Tuesday, 18 October 2011

Denisovans and Depilation: New Thoughts On Modern Human Migration

Over the past few weeks a bewildering number of papers on human genetics has been released.  I have endeavoured to keep up with the almost weekly revelations that occurred in September and early October, but will no doubt have missed some vital points along the way.   To summarize briefly; a newly recognized human species discovered in early 2010 has a genome that overturns one of the most widely accepted models for Homo sapiens provenance and migration. Add some additional genetic studies and a lock of 88-year-old human hair from Western Australia and the palaeoanthropological world turns upside down.  To explain further I have to go back to the beginning and describe the two polarized models for modern human evolution that previously held sway.

'Out of Africa' v. The Multiregional Hypothesis
Since the 1980s there have been two main models of human migration, which I will over simplify here.  The first 'Multiregional Hypothesis' states that Homo erectus evolved in Africa and moved into Eurasia about one million years ago.  It then explains that Homo sapiens independently evolved from these H. erectus populations spread out across the globe.  The idea of ‘gene flow’ or migration is proposed to explain how complete speciation didn’t take place in each region and that homogeneity was maintained.

Out of Africa also describes an initial waved of migration by Homo erectus, but suggests that about 100,000 years ago modern humans originating from Africa then spread out across the globe and replaced all other hominin species that had come before it. I would say that the majority of academics in recent times agree with the 'Out of Africa' model (or some variant of it), especially after a paper was published that seemed to prove a single origin for modern humans in a shared common female ancestor who lived in Africa about 200,000 years ago (Cann, Stoneking and Wilson 1987). 

Mitochondrial DNA, which is passed only down the female line, was examined in 147 individuals from five major ethnic groups across the globe.  Two major conclusions were reached from the investigation.  The first was that there was the existence of two major groups  - one with very similar mtDNA of African origin; the other contained the four non – African groups.  The second conclusion was that the African group had a far greater level of variation than the other four, indicating it was the oldest DNA as it had had the most time to accumulate mutations (ibid).   This led to the ultimate conclusion that there was one original ancestor of modern humans alive about 200,000 years ago, supporting the out of Africa hypothesis.  She is referred to as Mitochondrial Eve.
On the right is the Out of Africa model showing replacement of earlier popualtions. On the left is Multiregional Theory showing gene admixture (Source: Scientific American) 
Out of Africa is not completely inflexible.  It's supporters allowed that some degree of interbreeding might have taken place between modern humans and other similar species they encountered on the way, such as Neanderthals, but that such encounters had left no lasting genetic legacy in the world population today.  I even went as far as writing an undergraduate dissertation on the reasons that two such similar species wouldn't have mixed, suggesting a biological phenomenon known as Behavioural Character Displacement.

How wrong I was.

A possible encounter in Late Pleistocene Europe? (plawiuk.blogspot.com/2006_10_29_archive.htm)
The first alarm bells began to ring when Green et al (2010) published a paper showing that up to 4% of the human genome of modern non-Africans contained Neanderthal DNA.  In genetic terms this is a massive amount and was pretty conclusive evidence that Neanderthals had not only bred with us - they are us.  With the addition of the 'X-woman' find, 2010 heralded a whole new worldview in Palaeoanthropology...and a whole new species - the Denisovans.

Who are the Denisovans?!
A human population identified from a single bone in the finger belonging to individual nicknamed 'X-woman'.  

This bone was found in a cave in Denisova, Siberia and was dated to between 50,000 and 30,000 years ago, making it contemporaneous with modern humans and Neanderthals, previously thought to be the only two hominin species left on earth by this date.  The DNA sequenced from this finger suggests that it was significantly different from both Neanderthal and modern human DNA and should therefore belong to a group of its own, which palaeoanthropologists refer to as the Denisovans.  They would have shared a common ancestor with modern humans and Neanderthals between 750,000 and 1.3 million years ago (Krause et al 2010).

Since this first study was released subsequent genetic investigations have shown that the Denisovans contributed 5-6% of the DNA in the genome of present-day Melanesians (Reich et al 2010).  A further study published this year (Reich et al 2011) shows that many South East Asian populations contain Densiovan DNA. It suggests that with a range stretching from the far North of the globe to South East Asia, the Denisovans were as flexible to changes in environment as modern human populations famously are.  

Finally, Hammer et al (2011) have shown that even in present-day African populations, such as the San, genetic material from an unknown archaic hominin makes up 1-2% of the genome, further implying we are genetic mix of different species.  

It seems likely that this fraternization between species actually benefited Homo sapiens in the long run.  An investigation of Human Leukocyte Antigens (HLAs), proteins coded for by genes essential for a healthy immune system, shows that many were 'picked up' by non-Africans from Denisovans and Neanderthals they interbred with as they moved across the globe.  Migrating Homo sapiens would have not survived the new diseases they were exposed to without these vital genes.

A Lock of Hair and the Earliest Modern Human Migration
In 1923 an Australian Aboriginal man gave a lock of his hair to an anthropologist.   This year DNA was collected from a strand of the hair and the entire genome of the man analysed.  Rasmussen et al (2011) concluded that this individual's ancestors had separated from the ancestors of all other human populations between 64 and 75 thousand years ago.  Thus Aboriginal Australians were the first modern humans to leave Africa and migrate across the globe into Asia and then Australia.  This is great news for the indigenous populations of Australia, who can now prove that they have the longest association with their land, or indeed any human population has had with any area of land.  The diagram below shows how the earliest migrations might have been timed and which groups moved where.

Early spread of modern humans outside of Africa (Science/AAAS)
To Conclude...Rather than one sweeping migration of modern humans leaving Africa 100,000 years ago that trampled on all other hominins in their way, migration seems to have occurred in numerous fits and spurts with the existing populations interbreeding with new arrivals.  Each area of the globe has a unique genetics admixture, to the benefit of all - a veritable 'melting pot' of species.

A Little Disclaimer...I must stress that genetics studies are not always to be trusted.  Mistakes have been made in the past when sampling living human populations and when dealing with contamination of fossil material. However such work can certainly contribute a great deal to Palaeoanthropology if treated with caution.

Abi-Rached et al (2011): 10.1126/science.1209202

Cann, Stoneking and Wilson (1987): 10.1038/325031a0

Green et al (2010): 10.1126/science.1188021

Hammer et al (2011): 10.1073/pnas.1109300108

Krause et al (2010): 10.1038/nature08976

Rasmussen et al (2011):10.1126/science.1211177

Reich et al (2010): 10.1038/nature09710

Reich et al (2011): 10.1016/j.ajhg.2011.09.005

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