DID NORTH AMERICANS CAUSE AN OVERKILL?
Great work Karen, yes I sought of set this up, since I knew the overkill hypothesis was a hotly debated issue, I like the fact that you did describe the contrasting views of the two camps with peer-reviewed references, there is a similar situation in Europe – with the balance being that the changing climate was hugely influential in the mega-fauna extinctions, whereas the situation in Australia and especially New Zealand tends to sway to increasing support for an overkill hypothesis. In particular New Zealand being the last relatively large land mass to be populated with people showed no extinctions until the arrival of the Polynesians and is the most concrete case for human induced habitat change and overkill.
I think you were right – North America was nothing like the productivity of Serengeti when Clovis man arrived. Indeed one problem is we still do not accurately know who the earliest North American was? It does appear that the uniquely manufactured Clovian tools have only been around for some 11 000 to 13 000 years in North America – but see latter the South American evidence. This is actually quite critical, because at least in Europe and Asia the climate was changing and forest and tundra were replacing grasslands which were the preferred habitat of the mammoths. By 12 000 years ago the once widespread mammoths had isolated into several widely dispersed populations. Smaller and isolate population always puts stress on the evolutionary survival (subject to minimum viable population), but it would also mean that actually they were already likely quite rare in the environment when the Clovis people arrived.
One expert view on the existence of the overkill in Europe is that of Adrian Lister, Professor of Palaeobiology, University College, London who stated to the BBC that…
"our data suggests that the mammoth's range splintered into several small Populations hundreds or thousands of kilometres apart, a situation which - as is well known from endangered species today - leaves the animals very vulnerable to further stress. It is possible that they then succumbed to a mixture of further warming, and hunting by prehistoric humans, the last populations petering out one By one between about 11,000 and 4,000 years ago. Although humans may have played a role, this is a far cry from theories that suggest that hunting wiped out millions of animals across the entire original range of the species." 
If the mammoths were becoming rarer it is likely that other species might also become rarer – especially plains-living species such as the woolly rhinoceros, and even the giant deer (Irish elk). Obviously as they became rarer so would the predators including the saber-toothed cats. Of these latter predators – there appeared to be a great variety of them and that they had evolved some 10 times independently  only to go extinct each time as their prey items declined and those extinctions were unlikely to be human aided. The last saber-toothed cat lived 11 000 years ago. Accepting the 11 000 to 13 000 years ago arrival of humans to North America then I would speculate that it probably would have been quite sparsely populated with animals, frozen tundra and coniferous forest is not an ideal environment for lots of herds of mega fauna. The most common animals would probably have been the musk ox and reindeer or caribou (same animal).
If humans had arrived earlier than the presumably mongoloid Indian “Clovian Cultures” as suggested through Mitochondrial DNA – then who were these people? Earlier people of assumed “Caucasoid” have lead to the erroneous interpretation that the first Americans were possibly white! . This is clearly wrong and although Kennewick Man has been described as Caucasoid he is rather better description as not being of recent mongoloid origin. This person’s ancestry was mostly likely linked to the Ainu of Northern Japan, who are also not mongoloid and indeed share certain characteristics with the Caucasoid such as their general hairiness, but otherwise are quite distinct phenotypically . Reconstructions of Kennewick Man also suggested he looked more similar to people from Papua Guinea 
To complicate matters more at Monte Verde in southern Chile human sites have been positively dated to 12 500 years ago , but some archaeologists have argued that some human artifacts date to 33 000 years ago . The use of genetic markers for Y chromosome do not date North Americans nearly as early as this (Karen can you check in the Spencer Wells book here, but I think he put it down to about 12 000 years) but did not include South Americans. Llinguistic analyses suggesting that Asian and Native North American populations diverged closer to 30,000 years ago . To complicate matters even more South America Indians are usually considered to have different ancestors from North American Indians  and are not especially closely related and some people have even suggested that Polynesians came from a South America ancestry but this is not widely accepted. I have found one reference to a colonization of America dating back to 30 000 years ago based on mtDNA – but this time they are suggesting a possible route across the northern Atlantic  explaining a closer ancestry to European people. This research is based on the shared Haplogroup X (series of alleles at specific locations on a chromosome). Herein lies some interesting evidence, since the Haplogroup X group exists mostly in Europeans, but can occur in North America e.g. Ojibwa (25% occurrence), and even in South American Yanomami people (12% occurrence) that live in the Amazon, but is rarely associated with Central Asian groups  Does this explain how a Greek woman could be related to a North American Native Indian that we saw in the video (Real Eve) which confirmed a common ancestor 30 000 years? The explanation in the film was of a shared ancestor living 30 000 years ago in Central Asia and was pushing the credibility a bit here.
At the end of all of this I am also really confused, but I think it is safe to say that to portrait the Clovian invaders as solely responsible for the over-kill of the North America megafauna is misrepresentative, unless we accept the combined evidence of divergence of dialects the shared Haplogroup X and the South American artifacts all confirming colonization some 30 000 plus years ago, in which time a slower-paced over-kill might have occurred.
I noted that on visiting the BBC website they have attempted to re-dress this balance (mis-representation?) by including the views of Dr Ken Tankersley an anthropologist at Northern Kentucky University who concluded a combination of a very abrupt climate change and hunting .
"Data from these sites suggests that the ice age ended within the span of a single human life, within a period of 50 years when the climate was extremely unstable and rapidly and profoundly changing. Not every animal had the same tolerance to climatic change, so some were more vulnerable to human predation than others. Other animals, such as turtles and amphibians, were unaffected and still others such as the jaguar, pine marten, and water rat moved north, south, east, or west. Bison became smaller in size, but most of the mega-mammals, such as mammoths and mastodons could neither adapt nor move. They were hunted and became extinct.”
My conclusion is that we can only really start speculating on the North American overkill once we know who the first North Americans were and when they occurred and by what route? In attempting to piece together the mtDNA Eve and the Y Chromosomal Adam with the invasion of various continents and overkill of the mega fauna requires some spectacular leaps of faiths, even greater leaps than those that humans made when populating the world, and possibly the one explanation is that we actually used boats at a lot earlier stage that has been previously considered, and indeed Stinger 2002 suggested as far back as 65 000 years ago to reach Australia .
Thanks Karen for a very informative posting but it is important to realize that in science there is lots of room for scientific interpretation and egos at stake. – One of our colleagues here at UWC said about academia “never was the fighting more bitter and never were the stakes so low”. It is important to realize that science does not have answers for everything and an open mind and good observation and integration skills are also critical for interpretation.
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