2.17 Wrapping Up: Are Humans Evolving?

At the start of this chapter, we posed three questions about human evolution:

• How did adaptations to high altitude evolve?
• Are the adaptations the same or different across the three populations? Do they involve the same or different gene variants?
• Are humans still evolving?

How did adaptations to high altitude evolve?

Humans, as living inhabitants of the planet, are subject to the same natural laws as all other living organisms. Evolution is no exception: the diversity of humanity today is the result of millions of years of biological evolution—mutation, selection, migration, and drift. Let’s explore the role of each of these mechanisms in adaptation to high altitude.

Mutation | Biologists like Emilia Huerta-Sánchez have sequenced the genomes (all of the DNA) of many individuals from the Tibetan Plateau, the Andes Mountains, and the Ethiopian Highlands. They have found mutations in over 55 genes across all three populations ^[1]. These mutations arose randomly through errors in DNA replication, and then the frequencies of these mutations changed through the other three mechanisms of evolution–particularly through natural selection. Other random mutations that caused lower fitness in the face of high altitude were lost from these populations over time.

Natural selection | Variants of genes that caused adaptations to high altitude, like lower blood pressure at high altitudes and lower infant mortality, have increased dramatically in frequency over many generations. Biologists have compared the frequency of gene variants in populations that live at high altitude to closely related populations that live at lower altitude. For example, Yi Peng and colleagues compared the frequencies of gene variants of Tibetan people to Han Chinese people who live in low-lying regions near Tibet. There are at least 16 genes in which nearly all Tibetan people have a different variant than Han Chinese people ^[2]. Over many generations, frequencies of those gene variants changed from being extremely rare to nearly 100% in Tibetans.

Migration | There is evidence of migration and subsequent gene flow between populations in all three high-altitude locations. In the Ethiopian Highlands, Oromo people migrated around 500 years ago to areas where Amhara people had been living for 70,000 years. Many Oromo people now have gene variants found in Amhara people–evidence of those genes “flowing” from one group of people to another through migration and interbreeding [1]. Another amazing example is in Tibetan people. Around 48,000 years ago, a group of Homo sapiens in Asia interbred with a group of Denisovans. Denisovans are a now-extinct group of humans that is considered by some biologists to be a subspecies of Homo sapiens. Those Denisovans had a unique version of a gene called EPAS1. That gene was passed from Denisovans to the human population through interbreeding. Many generations later, that variant of EPAS1 became more common as the group of humans migrated to high altitudes ^[3]. It’s fascinating that the gene variant happened to be present long before it was advantageous.

Drift | As discussed in Section 2.12, random changes in gene variant frequencies are especially influential in small populations. The current population size of Tibetan people in the Tibetan Plateau is around 6.7 million. In the past, the population was much smaller. A group of humans migrated into the Tibetan Plateau when conditions were less treacherous–temperatures were warmer and water was available as freshwater in lakes instead of being inaccessible in glaciers. There were two waves of migration into the Plateau–one 30,000 years ago and a second around 10,000 years ago ^[4]. Both migration events likely caused founder effects, where gene variant frequencies changed because only a small number of people migrated to the Tibetan Plateau.

The interplay of all four mechanisms of evolution created the populations of humans present today.

Are the adaptations the same or different across the three populations? Do they involve the same or different gene variants?

In Section 2.5, we discussed convergent evolution. In convergent evolution, distantly-related organisms happen to evolve the same traits, not because the species are related but because they experience the same selection pressure. Adaptation to high altitudes is a fascinating example of convergent evolution. All three populations have evolved high fitness at high altitudes. Interestingly, the exact ways the populations have become adapted differs. Let’s look at each population’s adaptations and the genetic changes involved.

Differences and similarities at the trait level:

Tibetans: Tibetan people have lived at high altitudes for the longest period of time and have many different adaptations. They have higher breathing rates, wider blood vessels, and do not experience the spike in hemoglobin levels that most people experience at high altitudes. (High hemoglobin concentrations can be dangerous because it causes blood to become thicker, increasing the risk of heart attack and stroke.) [1]

Andeans: People in the Andes Mountains have different adaptations than Tibetans. They have higher hemoglobin concentrations compared to nearby sea-level populations–which may not be adaptive at high altitudes, and could be explained by Andeans having lived at higher altitudes for fewer generations. However, Andeans do exhibit higher blood flow to the uterus during pregnancy, and their red blood cells are larger. [1]

Ethiopians: Unfortunately, less is known about adaptations in the Ethiopian Highlands because research into this population has only recently begun. Early findings show that the specific adaptations in Tibetans and Andeans are not present in Ethiopians, so there must be other unique adaptations. One unique adaptation is greater oxygen delivery to the brain. ^[5]

Differences and similarities at the genetic level:

Evolutionary biologists and geneticists have identified many gene variants involved in adaptation to high altitudes. Examine the diagram in Figure 2.24. For each gene shown, people at high altitudes have a different variant compared to people at low altitudes. That is a signal that the gene is involved in adaptation to high altitudes. The placement of the genes shows whether that gene has undergone natural selection in one, two, or all three populations. For example, the gene EDNRA/B shows signs of natural selection in all three regions. This gene may increase blood flow in the uterus ^[6]. The majority of gene variants are unique to only one population.

Genetic analysis has revealed that while all three groups have evolved adaptations to high altitude, the specific adaptations and genes involved are different. This is consistent with convergent evolution.

Are humans still evolving?

To answer this question, let’s think about the requirements for natural selection. Is there genetic variation within humans? Absolutely. While humans are quite similar genetically, there are still many different gene variants across different individuals. Is this variation heritable? Yes, we pass on our gene variants to our offspring. Do humans differ in their reproductive success, because of these gene variants? Yes, sometimes.

While modern medicine, agriculture, and technology have reduced selection pressures to some extent, there are many signs that humans are still evolving. Evolutionary biologists track the changes in gene variant frequencies over time and across populations, and have found that some gene variants have become very common quite rapidly and are continuing to change. Gene variants that lead to survival at high altitudes are a great example. The EPAS1 gene has massively increased in frequency in Tibetans within only 3,000 years. In addition, gene variants that affect the ability to digest lactose as an adult have increased in frequency, particularly in European populations. Some of the gene variants that are involved in protection against UV radiation and the synthesis of vitamin D are changing in frequency. Gene variants that cause resistance to specific infectious diseases like Lassa fever also appear to be increasing in certain groups.

Evolution is also still occurring through mutation, migration, and drift. On average, a human has around 50 to 200 new mutations caused by errors in DNA replication while their parents produced eggs and sperm ^[7]. (Almost all of these mutations have no effect, but a handful might cause changes in traits.) Humans are migrating across the globe and interbreeding with people from other geographic origins at even higher rates than in the past. Drift plays an especially large role when small groups of humans move to a new location, like when a small group of Amish people moved from Europe to Pennsylvania.

Across the globe, our environment is changing. New selection pressures like higher temperatures, more frequent natural disasters like hurricanes, and widespread infectious diseases like COVID-19 may lead to the spread of new gene variants. The combination of mutation, natural selection, migration and drift will continue to cause evolutionary changes in humans over the coming generations.

Human similarity

Our exploration of adaptation to high altitudes may give the impression that human populations are completely separate and very different genetically. However, humans are actually very similar. Homo sapiens is a young species, evolutionarily. Two randomly chosen humans share 99.9% of their DNA, meaning that for every sequence of 1000 A’s, G’s, T’s, and C’s, only 1 nucleotide (letter) will be different. Humans have also migrated widely and interbred with different populations, which tends to decrease genetic diversity. Many species have much greater genetic variation than humans. For example, the two humans shown in Figure 2.25 are much more similar genetically than the two Gentoo penguins, which to our eyes look pretty identical. Modern humans typically differ in a few genes that are (or previously were) important for survival in the environments where their ancestors evolved (like at high altitudes), or just by random chance. Otherwise, we’re remarkably similar, at a genetic level.

Evolution of sex

Here we’ve explored adaptation to a feature of the physical environment. The same mechanisms apply to traits related to sex, from the brilliant feathers of male birds of paradise to the evolution of the human female orgasm. This is what we will explore in the remainder of the textbook.

As we conclude this chapter and prepare for in-class discussion, be sure to return to the chapter’s goals and objectives.