudes to absorb sufficient vitamin D from limited sunlight to remain healthy. Near the equator, where there is lots of sun available year around, vitamin D deficiency is not a problem but exposure to UV rays is. Hence, people with darker skin tones are found near the equator. Brown mentions this particular variation later in his article. So even though we are mostly alike, the differences in our details make a big difference — even if those differences are surprisingly small.
“Population geneticists expected to find dramatic differences as they got a look at the full genomes — about 25,000 genes — of people of widely varying ethnic and geographic backgrounds. Specifically, they expected to find that many ethnic groups would have derived alleles that their members shared but that were uncommon or nonexistent in other groups. Each regional, ethnic group or latitude was thought to have a genomic ‘signature’ — the record of its recent evolution through natural selection. But as analyses of genomes from dozens of distinct populations have rolled in — French, Bantu, Palestinian, Yakut, Japanese — that’s not what scientists have found. Dramatic genome variation among populations turns out to be extremely rare. Instead, it is ‘random genetic drift’ that appears to be more important in sculpting our genes. Drift describes the chance loss of genetic variation that occurred not only in the out-of-Africa migration, but through all of human history as famine, climate change or war caused populations to crash and then recover. Despite those calamities, it appears that all contemporary populations ended up largely the same, or only crudely distinguishable from one another, on the genome level.”
In addition to differences in skin tones mentioned above, Brown talks about other genomic signatures that have proved valuable.
“Among West Africans, a chance mutation in the blood protein hemoglobin turned out to partially protect against malaria. It rapidly became common in places where malaria was a huge threat to survival. Similarly, a mutation allowing adults to digest milk became valuable when Middle Easterners and Europeans domesticated cattle. About 90 percent of Scandinavians now carry it.”
Brown reports that researchers were surprised that such clear ethnic advantages are the exception rather than the rule. The reason they were surprised is because genetic studies using fruit flies result in widespread genetic mutations become pervasive in just a few generations when the flies are exposed to extreme conditions.
“Not so for people, it appears. … In human beings, natural selection appears to work most of the time on dozens of genes in small and hard-to-detect ways. In contrast to fruit flies in the lab, useful traits involving body size, immunity, metabolism and behavior do not come about because one or two genes become ascendant. The short stature of rain-forest dwellers such as the pygmies of central Africa, for example, appears not to be the product of a single derived allele for shortness carried by virtually everyone in the population. Instead, dozens of gene variants that slightly decrease height have each become slightly more common, and it is their total effect that results in the group’s dramatically shorter stature.”
Brown concludes by noting that when our first ancestors walked off the African continent they brought with them only a portion of the genetic diversity found there. As the expatriates went east and west, each group diluted that diversity even more. The logical conclusion is that there should be more genetic diversity found in Africa than anywhere else on earth — and there is [“Africans Have World’s Highest Genetic Diversity, Study Finds,” by Joel Achenbach, Washington Post, 1 May 2009].
“Africans are more genetically diverse than the inhabitants of the rest of the world combined, according to a sweeping study that carried researchers into remote regions to sample the bloodlines of more than 100 distinct populations. … Although the study’s main focus was on Africa, [Sarah Tishkoff, a University of Pennsylvania geneticist who led the international research team,] and her colleagues studied DNA markers from around the planet, identifying 14 ‘ancestral clusters’ for all of humanity. Nine of those clusters are in Africa.”
Alison Brooks, a professor of anthropology and international affairs at George Washington University, concluded, “The study shows that single sources of data, whether from archaeology, oral history, genetics or linguistic similarity, are not sufficient to understand the complex history of an African region — one can be transmitted without the others, and each has a different story to tell about the past.” The same can be said of any ethnic group. No single source of data ever tells the full story. As I travel around the world on business, I continue to be impressed with the cultures of many lands. The mystery and majesty of humankind are reflected in the fact that we have been able to survive (and often thrive) in every kind of condition. We have created rich cultures that have satisfied our most basic needs and we have created art, music, religions, literature, and science that have satisfied even deeper longings to understand the world around us. The challenge for everyone in the development sector is to help connect diverse populations together in a growing and vibrant economy while managing to preserve the uniqueness that makes us human. When I discuss Development-in-a-Box™, I stress the importance of economic diversity; but we shouldn’t forget the importance of cultural diversity as well.