Who killed off Australia's big birds?
New evidence suggests 50 000 years ago humans were cooking large eggs
Genyornis, shown here being chased by a giant extinct monitor lizard. Did humans cause their extinction 50,000 years ago? Illustration: Peter Trusler/Reuters
When the first modern humans reached the shores of Australia, they set foot in a world that was completely unlike anything they had ever seen before. More than 98 million years ago, the Australian continent started drifting apart from Antarctica, with which it had formed the southern supercontinent Gondwana, and set out to do its own thing. Australia’s flora and fauna developed in isolation, resulting in many groups of animals and plants that are unique to the Australian continent.
At the time of human arrival, Australia was home to a fauna dominated by large animals. There were giant marsupial mammals (mammals that carry their young in an external pouch) such as the hippopotamus-sized Diprotodon and the giant short-faced kangaroo Procoptodon, several species of egg-laying mammals (of which only two, the platypus and short-faced echidna, persists today), the largest of monitor lizards ever to stomp the earth, Varanus priscus, and a group of very large, flightless birds, the Dromornithidae. All these magnificent beasts have since disappeared. Are we to blame? Two new studies suggest that, indeed, we are.
It seems a straightforward case. Modern humans appear, animals disappear, ergo, humans are to blame. It is a scenario that we have observed over and over again, and all over the globe, in the past as well as today. But scientists have been debating the causes of the extinction of Australia’s megafauna for decades. Europe had been populated by various species of hominids before the arrival of modern humans, but modern humans were the first hominid to reach Australia. The arrival of modern humans in Australia provides us therefore with a unique case to understand the impact of humans on pristine ecosystems.
Scientists have pointed out that the disappearance of the megafauna coincided with a period of significantly drier climates that led to a shift in vegetation and resulting in the extinction of many species of megafauna. Therefore, they argue, climate must have been the bad guy. Others have argued that hunting by humans resulted in megafaunal extinction. Large-bodied species with low rates of reproduction would have been particularly susceptible to pressure from hunting, even at low human population densities. A third scenario involves a combination of these two main theories: megafaunal populations already under stress because of changing climates and vegetation could have easily been pushed over the edge by being hunted by humans.
The exact order and timing of events is crucial in solving the Australian megafaunal extinction case. What is the earliest evidence for the presence of modern humans in Australia, and when do we see species disappearing from the fossil record? This is where things become complicated. Fossil and archaeological evidence for the very first arrival of modern humans in Australia is plagued with uncertainties regarding age and origin, but most palaeoanthropologist agree that this must have happened at least 50 000 years ago. This time period sits right at the limit of where we can confidently date fossilized bones using radiocarbon dating (radiocarbon, or C14, dating becomes unreliable beyond ~50 000 years). Added to that is the fact that the fossil record is inherently incomplete; even when one does manage to reliably date the last (i.e. youngest) fossil specimen ofDiprotodon or a dromornithid, the Signor-Lipps effect makes it unlikely that your specimen was actually the very last of its kind. Things are never as easy as they seem.
The largest known marsupial, Diprotodon optatum, from the Pleistocene of Australia. Photograph: Alamy
In a new paper, Saltré and colleagues describe how they have tackled this issue by taking a metadata approach. They assessed the reliability of the fossil ages reported for a large number of megafaunal and modern human remains according to a list of criteria. Only those fossil ages deemed reliable were incorporated in to a statistical model that estimates the most likely time window for the first arrival of modern humans and the extinction windows for 16 genera of megafauna.
According to the model, modern humans arrived around 55 600 years ago. The extinctions of the 16 genera of mammals happened almost synchronously and peak at 42 100 years ago. This places the presence of modern humans well before the megafaunal extinctions. With roughly 13 500 years of overlap between the two, this leaves sufficient time for small groups of modern humans to disperse across Australia and extirpate the megafauna.
But what about the climate, you say? The calculated megafaunal extinction windows were held against a number of high-resolution climate variables, such as temperature and precipitation, but no evidence for a correlation between the timing of extinction and the observed variation in climate was found. In fact, climatic variations during the extinction event were less extreme than in the period prior to extinction, indicating that extinction of Australia’s megafauna was not correlated to changes in climatic variability.
Despite humans and megafauna coexisting for at least 13,500 years, direct evidence for humans preying on megafauna is rare. A second recent paper, by Miller and colleagues, takes a closer look at the extinction of Genyornis, a two-metre- tall member of the Dromornithidae, that family of large flightless relatives of ducks and geese. Eggshell fragments of Genyornis are present in a number of sites across Australia from almost 100,000 years ago to their extinction around 50,000 years ago, but burnt eggshell fragments only occur within a narrow time window between 54,000 and 43,000 years ago. These burnt eggshell fragments have been blackened irregularly and chemical analyses shows that amino acids within the eggshell have been denaturalized from heat along a steep gradient along the eggshell. This suggests that the eggs were exposed to a localized heat source, such as a human cooking fire, rather than a raging wildfire, and that humans were preying on Genyornis. The occurrence of burnt Genyornis eggshells overlaps with the estimated time window of the bird’s extinction, as well as the presence of modern humans across Australia, and points towards human predation as a major cause in the extinction of Genyornis.
Unburnt, and burnt fragment of Genyornis eggshells. Modified from Miller et al. 2016 Photograph: Gifford Miller
Despite the fossil record’s incomplete and biased nature, the available evidence indicates that human predation, rather than climate change, was the main driver of the disappearance of Australia’s megafauna. As scientists continue to develop more sophisticated tools to tackle these uncertainties and biases, the fog over the extinction of Australia’s magnificent beasts will slowly lift.
Saltré, F., Rodríguez-Rey, M., Brook, B.W., Johnson, C.N., Turney, C.S.M., Alroy, J., Cooper, A., Beeton, N., Bird, M.I., Fordham, D.A., Gillespie, R., Herrando-Pérez, S., Jacobs, Z., Miller, G.H., Nogués-Bravo, D., Prideaux, G.J., Roberts, R.G., & Bradshaw, C.J.A. 2016. Climate change not to blame for late Quaternary megafauna extinctions in Australia. Nature Communications 7, doi:10.1038/ncomms10511
Miller, G., Magee, J., Smith, M., Spooner, N., Baynes, A., Lehman, S., Fogel, M., Johnston, H., Williams,D., Clark, P., Florian, C., Holst, R., & DeVogel, S. 2016. Human predation contributed to the extinction of the Australian megafaunal bird Genyornis newtoni ~47 ka. Nature Communications 7, doi:10.1038/ncomms10496
This article was amended on 26 February to include the echidna in the list of modern egg-laying mammals.