Dating and explaining the extinction of megafauna in Australia and North America is a controversial pursuit. Research assigning blame either to humans or glacial cycles is felt to have political implications, and is sometimes assessed on these grounds. Onlookers seem to find the former argument more or less congenial according to whether or not they think indigenous peoples were responsible ‘custodians’ of the land, who respected a harmonious ‘balance of nature’, an equilibrium later disrupted by the arrival of European pathogens, agriculture and industry.
There are two basic models whereby hunter-gatherers are said to have caused enough ecosystem disruption to wipe out large mammals. For North America and New Zealand, the proximal cause is said to be a ‘blitzkrieg’ of rapid human predation. In Australia, the ultimate driver is usually claimed to be human burning practices leading to vegetation change. Evidence for the latter is:
- Many large herbivores became extinct, across a wide range of habitats and climates, around the time humans colonised Australia, about 55 to 45 ka (thousands of years) ago.
- Megafauna survived longer where human arrival was later (Tasmania).
- No unusual levels of aridity, or other climatic instability, around this time.
- Sediment cores show an increase in microscopic charcoal.
- Sudden changes in the diet of wombats and flightless birds, including the emu, suggest a corresponding change in vegetation.
- Specialised leaf-eating herbivores (browsers) were less successful than grazers, implying that undifferentiated scrub replaced a ‘habitat mosaic’ of trees and grasslands.
The leading alternative view attributes the extinctions largely to climate change. Its case against a human-induced event goes like this:
- Late survival of megafauna (notably at Cuddie Springs), with fossils dated to about 35 ka, refutes the notion of continent-wide extinction and indicates long-term coexistence between humans and large marsupials.
- The late-Pleistocene population of Australia was neither dense nor sedentary enough for fire-stick farming to have more than a local effect, or to control the lightning regime.
- Beyond the glacial cycle, there was a long-term trend towards greater aridity from 300 ka, along with increased ENSO activity from 60 ka, which produced drying of bodies such as Lake Mungo, and changed fire regimes and consequently vegetation. Accessible water became too scarce for megafauna to survive.
- Most megafauna were extinct by at least 80 ka, i.e. before human arrival. The extinctions occurring ca 46 ka were thus merely the tail of a staggered process.
Now Science has published a report showing that the Cuddie Springs deposits have been re-dated, and show no ‘late survival’ of megafauna. From the abstract:
Giant marsupials, reptiles, and flightless birds once inhabited Australia (see the first figure). But 23 of the 24 genera of these megafauna disappeared in the late Pleistocene (125 to 12 thousand years ago). Most Australian megafauna appear to have survived until 51 to 40 thousand years ago, with human impact by hunting or vegetation change proposed as the extinction drivers (1–4). Yet, one site has stood out as an anomaly: Cuddie Springs in interior New South Wales. Persistent claims have been made that this site contains megafauna fossils associated with stone tools in sediments deposited 40 to 30 thousand years ago (5–7), thus indicating prolonged overlap between people and megafauna. These claims have been challenged (2, 8) based on concerns about possible reworking of fossils from older deposits. To resolve this conundrum, Grün et al. (9) have now directly dated the fossils themselves. The results provide no evidence for the late survival of megafauna at the site.
More from ScienceDaily.
This has been received in some quarters as final proof that the theory of human-induced extinctions is correct. Findings are often reported this way. It’s certainly a strong result: there’s now no real evidence of megafauna persisting until the onset of higher aridity around 30 ka. It’s especially compelling when combined with the recent Tasmanian result, which shows that some megafauna did survive later (up to 41 ka, after their extinction on the mainland), until human arrival. Against the hypothesis of ‘staggered attenuation’, there’s not much fossil evidence for decreasing megafaunal diversity once sampling bias is taken into account. The worth of this new paper is that attention can now be devoted solely to the critical interval 50-40 ka, when the extinctions surely occurred. The exact mechanism, however, hasn’t been confirmed.