'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).
|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)|
How wrong I was.
|A possible encounter in Late Pleistocene Europe? (plawiuk.blogspot.com/2006_10_29_archive.htm)|
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)|
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
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