Human Origins Essay

submitted finally! This essay taps into the recent biological advancements made in genetics and its application to the early days of our species.  This has been my favorate module  in semester one of my first year at the univercity of southampton and has allowed me to make good use of my biology A-level.

I’m not saying this is a particularly good essay (I’ve almost certainly made many errors) but I found it so interesting to write and read about that that I had to share what I learnt.

Assess the contribution that DNA has made to our understanding of when modern humans (i.e. Homo sapiens) spread out of Africa and what routes they took?

Recent advances in genetic technology have illuminated the time and space Homo sapiens travelled through when leaving Africa.  The ‘Out of Africa’ theory has been widely accepted because of overwhelming genetic evidence; now the question of how and when our species left the continent is being deciphered through a range of genetic testing.  Pushing biology to the extremities of possibility, current research has led to the remapping of the route taken by our ancestors.  Whether this migration occurred as singular or multiple events is widely disputed.  The handful of possible escape route out of Africa has been explored through the comparison of haplogroups in mtDNA lineages. DNA evidence shows a gradual decrease in genetic diversity as the distance from Africa increases.  Genetic analysis has been the most significant contribution in our understanding of this first migration out of Africa.

The 1987 study of mtDNA variation by Cann et al. set a president for the study of archaeogenetics and effectively re-wrote the theory of early modern expansion by disproving the ‘Multiregional Hypothesis’.  The benefit of analysing mitochondrial DNA is that it is non-recombining, only passed on from mother to child; its application in genetic history is therefore more substantiated then nucleotide DNA that is prone to greater variety and mutation.  By tracking the maternal line an original Cann’s study established a “Mitochondrial Eve”. She has been dated back to around 150,000 years and was argued to have given rise to 134 mitochondrial haploid types (Cann 1987).  The work was heavily criticised due to issues in method and computer programming but has since been proven essentially accurate in its conclusions.  These conclusions provided strong support for the ‘Out of Africa’ or ‘single-origin model’ by establishing a single hypothetical ancestor that originated in sub-Saharan Africa who that spread out of the continent 100,000 years ago and replaced all other Homo populations. This infers that only a select few ventured out of Africa; here we can see the restricted variation in Haploid groups as a result of a founder effect whereby a small population has gone on to colonise the rest of the world. There has been evidence that as few as ten “daughters” make up the mitochondrial lineages within modern European populations (Gusar 2004).  One of the prominent points of evidence is the similar age between the three main haplogroups in Eurasia M,N and R ; this indicates they were part of the same colonisation (Macaulay et al. 2005).

By comparing mtDNA variety between populations of modern humans and our “Cousins” the chimpanzee, Cann was able to establish a significant lack of variety within modern populations (Cann 1987). The existence of a small pioneering expedition of Homo sapiens who ventured out of Africa 100,000 years ago has now been conclusively accepted; this is largely due to irrefutable genetic evidence.  The study of nuclear DNA, especially of strand CD4 locus on chromosome 12 (Tishkoff et al. 1996), has shown a different variety of mutations in Africa whilst having a heavily restricted variation throughout the rest of the world. This supports the ‘Out of Africa’ theory and posits that non-African descendants originated from a small pioneering group that left Africa at a maximum age of 102,000 years ago given the inferred date of initial mutation (Tishkoff et al 1996).

The contribution of DNA in establishing an approximate date of dispersal, to the point where a few thousand years can be argued either way is one of the greatest archaeological advances of our time.  Our understanding of when this occurred allowed us to irrevocably disprove the traditional ‘Multiregional Hypothesis’ of human evolution and forgo some of the polycentric ideas that became associated with it – disproving the idea of ‘race’.  Analysis of Nuclear DNA has gone a long way in our understanding of the flow of genetics across the world.  Evidence of a restricted genetic pool in the initial colonisation of the world outside of Africa insinuates the occurrence of the founder’s effect or potentially a genetic bottleneck where diversity is heavily restricted.  The evidence resulting from the complete mapping of the Human genome in 2004 emphasised greater variation within sub-Saharan populations than the rest of the world; there was no exodus out of Africa.

Work in the 1980-90’s almost exclusively debated the plausibility of the two main models of human global colonisation however the discussion has turned towards the more intricate task of understanding the relationship between our genetic ancestry and demographic history (Groucutt et al. 2015). The movement of Homo sapiens out of Africa lies at the cusp of scientific possibility; advances in our technical capabilities have expanded our genetic history book by several chapters.  The new millennia now accessible for archaeogenetic study have allowed us to analyse the relationship between modern populations in order to determine the route of the first settlers.  The debate is now largely centred on the mapping of routes our ancestors most plausibly took on their exit from Africa.  Some have speculated that this early migration may have occurred multiple times and not necessarily followed the same path.

The most prominent theories of human dispersal suggest a northerly exit from the African continent seeing three possible escape routes; across the Mediterranean to southern Europe and Italy, across the straits of Gibraltar and across the Levant. Each theory insinuates a sea crossing there being no immediate land bridge between the continents in geographic reconstructions at the proposed time of crossing ~100,000 years ago (Cann 1987).  However others have posited a southerly route across the Indian Ocean is a more probable route as a single dispersal model. The analysis of haplogroups in mtDNA has shown the time periods in which these divergences of populations occurred and archaeological evidence along coastal Africa signify the capability to move across bodies of water in order to populate the new land.

This path out of Africa has been been a matter of great debate however the most prominent thought sees a movement out of the to the Horn of Africa to the Arabian Peninsula.  DNA from  southern Indian and southeast Asia comply with the theory that the people who first left Africa came not across the Indian Ocean. This southern route has been supported by the analysis of mtDNA samples from New Guinea, mainland India and the Andaman Islands (Macaulay at al. 2005).  These “relic” populations have been argued to have been the result of a rapid dispersal out of Africa ~65,000 years ago.  The dating suggests that this may have been a secondary route taken at a later stage aiding the theory of multiple dispersion.  However Macaulay makes the argument that the northern routes are less likely exits. Geographic models calibrate a more arid and harsh environment as the ‘ice age’ widens deserts by locking moisture in glaciers and increased the distance to water by reducing sea levels ; the northern passages were probably blocked by desert (Macaulay et al. 2005).  This is backed up by the lineage of haplogroups; the oldest N and R have the oldest date outside of Africa and are found almost exclusively in western Asia and India and arise ~10,000 years before the development of another haplogroup U in northern Africa. This so suggests that the southerly route across to Asia was the route taken by the first global explorers.  The route along this latitude was also a lot more comfortable and ecologically rich. It is plausible that our ancestors took the path of least resistance and remained in the subtropical environments rather than push closer to the ice sheets; models have shown that southern Arabia was in a wetter phase and well within the biological niche that Homo sapiens inhabit (Armitage 2011).

Genetic studies have allowed us to trace the migration of all humans across the globe.  It has contributed a great deal to our concept of ‘race’ and social derivation. Outside of sub-Saharan Africa we have all originated from one small group of pioneering humans who 100,000 years ago decided to venture outside of the continent they were born. Our understanding of genetics and especially interpretation of mtDNA has allowed us to rewrite the earliest pages of our species history.  We have retraced the footprints of the first sapiens to step outside of the “cradle of life” and conquer the word. Geneticists have been able to tell us more about our origins in the last 50 years then we could ever have imagined. Possibly one of the greatest human achievements; the complete mapping of the Human genome in 2004 aimed to provide data in order to aid our understanding of the movement and evolution outside of Africa. We have been able to narrow the time period and space in which the human race could have left the continent. We have much to credit genetic studies for in our understanding of the first migration/s out of Africa.

Reference list :

Armitage, S.J., Jasim, S.A., Marks, A.E., Parker, A.G., Usik, V.I. and Uerpmann, H.-P. (2011) ‘The southern route “Out of Africa”: Evidence for an early expansion of modern humans into Arabia’, Report, 331(6016), pp. 453–456. doi: 10.1126/science.1199113.

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