Genetic Evidence For Theories Of Human Dispersal


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Slideshow comparing the theories for the origin of modern humans.

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Genetic Evidence For Theories Of Human Dispersal

  1. 1. Theories of Human Dispersal. The Replacement Theory (or Mitochondrial Eve and Y chromosome Adam ) vs The Multiregional Theory.
  2. 3. Examining the evidence
  3. 7. The Replacement Theory (Out of Africa Hypothesis)
  4. 9. This section is the part that both theories have in common, but in the Multiregional Hypothesis gene flow continues, while in the Replacement Theory there is little or no gene flow.
  5. 13. African Origin European Homo erectus African Archaic EXTINCT EXTINCT Asian Modern African Modern European Modern Asian Homo erectus African Homo erectus European Archaic Asian Archaic
  6. 14. The Out of Africa model contains the following components: <ul><li>After Homo erectus migrated out of Africa the different populations became reproductively isolated, evolving independently, and in some cases like the Neanderthals, into separate species. LITTLE OR NO GENE FLOW. </li></ul><ul><li>Homo sapiens arose in one place, probably Africa (geographically this includes the Middle East) </li></ul><ul><li>Homo sapiens ultimately migrated out of Africa and replaced all other human populations, without interbreeding. </li></ul><ul><li>Modern human variation is a relatively recent phenomenon. </li></ul>
  7. 15. The Replacement Theory is most strongly supported by genetic evidence <ul><li>Allan Wilson contributed much research on the mitochondrial “Eve” theory – tracing human women of today back to a common ancestor (really a group of women) who lived in Africa about 150,000 years ago. </li></ul>
  8. 16. Mitochondrial DNA <ul><li>DNA is present inside the nucleus of every cell of our body but it is the DNA of the cell’s mitochondria that has been most commonly used to construct evolutionary trees. </li></ul>
  9. 17. <ul><li>Mitochondria have their own genome of about 16,500 bp that exists outside of the cell nucleus. Each contains 13 protein coding genes, 22 tRNAs and 2 r RNAs </li></ul><ul><li>Mitochondria are present in large numbers so fewer samples needed. </li></ul><ul><li>Mitochondrial DNA has a higher rate of substitution mutations than nuclear DNA making it easier to resolve differences between closely related individuals. </li></ul>
  10. 18. <ul><li>Mitochondrial DNA is inherited only from the mother, which allows tracing of a direct genetic line. (Mitochondria are found in the midpiece of the fertilising sperm, which remains outside the fertilised egg) </li></ul><ul><li>Mitochondrial DNA does not recombine – the process of recombination in nuclear DNA mixes sections of DNA from the mother and the father creating a garbled genetic history. </li></ul>
  11. 19. <ul><li>More recently, research on Y chromosome markers has supported the replacement hypothesis and traced origins of today’s human males to a common ancestor ( or group of men) approximately 60 – 90,000 years ago) </li></ul><ul><li>Both of these groups are placed in Africa </li></ul>
  12. 20. The Y Chromosome has certain Unique Features <ul><li>The presence of the Y Chromosome causes maleness. This little chromosome – about 2% of a father’s genetic contribution to his sons – programs the early embryo to develop into a male. </li></ul><ul><li>It is transmitted from fathers only to their sons. </li></ul><ul><li>Most of the Y chromosome is inherited as an integral unit passed without alteration from father to sons and to their sons. It is unaffected by recombination and is the only nuclear chromosome that escapes the continual reshuffling of parental genes during the process of sex cell production. </li></ul>
  13. 21. <ul><li>The Y chromosome is estimated to carry about 78 working genes with 97% of its length “junk” DNA </li></ul><ul><li>Junk DNA is not subject to change by natural selection so variation is only due to mutation. </li></ul><ul><li>Thus, Y chromosomes pass down from father to son largely unchanged except by the gradual accumulation of mutations (polymorphisms) </li></ul><ul><li>Certain polymorphisms are selected to act as genetic markers and the changes in them tracked over generations. </li></ul>
  14. 24. What’s on the Y Chromosome?
  15. 26. Multi – regional (Regional Continuity) Theory.
  16. 27. Multi-regional Theory – contains the following components: <ul><li>Some level of gene flow between geographically separated populations prevented speciation, after the dispersal. </li></ul><ul><li>All living species derive from the species Homo erectus that left Africa nearly 2 million years ago. </li></ul><ul><li>Natural selection in regional populations, ever since their original dispersal, is responsible for the regional variants (sometimes called races ) we see today. </li></ul><ul><li>The emergence of Homo sapiens was not restricted to any one area but was a phenomenon that occurred throughout the entire geographic range where humans lived. </li></ul>
  17. 28. African Origin
  18. 31. <ul><li>The Multiregional Hypothesis is more supported by fossil evidence </li></ul>
  19. 32. <ul><li>The current best explanation for the biginning of modern humans is the Out of Africa Model that proposes a single African origin for Homo sapiens. The major neurological and cultural innovations that characterized the appearance of fully modern humans has proven to be remarkably successful, culminating in our dominance of the planet at the expense of all earlier hominid populations. </li></ul><ul><li>This theory is best supported by the genetic evidence while the multiregional theory is best supported by the fossil evidence. </li></ul>
  20. 33. <ul><li>But the genetic evidence has a crucial advantage in determining the structure of family trees: living genes must have ancestors, whereas dead fossils may not have descendants. </li></ul><ul><li>Molecular biologists know the genes that they are examining must have been passed through lineages that survived to the present; paleontologists cannot be sure that the fossils they examine do not lead down an evolutionary &quot;blind alley&quot;. </li></ul>
  21. 35. Questions relating to the article by Donald Johanson “Origins of Humans: Multiregional or Out of Africa?” <ul><li>What are the scientific names of all the early ancestors of humans including the name for modern humans? </li></ul><ul><li>Define the following terms: gene flow; speciation; taxonomic diversity; genetic drift; occipital region of the skull; molars; cranial vault; Paleolithic era; mtDNA; Neanderthals. </li></ul><ul><li>How many years ago did all modern humans become anatomically and behaviourally similar.? </li></ul>
  22. 36. <ul><li>What are the three types of evidence that are used to debate the 2 models of human origins? </li></ul><ul><li>What was the “significant innovation” that occurred in the Upper Paleolithic of Eurasia? </li></ul><ul><li>When did Neanderthals disappear? </li></ul><ul><li>Describe the physical characteristics of each of the early ancestors of humans and compare those characteristics to those of modern humans, as described in the article. </li></ul><ul><li>Summarize Klein’s idea of what brought about the emergence of behaviourally modern humans. </li></ul>
  23. 37. <ul><li>What countries are represented in the evidence on the origins of modern humans? </li></ul><ul><li>How are Peking Man and Java Man different from modern humans? </li></ul><ul><li>What is interbreeding and what role did it play in the development of modern humans, according to the Out of Africa model? </li></ul>