Using DNA to Trace Human Migrations

1. Using DNA to Trace Human Migrations Investigating the Multimillennial Trek Out of Africa

2. Human Migrations • Most evidence indicates the road to the evolution of our species began in Africa. • Anthropologists estimate that the human lineage diverged from other primates about 5 million years ago, with chimps being our closest living relative. • The most primitive human ancestors yet discovered belong to the genus Australopithecus, which lived about 3 million years ago. Remains of Australopithecines – including "Lucy" – have been discovered primarily in the Rift Valley of Africa.

3. Human Migrations • Early members of our own genus, Homo erectus, and its near relative, Homo ergaster, arose in the same region about 2.5 million years ago. • These "archaic" hominids migrated out of Africa approximately 1.5 million years ago to found populations in Europe, the Middle East, and Asia.

4. Human Migrations • Fossils of modern humans, dating to 40,000-100,000 years ago, have been found throughout the "Old World" – Africa, Europe, and Asia – and in Australia. \ • Members of our own species, Homo sapiens, defined by • anatomical features (skull shape and size) and • behavioral attributes (use of blades, bone tools, pigments, burial goods, art, trade, hunting, and varied environmental resources). • These humans subsequently spread to Micronesia, Polynesia, and the "New World" (North and South America).

5. Theories of human emergence • How and where modern humans emerged is a matter of debate between proponents of two opposing theories.

6. Multiregional Theory • Supporters of the multiregional theory contend that modern human populations developed independently from archaic hominid (Homo erectus or Homo ergaster) populations in Africa, Europe, and Asia. Early modern groups evolved in parallel with each other and exchanged members to give rise to modern population groups

7. Displacement Theory • Supporters of the displacement theory, commonly known as "out of Africa," contend that modern human populations are derived from a single modern population group that left Africa about 80,000 years ago. • This founding group migrated throughout the Old World, displacing any surviving archaic hominids.

8. The emergence of H.sapiens So, scientists all agree that our early hominid relatives arose in Africa, but disagree on when the direct ancestors of living humans left Africa to populate the globe.

9. Human Genetic Variablity Historical Global Positioning • First recognized in WW I when soldiers being treated in Greece showed differing blood groups • 1950’s studies of genetic differences among population groups by examining specific blood proteins.

10. Tracing the path of human migration • Bones • Artifacts • DNA • Mitcondrial—the first– • Y chromosome • Entire Genomes used in latest studies • DNA from contemporary humans can be compared to determine how long an indigenous population has lived in a region

11. T H E O R YThe Molecular Clock and Anthropology • Like virtually all fields of biology, anthropology has been revolutionized by the incorporation of methods from molecular genetics and bioinformatics. • By comparing inherited DNA variations, one can determine relationships between different primate species or between different population groups within the same species. • However, since the oldest intact DNA yet isolated dates back only about 40,000 years, the study of the early genetic history of hominids is based primarily on comparisons of DNA from humans and primates alive today

12. Molecular record of changes . At first this might seem a contradiction, but remember that the DNA of any individual bears the accumulated genetic history of its species.

13. Molecular Clock • When two groups split off from a common ancestor, each accumulates a unique set of random DNA mutations. • Provided that mutations accumulate at a constant rate, then the number of mutations is proportional to the length of time that two groups have been separated. • This relationship (number of mutations over time) is called the molecular clock

14. Molecular Clock • An event that has been independently established by anatomical, anthropological, or geochronological data is used to attach a time scale to the clock. • For example, fossil evidence shows that humans and chimps diverged about 5 to 6 million years ago. • Inserting this number, and the number of sequence differences between humans and chimps, into the equation above sets the clock. The clock is then used to gauge other events in human evolution.

15. Mt DNA –Does it mutate too easily to be used as a clock? • The mt control region mutates at a much higher rate than nuclear DNA (WHY?) • Patterns of single nucleotide polymorphisms (SNPs) are stably inherited in a Mendelian fashion over thousands of generations. • However, the apparent mutation rate - based on the number of observed mt SNPs between population groups - does not reflect the true mutation rate.

16. Mt DNA • The high mutation rate of mt DNA predicts that • there is a statistical likelihood that many positions have "back-mutated" from a new mutation to the original state or • mutated several times during the course of primate evolution. • counting observed mutations underestimates the actual mutation rate.

17. Mt DNA • mt DNA polymorphisms first used in 1987 to create a human family tree showing ancestral relationships between modern populations. • Based on the reasoning that all human populations arose from a common ancestor in the distant evolutionary past. • It calculated how long it would take to accumulate the pattern of mutations observed in modern populations. • They concluded that the ancestor of all modern humans arose in Africa about 150,000 years ago.

18. mt DNA • This common ancestor was widely reported as the "mitochondrial Eve…of a female from Africa who lived 200,000 years ago • Eve was not the first woman…but the linage she created is all that we have found or all that survived • A simplification - which appeared to leave out Adam - is due to maternal inheritance of mt DNA.

19. Y chromosome shows paternal line • Within the last decade, Adam has been added to the evolutionary analysis through examination of Y chromosome polymorphisms. • Y chromosome SNPs are the male counterpart to Mt SNPs, and are inherited in a paternal lineage. • However, unlike mt SNPs, which have a high rate of back mutation, each Y chromosome SNP is believed to represent a unique mutation event that occurred once in evolutionary history.

20. Ancient DNA • In 1997, Svante Paabo, revolutionized human molecular anthropology when he isolated the mt control region sequence from a 40,000 year-old specimen of Neandertal.

21. Ancient DNA • This added an actual ancient DNA sample to the reconstructions of hominid evolution based on modern DNA.

22. Ancient DNA • Additional Neandertal samples now have been sequenced, which makes it possible to begin to assess the species diversity of this extinct hominid.