OUT OF AFRICA HYPOTHESIS

=The Out of Africa Hypothesis= [image:http://i.imgur.com/ccWlElv.jpg?1] Also known as: • ''Recent African Origin hypothesis'' • ''Recent Single-Origin hypothesis'' • ''Replacement hypothesis'' This theory suggests that anatomically modern humans originated in South-eastern Africa and dispersed throughout the rest of the world replacing existing human populations in Asia (''erectus'') and Europe (''neanderthals''). Evidence suggests anatomically modern humans (''Homo sapiens'') left Africa somewhere between 120,000 and 60,000 years ago. Scientific consensus favours this theory (over the multi-regional theory), however, it is still debated whether there was a single exodus out of Africa or several. ==Supporting Evidence== '''Fossil Evidence:''' The oldest ''Homo sapiens'' fossils are found in Africa around 195,000 years ago. ''Homo sapiens'' fossils found outside Africa are not nearly as old. As a general rue, the closer a country is to Africa the older some of it's ''Homo sapien'' fossils are. This supports an African origin. [image:http://i.imgur.com/Pg6TzaI.png?3] '''Mitochondrial DNA (mtDNA):''' Because our mitochondrial DNA is inherited solely from our mother, variation in our mtDNA sequences is purely the result of mutations. Over time these mutations accumulate and therefore older populations tend to have greater variation in their mtDNA. For instance, anatomically modern humans (''Homo sapiens'') in Africa would have accumulated a significant degree of genetic variation. This is represented by the different coloured dots in the diagram (left). [image:http://i.imgur.com/gUscFqM.png?2] When a small group of ''Homo sapiens'' left Africa they would have take only a small part of this variation with them. As a result, regions that were populated more recently should have less variation in their mtDNA sequences. They would have started with a limited sample from the ancestral variants and would have had less time to develop new variants than the ancestral population. This trend has been observed with mtDNA analysis showing the greatest diversity in African population, whilst those regions that are geographically furthest from Africa show the least variation. In fact evidence suggests we originated from a single female, nick-named ‘'Mitochondrial Eve'’ who lived around 100,000 - 200,000 years ago in Africa. [image:http://i.imgur.com/IcQdYvR.png?1] '''Y-Chromosome Analysis:''' Because the Y chromosome is always paternally inherited (from the father), variation in Y-chromosome sequences are also generally the result of mutations (rather than independent assortment and crossing over). Y-chromosome analysis shows a similar trend to mtDNA, with African populations demonstrating the greatest diversity and decreasing diversity in Y-chromosome sequences the further a population is from Africa. '''Nuclear DNA:''' Analysis of nuclear DNA from Neanderthal, Denisovan and Cro-magnon remains suggests that they were distinct species. Common sequence elements suggest there was some interbreeding and the Neanderthals & Denisovans made some contribution to the genome of non-African populations. However, interbreeding appears to have been quite limited and the evidence still supports the notion that anatomically modern humans (''Homo sapiens'') originating from Africa largely replaced the other species in Asia and Europe. There simply doesn't seem to have been enough gene flow (interbreeding) between continents to support the idea that anatomically modern humans evolved simultaneously in each continent.