Conservation Biology

Sunday, July 30, 2006


What were the impacts on the environment and its biodiversity after humans entered the “New World” some 13 000 - 11 000 years ago? This was the question I was left to explore after watching “Monsters we met”… The message in the video was quite clear. Where ever humans went, they quickly changed the environment with dire consequences to many species; plants and animals alike.

But what I found when searching for material pertaining to the late Pleistocene mammal extinctions of North America, was certainly not what I had expected! Rather than finding numerous scientific papers with empirical evidence of what caused these extinctions, I came upon two “scientific camps” fighting it out in “scientific papers”. On the one side the anti-overkill hypothesis camp (Donald Grayson, David Meltzer and a few others) and on the other side the pro-overkill hypothesis camp (Paul Martin, Gary Haynes, Stuart Fiedel, Jared Diamond and many others).

So what is this fight all about? Paul Martin’s overkill hypothesis first became popularized in 1967 (Grayson and Meltzer 2003). Over the years his hypothesis changed somewhat according to Grayson and Meltzer (2003), but always included the following:

1. Archaeological and paleontological evidence indicate that the prehistoric human colonization of islands was inevitably followed by massive vertebrate extinctions
2. The Clovis people, identified in the archaeological record by their well-made and distinctive fluted spear points, were in all likelihood the first humans to have entered North America. They were the first people known to have hunted large-mammals in this vast continent and their arrival is dated to about 11 000 radiocarbon years ago.
3. Clovis people hunted a diverse variety of now-extinct mammals.
4. The North American extinctions of most large mammal species coincided with the arrival of the Clovis people occurred during the late Pleistocene, around 11 000 radiocarbon years ago. (Grayson and Meltzer 2003, p586)

The overkill hypothesis concludes that the Clovis people hunted many Pleistocene mammals into extinction (Martin 1967, 1973, 1984; Mead et al. 1986; Martin and Steadman 1999; Alroy 2001; Steadman et al. 2005). To be exact, 35 genera became extinct in North America by the end of the Pleistocene, of which 6 genera still exist outside of North America (Grayson and Meltzer 2002, 2003).

Grayson and Meltzer do not deny that human hunting might have played a part in these extinctions ( Meltzer 1983, 1989), they do however strongly disagree, unlike Martin, that hunting was the sole cause (Grayson and Meltzer 2002, 2003, 2004). In their critical review of the evidence in 2002, Grayson and Meltzer found convincing evidence linking the Clovis people to the hunting of large mammals at only 14 out of 76 archaeological sites; only two for mastodons and the rest for mammoths. That still leaves 33 genera unaccounted for.

There is enough evidence out there that humans have had a huge impact on islands (Mc Arthur and Wilson 1967; Olson and James 1982(a), 1982(b), 1984; Steadman 1989; Paulay 1994; Rosenzweig 1995; to mention but a few…). I also remember reading somewhere that Alfred Russell Wallace already came to that conclusion more than a hundred years ago! However, the massive impact humans have had on island vertebrate species does not necessarily imply that they would have had the same influence in a continental setting (Grayson and Meltzer 2003). In addition, it is clear that island extinctions were rarely attributable to hunting alone, but rather by a combination of factors such as habitat destruction (McGlone 1989; McGlone and Wilmshurst 1999), the introduction of predators, parasites and competitors (Holdaway 1999), as well as hunting. All agree however, that the arrival of the first humans heralded the demise of many island species. Grayson and Meltzer (2002, 2003) argue that if one wants to apply the island theory to a continental setting, then it is even more likely that a combination of factors will be at play, as it is expected that animal populations and the habitat available would be far bigger, making it more resilient to extinction by hunting alone. Up to date there is however no evidence of large-scale human–caused alterations to the landscape, like fire or deforestation, for the time period the Clovis people roamed this continent (Walker 1982; Grayson 2001). Fiedel and Hayes (2004) however disagree. They have referenced several papers that do indicate burning, but add that it is difficult to distinguish between natural fires and human-induced fires. They also indicate that maybe North America at the end of the Pleistocene did not exactly look like the Serengeti. We simply do not know if there were massive herds of animals or mere pockets here and there. Somehow my logic tells me that if Fiedel and Haynes (2004) are right about it not being the Serengeti, then that could explain why the Clovis people populated this vast stretch of land in such a short period of time. Why would they have scattered all over if they had arrived in the land of plenty? Did they move because it was their way of life as hunter-gatherers or were they forced to move because there wasn’t enough to share amongst them?

The mere fact that Clovis artefacts are dated to more or less the same time as the bones of many Pleistocene mammals, does not imply that they were killed by these people. It merely shows that they occupied the same space at the same time, but it certainly does not exclude the possibility either. Grayson and Meltzer (2002, 2003) seem to exclude the possibility due to lack of evidence. Martin claims that the Clovis hunted a great variety of now extinct mammals, although the only positive evidence found by Grayson and Meltzer (2002, 2003) confirmed that they hunted mammoths and mastodons. In a later review, Fiedel and Haynes (2004) do not dispute the lack of conclusive evidence, but they indicate that very few bones from that period have been preserved overall, so the few bones found is not unexpected. In conclusion they emphasize that a lack of conclusive evidence does not disprove the overkill hypothesis, in the absence of conclusive evidence the most likely scenario should be considered.

Grayson and Meltzer (2002, 2003) also indicate that only 15 of the 35 now extinct genera can be positively dated to have lived beyond 12 000 radio-carbon years before present. That means that 20 genera must have become extinct even before the Clovis arrival. I must admit I do not understand the different methods of dating, and how exactly that relates to present dates, but what is evident is that at least 15 genera were present when the Clovis entered the North American arena and that within a relatively short period of time they had become extinct. (The two camps seem to differ on how long it took, but by now I am so fed up with contradicting papers that I will leave it at that!!!)

From what I have read over the past few days, it seems that Fiedel and Haynes (2004) give the most balanced presentation of ALL the available evidence. According to this evidence or lack of it, the most likely hypothesis is that humans played a big part in at least some of the mega-fauna extinctions that occurred during the late Pleistocene. Would all 35 genera in question have become extinct in the absence of man? I don’t know and what is clear to me is that nobody exactly knows what happened. Technology however keeps on developing as Kerr (2003) points out. In this study scientists have used spores from a fungus found in dung to try and recap the order of events and according to their results it seems as if humans played a bigger role than the climate. This topic is surely to be continued…

What have I learned from all of this? Whether it was humans alone (new evidence shows that the Clovis were possibly not the first humans in North America….(Dillehay 1997; Meltzer 1997; Meltzer et al. 1997) ) or a combination of factors that caused the extinctions will most probably never be known conclusively! What I do know is that “science” can also become a dirty game played by self-important people…


Alroy J. 2001. A multispecies overkill simulation of the end-Pleistocene megafaunal mass extinction. Science 292:1893-1896

Dillehay TD. 1997. Monte Verde: a late Pleistocene settlement in Chile. The Archaeological Context and Interpretation vol. 2. Washington D.C.: Smithsonian Institution Press.

Fiedel S and Haynes G. 2004. A premature buriel: comments on Grayson and Meltzer’s “Requiem for overkill”. Journal of Archaeological Science 31:121-131.

Grayson DK. 2001. The archaeological record of human impacts on animal populations. Journal of World Prehistory 15:1-68.

Grayson DK and Meltzer DJ. 2002. Clovis Hunting and Large Mammal Extinction: A Critical Review of the Evidence. Journal of World Prehistory 16(4):313-359.

Grayson DK and Meltzer DJ. 2003. A requiem for North American overkill. Journal of Archaeological Science 30:585-593.

Grayson DK and Meltzer DJ. 2004. North American overkill continued? Journal of Archaeological Science 31:133-136.

Holdaway RN. 1999. Intoduced predators and avifaunal extinction in New Zealand. In: MacPhee RDE, editor. Extinctions in Near Time. New York: Kluwer Academic/Plenum Publishers. p 189-238.

Kerr R. 2003. Megafauna died from big kill, not big chill. Science 300:885.

MacArthur RH and Wilson EO. 1967. The Theory of Island Biogeography. Princeton: Princeton University Press.

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Martin PS. 1973. The discovery of America. Science 179:969-974.

Martin PS. 1984. Prehistoric overkill: the global model. In: Martin PS, Klein RG, editors. Quarternary Extinctions: A Prehistoric Revolution. Tuscon: University of Arizona Press. p 354-403.

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Meltzer DJ, Grayson DK, Ardila G, Barker AW, Dincauze DF, Haynes CV, Mena F, Nuñez L and Stanford DJ. 1997. On Pleistocene antiquity of Monte Verde, Southern Chile. American Antiquity 62:659-663.

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Rosenzweig ML. 1995. Species Diversity in Space and Time. Cambridge: Cambridge University Press.

Steadman DW, Martin PS, MacPhee RDE, Jull AJT, McDonald HG, Woods CA, Iturralde-Vincent M and Hodgins GWL. 2005. Asynchronous extinction of late Quaternary sloths on continents and islands. Proceedings of the National Academy of Sciences 102(33):11763-11768.

Walker DN. 1987. Late Pleistocene/ Holocene environmental changes in Wyoming:the mammalian record. In: Graham RW, Semken H and Graham M, editors. Late Quaternary Mammalian Biogeography and Environments of the Great Plains and Prairies. Illinois State Museum, Scientific Papers 22:334-393.

Image credit:

Karen Marais
BCB Hons NISL student
University of the Western Cape
Private Bag X17