What happened to the Australian megafauna?

Australia is well known for its unique animals. It has the most extensive diversity of marsupial mammals found anywhere in the world. Together with nearby New Guinea, it is home to the only monotreme mammals in the world, the platypus and echidna. It has not one but two species of very large flightless birds, the emu and cassowary. And until historic times, its only placental mammals were humans, dingos, and bats. So much is common knowledge.

But before the arrival of humans, the Australian fauna was even more extensive for the inclusion of many kinds of very large animals, or megafauna. These include Diprotodon, which is often compared to a rhinoceros in size and adaptive pattern, Zygomaturus, which was a somewhat smaller, bison-sized herbivore, the "marsupial lion" Thalacoleo carnifex, the giant 9-foot kangaroo Procoptodon goliah, and the large flightless bird Genyornis. A guide to the extinct Australian megafauna is available from the Museum Victoria site.

What happened to these large animals? Some archaeologists have hypothesized that the appearance of humans in Australia by 50,000 years ago may have led directly to the extinction of the megafauna. This hypothesis would be essentially like that proposed for the disappearance of the American megafauna, including mammoths, giant ground sloths, horses, and many other species in the terminal Pleistocene, called the "overkill" hypothesis. A new paper in PNAS by Clive Trueman and colleagues (2005) addresses this hypothesis by examining the archaeological evidence for megafaunal extinction.

In Australia, the evidence for the "overkill" hypothesis has mainly been derived from the coincidence of the arrival of humans and the extinction of the megafauna. As described by Trueman et al. (2005), most studies have placed the most recent evidence of megafaunal elements at around 46,500 years ago. The present evidence for human occupation of Australia places it certainly by 45,000 years ago, possibly as early as 60,000 years ago. Some archaeologists have persistently suspected that the date of occupation may be even earlier, and occasionally dates for archaeological sites have emerged to support an earlier date, only later to be questioned and retracted. In any event, there has been relatively little evidence for a substantial period of interaction of humans and megafaunal species, as would be documented by finding these species together with archaeological remains for a substantial period of time.

The current study examines one such site. From the abstract:

A number of key sites with megafauna remains that significantly postdate 46.5 ka have been excluded from consideration because of questions regarding their stratigraphic integrity. Of these sites, Cuddie Springs is the only locality in Australia where megafauna and cultural remains are found together in sequential stratigraphic horizons, dated from 36-30 ka. Verifying the stratigraphic associations found here would effectively refute the rapid-overkill model and necessitate reconsideration of the regional impacts of global climatic change on megafauna and humans in the lead up to the last glacial maximum. Here, we present geochemical evidence that demonstrates the coexistence of humans and now-extinct megafaunal species on the Australian continent for a minimum of 15 ka (Trueman et al. 2005:8381).

The chemical analysis in the paper was concentrated upon demonstrating that the bone remains actually belonged within the sediments where they were found, so that the site stratigraphy is a true record of the intercalation of human artifacts and megafaunal remains. A 1996 report at the University of Sydney Archaeology department documents the Cuddie Springs site in some detail, including the evidence for butchering of megafaunal species. So the find is not new, but its improved documentation has some substantial importance in the context of megafaunal extinction in Australia.

Here is an excerpt from the discussion:

The verification of late survival for at least some Australian megafauna has broad ramifications. Although prolonged persistence of the megafauna after human arrival certainly does not rule out a role for humans in their extinction, it does demonstrate that the extinction of the megafauna occurred over a time scale of many thousands of years. In Australia, there is no evidence for either megafaunal kill sites or contemporaneous technologies typically associated with big-game hunting (such as spear-throwers or stone-tipped projectile points). Arguments for human-mediated megafaunal extinction have commonly rested on the strength of a circumstantial case (i.e., that extinctions preceded significant climate change); therefore, humans must have been responsible. However, sites other than Cuddie Springs have yielded megafauna remains that are significantly younger than 46.5 ka, and an increasing body of evidence attests to the onset of climatic instability in Australia from ~50 ka, culminating in full glacial conditions as early as 30 ka. Climate instability is characteristic of the Late Pleistocene and is coincident with faunal extinctions on all continents (Trueman et al. 2005:8383-8384, citations elided).

In other words, climate change is very much in the game for explaining these extinctions. On the other hand, there is no real way to take humans out of the game. People were clearly hunting these animals as they expanded in population size at an unknown rate. In other regions of the world, increases in human population size were associated with increases in the breadth of prey species and apparent impacts on low-recovery rate species. Whether Australia is another example of this phenomenon will require not only a better understanding of the timing of megafaunal extinctions, but also a better modeling of ancient human population dynamics.

References:

Trueman CNG, Field JH, Dortch J, Charles B, and Wroe S. 2005. Prolonged coexistence of humans and megafauna in Pleistocene Australia. Proc Nat Acad Sci USA 102:8381-8385. PNAS online