Human evolution and adaptation

This reading is under revision for the Fall 2024 semester. This blue text will disappear when the reading is ready.

Learning Objectives

1. Recount the current hypothesis of the geographic origin and migration of modern humans
2. Describe which DNA sequences can be used to trace matrilineal and patrilineal descent
3. Describe how population genetic data can be used to formulate and test hypotheses about recent adaptive evolution of various human populations
4. Explain how haplotype groups or “haplogroups” can help identify probable ancestry, and identify some ethical and societal implications of DNA testing

Evolutionary geneticists have performed phylogenetic analyses of mitochondrial DNA and Y-chromosomal DNA sequences from people in various populations around the globe. Mitochondrial DNA is inherited only from the mother, and Y-chromosomes inherited only from the father. Because mitochondrial DNA or Y-chromosomes do not undergo assortment, shuffling and recombination during meiosis like autosomes, they facilitate reconstructing evolutionary lineages. Both mitochondrial DNA studies and Y-chromosome studies have yielded congruent results, consistent with the origin of modern humans in Africa, less than 200,000 years ago. An extensive survey of genetic variation within Africa revealed that Africa has greater human genetic diversity than the rest of the world combined (Tishkoff et al. 2009), another result consistent with Africa being the birthplace of modern humanity.

As modern humans migrated out of Africa into the Middle East, Europe and Asia, they interbred to a limited extent with archaic humans: Neanderthals and Denisovans. The TedTalk below by Svante Paabo explains these findings.

As human populations settled different areas around the world, they evolved to adapt to their local environment. Such adaptations included changes in skin pigmentation, retention of lactase expression as adults, and physiological adaptations to cold or to high altitudes. This NY Times article summarizes a number of such findings as of 2010:

Adventures in Very Recent Evolution by Nicholas Wade

Other signs of human evolution involve adaptation to the threat of infectious diseases. High prevalences of sickle cell disease and thalassemias in regions where malaria is endemic arise from balancing selection, as the heterozygotes have significantly less susceptibility to malaria.

Similarly, deviations from expected allele frequencies provide clues that some genes have undergone selection. In Europe, the high frequency of cystic fibrosis disease is puzzling, considering that homozygotes have died in infancy until the mid-20th century. Did heterozygosis for the CF allele confer resistance to some chronic diseases or epidemics?

Another recently discovered puzzle is the CCR5 delta-32 allele (a deletion of 32 nucleotides in the coding sequence of the CCR5 protein). Individuals homozygous for this allele are resistant to HIV infection, because they lack the CCR5 cell surface protein that HIV uses as a co-receptor to gain entry into CD4+ T cells. The delta-32 allele occurs with a frequency of 0.1 among Northern Europeans, with diminishing frequency among Southern Europeans and Mediterranean populations. This allele is extremely rare or non-existent in African populations. Since HIV arose only in the 20th century, what could account for this frequency distribution of the CCR5 delta-32 allele?

Humans can also alter our environment at scales and speeds unprecedented in Earth history. Photosynthetic cyanobacteria needed 2 billion years to pump enough oxygen into the air to support life on land. Human activity has significantly altered our atmosphere in less than 100 years. And humans now have the capability to take a direct hand in evolution of species, including our own, via artificial selection (animal and plant breeding, antibiotics and insecticides) and now with genetic engineering and genome editing.

Sources

Naturally blond in the Solomon Islands: https://blogs.discovermagazine.com/gnxp/2012/05/case-closed-blonde-melanesians-understood/

Adaptation to high altitudes: https://blogs.discovermagazine.com/gnxp/2012/01/how-the-amhara-breathe-differently/

https://blogs.discovermagazine.com/gnxp/2010/10/genetic-watersheds-on-the-great-himalaya

Scientists cite fastest case of human evolution https://www.nytimes.com/2010/07/02/science/02tibet.html?scp=1&sq=Tibetans%20Beijing&st=cse

Skin pigmentation: https://www.biologged.com/biology/skin-color-handy-tool-for-teaching-evolution/

https://abcnews.go.com/Health/blue-skinned-people-kentucky-reveal-todays-genetic-lesson/story?id=15759819

Dry earwax in East Asians: https://blogs.discovermagazine.com/gnxp/2010/10/east-asians-dry-earwax-and-adaptation/

Shilpi Aggarwal, Sapna Negi, Pankaj Jha, Prashant K. Singh, Tsering Stobdan, M. A. Qadar Pasha, Saurabh Ghosh, Anurag Agrawal, Indian Genome Variation Consortium, Bhavana Prasher, & Mitali Mukerji (2010). EGLN1 involvement in high-altitude adaptation revealed through genetic analysis of extreme constitution types defined in Ayurveda. PNAS : 10.1073/pnas.1006108107

Bigham A, Bauchet M, Pinto D, Mao X, Akey JM, Mei R, Scherer SW, Julian CG, Wilson MJ, López Herráez D, Brutsaert T, Parra EJ, Moore LG, Shriver MD, 2010. Identifying signatures of natural selection in Tibetan and Andean populations using dense genome scan data. PLoS Genet. 6: e1001116. doi:10.1371/journal.pgen.1001116

Angela M. Hancock, David B. Witonsky, Edvard Ehler, Gorka Alkorta-Aranburu, Cynthia Beall, Amha Gebremedhin, Rem Sukernik, Gerd Utermann, Jonathan Pritchard, Graham Coop, Anna Di Rienzo, 2010. Human adaptations to diet, subsistence, and ecoregion are due to subtle shifts in allele frequency. Proc. Natl. Acad. Sci. USA 107: 8924-8930 doi: 10.1073/pnas.0914625107

Jablonski, NG and G Chaplin 2010, Human skin pigmentation as an adaptation to UV radiation, Proc Natl Acad Sci U S A.: 8962–8968. https://dx.crossref.org/10.1073/pnas.0914628107

Norton et al. 2007, Genetic evidence for the convergent evolution of light skin in Europeans and East Asians, Mol Biol Evol 24: 710-722. doi: 10.1093/molbev/msl203

Ohashi J, Naka I, & Tsuchiya N (2010). The impact of natural selection on an ABCC11 SNP determining earwax type. Molecular biology and evolution PMID: 20937735

Hong Shi and Bing Su 2011, Molecular adaptation of modern human populations. Int. J. Evol. Biol. 2011. doi:10.4061/2011/484769

Storz, JF 2010, Genes for high altitudes. Science 329: 40-41

DOI: 10.1126/science.1192481

https://arxiv.org/abs/1211.3053

Sturm, RA, 2009, Molecular genetics of human pigmentation diversity, Hum. Mol. Genet. 18 (R1): R9-R17. doi: 10.1093/hmg/ddp003

Sarah A Tishkoff, Floyd A Reed, Alessia Ranciaro, Benjamin F Voight, Courtney C Babbitt, Jesse S Silverman, Kweli Powell, Holly M Mortensen, Jibril B Hirbo, Maha Osman, Muntaser Ibrahim, Sabah A Omar, Godfrey Lema, Thomas B Nyambo, Jilur Ghori, Suzannah Bumpstead, Jonathan K Pritchard, Gregory A Wray, and Panos Deloukas, 2007, Convergent adaptation of human lactase persistence in Africa and Europe. Nat Genet. 2007 January; 39(1): 31–40

Tishkoff SA, Reed FA, Friedlaender FR, et al. 2009. The Genetic Structure and History of Africans and African Americans. Science 324(5930):1035-1044. doi:10.1126/science.1172257.

Sustainable Development Goal

UN Sustainable Development Goal (SDG) 3: Good Health and Well-being- Population genetic data can be used to formulate and test hypotheses about recent adaptive evolution of various human populations, where historic and current similarities between populations can shape the health treatments pursued. Characterizing the genetic basis of disease susceptibility and responses to drug and other therapies, combined with information on health disparities of groups of people, can inform public health policies and interventions to improve health outcomes for different populations.