Late Pleistocene mass extinction

The recent fossil records of the Americas and Oceania are littered with species that became extinct in the last 100 000 years.  The majority of them are large animals whose body weights were greater than 45 kg – part of the megafauna.  While controversy rages about the date of entry of the first humans into both vast regions, for a long time archaeologists have suspected that the appearance of sophisticated hunters was somehow connected with the rapid decline in what would have been prey species.  One theory is that having never encountered weapon-bearing bipeds, large mammals were “naïve” and thus easily slaughtered.  Most visitors to the Americas and Australia soon notice how unafraid many animals are of humans, compared with their behaviour in Europe and Africa.  The suddenness of  the selective extinctions (around 15 to 11 000 and 47 000 years ago in North America and Australia respectively) is astonishing, if the cause was small bands of hunters, and other workers have suggested that human entry brought diseases that wiped out species susceptible to them, but with no immunity. The third main theory is that a sudden shift in climate wrought havoc among large herbivores and predators, by producing a change in vegetation.  The last is difficult to support for the Americas as the extinctions were in a period of increasing warmth and humidity following the termination of the last glacial period.  As always, new information from research directed at the problem has narrowed the choices, but revealed complexities.

Modelling the influence of changing predation on prey stems from the mathematical simplification of reality by Lotka and Volterra, in which “boom and bust” events pop out of the simulations.  John Alroy of the University of California applied an advanced version of the basic model to the likely effects of advanced hunters appearing suddenly in North America (Alroy, J.  2001.  A multispecies overkill simulation of the end-Pleistocene megafaunal mass extinction.  Science, v. 292, p. 1893-1896).  His model assumes slow human population growth, random hunting and the least possible effort – a conservative approach.  The results closely parallel the record, if human population expanded from 100 first entrants about 14 000 years ago to almost 1 million 750 years later, and suggest that a steady state population of around half that co-existed with the surviving fauna until the appearance of Europeans and their culture.  It is an entirely mechanistic model, but mimics what happened without recourse to any other influence, such as climate change.  So far, no human site in the Americas has been convincingly dated before 14 000 years ago.

Dating is even more of a problem in Australia, particularly for human arrival.  The earliest dated fossil is 60 000 years old (see Out of Africa hypothesis confounded? EP Feb 2001), but claims have been made for artefacts at least twice that age.  Alroy’s model applied to Australia would demand extinction (24 out of 25 genera Pleistocene megafaunal species) shortly after earliest arrival.  A large team of Australian scientists (Roberts, R.G. and 10 others 2001.  New ages for the last Australian megafauna: continent-wide extinction about 46 000 years ago.  Science, v. 292, p. 1888-1892) have systematically dated the age of burial of extinct faunas at 27 sites in coastal areas and the more humid SE of the continent (none from the vast, arid “red centre”), and one in Papua New Guinea.  The most likely interval for the extinctions, between 39 800 and 51 200 years ago, bears no relation to extreme aridity during the last glacial maximum, so the data weigh against that climatic cause.  However, the last 100 000 years have seen lesser, but still extreme shifts in climate, so climate change cannot be ruled out.  Though the authors also do not rule out humans eating their way through Australias bizarre megafauna, the lag between evidence for first entry and the extinction seems far longer than that in the Americas.  Closer inspection of their data, however,  does show precise 230Th/234U ages (+ 600 to 2 200) from 33 600 to 60 000 from 3 sites, and less well-constrained luminescence ages (+ 200-21 000) from 16 000 to 171 000 years from all the sites.  Applying simple statistics to samples from such a wide spread of localities does not seem justified to me – normal practice is for ages at individual sites to be accepted as dates within the errors of the method used.  Australia’s megafaunal extinction seems to have been protracted.  Using fuzzier dating of the extinction, earlier workers correlated it with evidence for an increase in bush fires marked by ash in offshore sediments.  Much of Australia’s flora is fire resistant, and the seeds of some species require light burning before they will germinate.  The most popular theory for the extinctions there is through deliberate fire setting by hunters – a culturally induced decline unique to Australia’s peculiar climate and terrain.

See also:  Dayton, L.  2001.  Mass extinction pinned on Ice Age hunters.  Science, v. 292, p. 1819.

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