Despite headlines proclaiming that it has been ‘definitively’ proven that all humans alive today arose from a single population of south-central Africans from the Rift Valley around 50,000 bp, John Hawks, Erik Trinkaus and others would beg to differ, claiming that the research conducted into skull morphology by Andrea Manica, of Cambridge University, doesn’t indicate what the authors claim. Here’s what National Geographic have to say on the matter…
Scientists who compared the skulls and DNA of human remains from around the world say their results point to modern humans (Homo sapiens) having a single origin in Africa.
The study didn’t find any evidence to suggest that human species living elsewhere in the world contributed to our direct ancestors’ make-up.
A team led by Andrea Manica at the University of Cambridge, England, combined analysis of global genetic variations with comparisons of more than 6,000 skulls from more than a hundred ancient human populations.
The team found that loss of genetic diversity was very closely mirrored by reduced physical variation the farther away people lived from Africa.
Regardless of what morphological or genetic data are offered, I’m automatically sceptical of any research that claims final proof of a single origin for modern humankind, particularly at the date of 50,000 bp, as there are clear indications from Eurasia, Australia and possibly even America, that by this date, modern humans were already living at all these locations, thousands of miles from Africa, and could not possibly have been part of this presumed African exodus. Here’s a look at why Manica et al believe they have identified the origin of their single exodus…
“We have combined our genetic data with new measurements of a large sample of skulls to show definitively that modern humans originated from a single area.”
Previous studies have found that genetic differences in human populations can be explained by distance from Africa.
The new study also looked at 37 measurements from male and female skulls from around the world. The chosen skulls were all less than 2,000 years old, making them better preserved and more likely to give accurate measurements than older skulls.
That last sentence is puzzling, in that all the skulls chosen are so young in comparison to the populations that were extant at 50,000 bp – if they had examined some of the 30,000 – 36,000 year-old skulls from Romania that Erik Trinkaus has been looking at, they would surely have gained a different impression from the uniform model of skull morphology in moderns than they propose here…
The lowest amount of variation was found in ancient populations from South America and Australia, the two main inhabited regions most remote from Africa.
The study team, writing in the latest issue of the journal Nature, argues that this low variation in remote regions relative to Africa would be expected if Homo sapiens arose solely in Africa.
“The more you move away from that center of diversity where you started, the less diversity you have,” Manica said.
“What we can confidently say is that there has not been a wave [of anatomically modern humans] starting from somewhere else, because then you’d find a second area with more variability,” Manica said.
What Manica can’t say is “that matings with the Neandertals never ever happened, but if it did happen, none of the descendants stayed around.” Effectively, any mating had no contribution whatsoever to modern humans, he added.
All of which sounds well and good, but objections have been raised which question the validity of the research, its results, and by implication, the conclusions derived therefrom. Again, I would refer him back to Erik Trinkaus, who by no mere coincidence, is quoted in the next part of the article…
Certain genetic and anatomical traits “cannot be explained as a simple and complete expansion of modern humans out of Africa,” he said.
“The idea that humans get more uniform further from Africa is simply ludicrous,” he added, noting that modern-day Chinese and Australian Aborigines look no more similar to each other than do Africans and Europeans.
And even two anthropologists, Fred Smith and Charles Roseman, who are sympathetic with the idea of an African exodus, contend that interbreeding between archaic and modern populations probably occurred to greater and lesser extents, though without necessarily leaving a discernible mark on the extant populations of today.
Next, it’s off to MSNBC.com, who not only add a little more detail to the claims of the research, but have thoughtfully turned to John Hawks for his take on all this – first some more from the research team themselves…
Manica and colleagues took multiple measurements of more than 4,500 male fossil skulls from 105 populations around the globe. They combined the results with data from studies of global genetic variations in humans, finding that both genetic and skull variability decreased with distance from Africa. So populations in southeastern Africa held the highest variability compared with populations in other countries.
“Humans seem to have poured out of Africa, spread out across the world, but at a really quite uniform rate such that you get this lovely gradual loss of diversity,” said study team member William Amos of the University of Cambridge.
The results held even when the scientists accounted for climate, since climate conditions can lead to changes in skull features. “In very cold climates you tend to generate a slightly thicker brow ridge. Whether or not that’s to keep horrible blizzards out of your eyes, I don’t know,” Amos said.
My immediate response to that last statement would be along the lines that for example, Australian and Tasmanian aborigines have had thicker brow ridges than many of us, but there’s no evidence they’ve been living in a chilly climate any time in the 50,000 years they’ve been resident in their respective lands, which escaped the worst ravages of the last Ice Age. John Hawks opines thus…
“…the paper is “mistaken.” A major flaw is that the current research is largely based on skull variability. You can’t find the origin of people by measuring the variability of their skulls,”
Differences in skull features are related to genetics, and genetic variation depends on how much mixing occurs with other populations. “The main problem with the paper is that it takes some assumptions from genetics papers of 10 to 15 years ago that we now know are wrong,” Hawks said.
“Africa is ecologically diverse, and cranial variation is a function of environments,” he said. In environments supporting hardy foods such as roots, people would need bigger jaw muscles, and thus larger areas for muscle attachments.
Also, correcting for climate is not a good idea, according to Hawks. “The most important feature that is related to climate is skull size. So by correcting for climate, they are subtracting a major component of variability,” he said.
I’m not up enough on the study of or research into genetics to be aware of the allegedly erroneous genetics studies of the past to which Hawks refers, but it does seem pretty clear from what he and Trinkaus are saying that the study by Manica is far too simplistic to make any sense, which in my opinion accords with the similar nature of the single African exodus theory in general. Even a cursory glance at relevant news articles in the fields of palaeoanthropology and archaeology, amongst others, over the past few years, reveals an ever more complex picture of human evolution unfurling, wherever in the world we look, be it Africa, Australia the Americas, Asia or Europe.
In his own research, Hawks is finding that natural selection has led to changes in thousands of genes during only the past few thousand years.
“I’m really thinking just the opposite of this paper,” Hawks said. “There are differences in the skull between populations, including their variability, but it is mostly due to very recent effects and not the origin of modern humans.”
At the end of the day, a resolution to the Out of Africa debate may be impossible, he said. Most of the evidence can be interpreted as supporting both human-origins theories. “It’s really hard to find observations that distinguish the two,” Hawks said.
“The multiregional idea is identical to the recent African origin idea, except for its prediction that Europeans and Asians were part of the single population of origin and didn’t become extinct.”
Hawks has written up some more thoughts on his blog, in which he gives vent to his frustrations at what he clearly perceives as being a study that is fatally flawed from the outset, but has nevertheless grabbed the headlines, as so many similar ‘single exodus’ claims have done in the past. Not so much a ‘final blow’ against single exodus dissenters, rather an ineffectual breeze blowing quietly in the background.
The idea that much of the Pleistocene was populated by people living in a technical and cultural coma, only wakened from their collective slumber in the Upper Palaeolithic by the sudden pinprick of a modern intelligence which arose from a single location at a specific point in time, (enabling the florescence of our über-selves), is itself long overdue for consignment to the compost heap of old theories based on out-moded concepts. (TJ)
see also: Science Daily, Feb 2006 – “New Analysis Shows Three Human Migrations Out of Africa“