I have always been captivated by history. I find the significant events, both natural and human induced, that have shaped our existence as well as the day-to-day living of generations past, truly fascinating. While living in Scotland I loved spending time wandering the hallways of ancient buildings, like the Medieval Abbey on the isle of Iona and the 15th century Rosslyn Chapel in Midlothian, structures that are steeped in fascinating history. I used to think about all the types of people that had walked the hallowed halls for centuries and try to imagine their stories. When walking in the South African bush I try to envisage what the landscape would have looked like to newly-landed settlers and what day-to-day life was like for the indigenous peoples of this country. It is interesting to think that these newly-landed “pioneers” were in fact returning to lands that their ancestors had migrated from eons ago. In these more recent times DNA is now being used to substantiate emerging theories of the origins of modern humans, such as the ‘Real Eve’ philosophy.
‘Real Eve’ represents the most recent common ancestor from which all modern humans have descended. It is postulated that as mitochondrial DNA (mtDNA) is maternally inherited without recombination, all current variation is derived from a single common female ancestor (Templeton 1993). mtDNA is uniquely suited in the analysis of evolutionary relationships, ancient lineages and the overall history of the modern human population due to the fact that it contains a relatively high copy number and mutation rate (Pakendorf & Stoneking 2005). This coupled with the aforementioned properties of maternal inheritance and the lack of recombination is used to successfully trace lineages backward through time to uncover a single female ancestor inhabiting the earth between 100 000 and 200 000 years ago (Soares & Ermini et al. 2009).
Using mtDNA variation between individuals within a population it is possible to closely estimate the date of lineage convergence because of the known approximate mutation rate of one every 3 500 years occurring in the mtDNA (Soares & Ermini et al. 2009). Additionally this process has been used to support the ‘Out-of-Africa’ migration theory describing the origin and subsequent radiation of modern humans (Forster 2004). It has been hypothesised that the rapid expansion of these African populations into other regions could simply be driven by the ability of these ‘modern humans’ to successfully exploit the environment in a far wider range of habitats due to advances in technological efficiency and productivity (Mellars 2006).
Through the analysis of fossil evidence and mtDNA it can be illustrated that the evolution of the anatomically modern human population occurred in East Africa approximately 60 000 to 200 000 years ago and that a portion of this group migrated from Africa between 60 000 and 90 000 years ago (Reid & Hetherington 2010). Because of the dominance of modern human mtDNA present in today’s population, competitive exclusion of earlier human populations such as Homo erectus and Neanderthals has been suggested (Forster 2004). There is evidence of an earlier migration through Egypt and the Nile Valley to Israel but this group died out approximately 90 000 years ago and never made it to Europe (Oppenheimer 2012). The most supported theory depicts a single exodus of a southern route through the ‘Gate of Grief’, a strait connecting the Horn of Africa to the Arabian Peninsula via the Red Sea (Mellars 2006).
This southern route theory is further reinforced by the climatic conditions experienced at the time of this proposed migration. The glacial events within the last 100 000 years have played an influential role in the times and routes opted for during migration occurrences (Forster 2004). The unprecedented low in sea levels (70m lower than present day levels) experienced during these periods would vastly decrease the width of the strait thus opening a route across the Rea Sea to Yemen on the Arabian Peninsula (Oppenheimer 2012). Fossil evidence found embedded within an exposed coral reef on the coast of Eritrea adds significant weight to the theory that as the game moved north so these small human populations were forced into a life of beachcombing surviving on the bounty of the sea. This way of life continued along the coastline bordering the Indian Ocean.
Once on the Eurasian continent subsequent expansion was experienced across Asia, Australia (via Timor) and, using the Bering Land Bridge, into the Americas. The routes followed by these populations were determined by climatically dependent corridors, such as the Bering and Red Sea Straits, and geographic barriers including oceans and mountain ranges as well as being dependent on the availability of both drinking water and sources of nutrition (Oppenheimer 2012). This extensive radiation occurred over a time period of approximately 70 000 years during which a coastal route through southern Asia and down into Oceania was followed, congruent with inland treks spanning from Europe and Iran to China and Russia (Goebel & Waters et al. 2008).
Approximately 22 000 years ago it was possible for groups of these modern humans to track big game across the Bering Land Bridge from Siberia to the western coast of Alaska due to the global glacial state and thus initiating further expansion into the Americas (Forster 2004). Due to extensive ice sheets covering most of the American land mass, these early colonists were forced south along the west coast resulting in the Gulf of Panama being breached and the consequential settlement of South America roughly 12 500 years ago (Goebel & Waters et al. 2008).
The driving forces behind the migration of modern humans out of Africa range in possibilities from advancements in technological efficiency and productivity; resulting in the ability to successfully exploit previously uninhabitable environments, to simple wanderlust, still evident in the nomadic tribes of today. The rates of dispersal together with the routes navigated by these early travellers were shaped by the dynamic climatic conditions and natural disasters experienced during the various periods as well as the essential availability of sufficient food and water. By following the genetic footprints left by our ancestors, evidence of these significant journeys across a vast expanse of territories can be found embedded within individuals of every population presently inhabiting the globe.
Explore. Dream. Discover.
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GOEBEL, T. WATERS, M.R. & O’ROURKE, D.H. 2008. The Late Pleistocene Dispersal of Modern Humans in the Americas. Science 319(5869): 1497-1502.
MELLARS, P. 2006. Why did Modern Human Populations Disperse from Africa ca. 60,000 years ago? A New Model. PNAS 103(25): 9381-9386.
OPPENHEIMER, S. 2012. A Single Southern Exit of Modern Humans from Africa: Before or After Toba? Quaternary International 258: 88-99.
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REID, G.B.R. & HETHERINGTON, R. 2010. The Climate Connection: Climate Change and Modern Human Evolution. Cambridge University Press: Cambridge.
SOARES, P. ERMINI, L. THOMSON, N. MORMINA, M. RITO, T. RÖHL, A. OPPENHEIMER, S. & MACAULAY, V. 2009. Correcting for Purifying Selection: an Improved Human Mitochondrial Molecular Clock American Journal of Human Genetics 84(6): 740-59.
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