The Sixth Mass Extinction

In between 80,000 and 2,000 years ago there have been several massive megafaunal extinction events that have completely reshaped continental-sized ecosystems.  These extinction events are collectively known as the Quaternary Mass Extinction.  Although these mass extinction events directly accounted for only a 0.01% decrease in global biodiversity (Levy, 2011), they may have been the largest disruptions to terrestrial faunal continuity since the Cretaceous-Tertiary (K/T) catastrophe.  Over the past 50,000 years, several genera of large mammals (>40 kg adult body mass) have disappeared (Levy, 2011):  Australia has lost 15/16 genera, North America 33/45 genera, and South America 46/58 genera.  Madagascar, New Zealand and several other isolated island environments have also suffered major megafaunal extinction events.  In total, the world lost 178 species of the largest mammals (Hortola & Martinez-Navarro, 2012).

The pace of these events is their most striking aspect.  Geologically speaking, all of them happened suddenly.  In Australia the extinctions occurred in between 50,000-40,000 years ago, in the Americas 14,000-10,000 years ago and in Madagascar, New Zealand, the Caribbean, and other islands around 2,000 years ago.  As a result, many ecosystems have been left without any terrestrial megafauna (Johnson, 2009).

What caused these ecosystems to collapse?  Why was there mass extinction among populations of terrestrial megafauna that left smaller fauna and aqueous fauna unaffected?  Why were African and South Asian megafauna unaffected?  Scientists have struggled to definitively answer these questions since the time of Darwin (Levy, 2011).  The two predominant (and mutually exclusive) hypotheses target natural climate change and human overkill.

Climate Change Hypothesis

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The natural climate change hypothesis posits that megafaunal extinctions occurred because of the Earth’s current interglacial phase.  Under this model, large continental-sized ecosystems changed too quickly for mammalian megafauna to adapt.  For me, this hypothesis has always had major flaws:

  • Why were Australia and South America so negatively affected?
  • Why were African megafauna unaffected?
  • Why did the extinctions happen very suddenly at different time periods in different geographic locations?
  • Why didn’t megafaunal mass extinction accompany previous interglacial phases?

These are all important questions that need to be answered, and I don’t feel as though the climatic change hypothesis adequately addresses any of them.  If the last interglacial caused mass megafauna extinction, we should expect to see far different patterns.  For one, the extinctions should have happened disproportionately in the northern hemisphere.  Australian and South American climate has not changed substantially over the past 100,000 years (Rule et al., 2012), so their fauna should have been relatively unaffected.  Furthermore, the extinction events should have happened at roughly the same time throughout Northern Eurasia and North America.  However, this is not what is observed.  As stated above, the extinction events happened at distinctly different times throughout the past 50,000 years.  Finally, why hadn’t previous interglacial’s cause megafaunal mass extinction?  It seems quite implausible that the most recent interglacial disrupted ecosystems disproportionately, since the transition from glacial to interglacial phase was not particularly quick or severe when compared to previous interglacial transitions.

OVERKILL

Prehistoric-life

The human overkill hypothesis on the other hand, seems to explain the events of the Quaternary Mass Extinction perfectly.  When humans first left Africa, we migrated into the Arabian Peninsula and down into South Asia.  This occurred in between 70,000-60,000 years ago.  During this time period there were no megafaunal mass extinctions, just as there had been no megafaunal mass extinctions in Africa.

Why?  Modern humans, and our ancestors had been living in Africa for millions of years, and Homo erectus (and potentially other ancestral humans) had been living in Asia for 1.8-1.3 million years (McHenry, 2009).  During this time, different species of humans had been evolving with the ecosystems on a geological timescale.  This allowed the megafauna of Africa and South Asia to evolve adaptations to deal with the human presence.  So when modern humans began quickly migrating throughout the world, they did not disrupt these ecosystems and cause mass extinction events.  As a result, Africa and Asia are the only regions with megafauna weighing over 1000kg today.  The same cannot be said for Northern Eurasia, Australia, the Americas and the many other isolated island environments that had evolved human-free ecosystems unable to cope with our presence (Johnson, 2009).

Northern Eurasia

The beginning of the Quaternary extinction events first began in Northern Eurasia about 80,000 years ago.  These extinction events occurred most gradually when compared to Australia, the Americas and other isolated island environments.  This is likely because the modern human migration into Northern Eurasia itself was very gradual.  Modern humans had adapted to life in tropical environments, and adaptation to northern climes posed very serious challenges.  However, the deeper modern humans penetrated, the more megafauna disappeared.  During the first Northern Eurasian migrations wooly mammoths, wooly rhinoceros, cave lions, cave bears and hyenas all vanished.  Recent studies have even incorporated the disappearance of Neanderthals into the larger megafaunal overkill hypothesis (Hortola & Martinez-Navarro, 2012).

Australia

The second of the Quaternary mass extinctions occurred in Australia starting 50,000 years ago.  These extinctions coincided with the arrival of the first Australians.  Marsupial rhinos, giant wombats, giant kangaroos, lioness-sized marsupial carnivores and giant monitor lizards were among some of the most striking casualties.  The eradication of these fauna completely changed the ecological landscape (Rule et al., 2012), however some equilibrium was reached 40,000 years ago when the rest of Australia’s megafauna adapted to the human presence.

The Americas

The next major Quaternary mass extinctions occurred in the Americas in between 14,000-10,000 years ago.  This event seems to mirror what happened in Australia, only on a larger and quicker scale.  The fact that the extinctions happened on a larger scale is self-explanatory; the Americas are much larger landmasses than Australia.  However, the speed at which these extinctions happened (~4,000 years as opposed to ~10,000 years) is striking and needs to be explained.  For me, I think it happened quicker because the modern humans that migrated into the Americas had more complex tool technology than the modern humans that migrated into Australia.  Either way, after humans radiated throughout the Americas, American horses, western camels, the giant beaver, saber-toothed cats, American lions, American cheetahs, mammoths, mastodons, South American bears and many other megafauna that have no contemporary analogues disappeared within 4,000 years.

Island Extinctions

When we move into the island extinctions that started 2,000 years ago, the extinction events speed up even faster.  Technically, when we discuss these extinctions we are moving away from “Quaternary” mass extinctions because they occurred in the geologic epoch known as the Holocene.  However, the extinction events on many Mediterranean Islands, New Zealand, Pacific Islands, Madagacar, Indian Ocean Islands and the Caribbean Islands display the same extinction patterns that occurred in Northern Eurasia, Australia and the Americas.  These extinction events are perfectly correlated with human arrival.  In Madagascar this happened 2,000 years ago, in New Zealand 1,500 years ago and in the Indian Ocean 500 years ago.  During these events megafauna was exterminated within decades, as opposed to millennia or centuries.  Madagascar lost giant tortoises, giant fossa, Malagascar crowned eagles and 17 of the largest lemurs, almost immediately on a geologic scale.

The Sixth Mass Extinction

Is it best to view the Quaternary Mass Extinction as one-continuous mass extinction that is still happening today?  Are we in the middle of the sixth major mass extinction in the natural history of life on Earth?  Some scholars are beginning to envision it as such (e.g., Barnosky, 2008; Levy, 2011).  Humans are a new type of species.  When we migrated into new landscapes the total megafauna biomass crashed and suddenly disappeared.  When the ecosystems reorganized it was in a new state concentrated around one species, instead of multiple (Barnosky, 2008).  Today, regions of Africa and Asia still have megafauna.  However, we demand so much space, energy, and, resources that almost all megafauna are on the brink of extinction.  If we don’t rethink our relationship to our environment, the 21st century may represent the true end point of a megafaunal extinction event that has been 80,000 years in the making.

References

Barnosky, A.D.  2008.  Megafauna biomass tradeoff as driver of Quaternary and future extinctions.  Proceedings of the National Academy of Sciences, 105: 11543-11548.

Grund, B.S., Surovell, T.A. & Lyons, K.  2012.  Range sizes and shifts of North American Pleistocene mammals are not consistent with a climatic explanation for extinction.  World Archaeology, 44: 43-55.

Hortola, P. & Martinez-Navarro, B.  2012.  The Quaternary megafaunal extinction and the fate of Neanderthals: An integrative working hypothesis.  Quaternary International doi: 10.1016/j.quaint.2012.02.037

Johnson, C.N.  2009.  Ecological consequences of Late Quaternary extinctions of megafauna.  Proceedings of the Royal Society.  doi: 10.1098/rspb.2008.1921

Levy, S.  2011.  Once and Future Giants: What Ice Age Extinctions Tell Us About the Fate of Earth’s Largest Animals.  Oxford University Press.

Lime-Ribeiro et al. 2012.  Human arrival scenarios have a strong influence on interpretations on the late Quaternary extinctions.  Proceedings of the National Academy of Sciences.  109, E2409-E2410

McHenry, H.M.  2009.  Human Evolution.  In: Michael Ruse & Joseph Travis.  Evolution: The First Four Billion Years.  Cambridge, Massachusetts: The Belknap Press of Harvard University Press.

Prescott et al. 2012. Quantitative global analysis of the role of climate nad people in explaining late Quaternary megafaunal extinctions.  Proceedings of the National Academy of Sciences.  109, 4527-4531.

Rule, S., Brook, B.W., Haberle, S.G., Turney, C.S.M., Peter Kershaw, A. & Johnson, C.N.  The aftermath of megafaunal extinction: ecosystem transformation in Pleistocene Australia.  Science, 335: 1483-1486.

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About Cadell Last
I am a science educator, freelance science writer, and founder of The Advanced Apes based in Toronto, Ontario. In the past my academic research focused on the evolution, ecology, and behaviour of non-human primates (i.e., chimpanzees, gorillas, ring-tailed lemurs). Currently, my official blog, The Ratchet, can be found via The Advanced Apes and Svbtle. I enjoy exploring recent research in human evolutionary sciences, as well as biology, ecology, astronomy, physics, and computer science. My work has been featured in Scientific American, American Humanist, Richard Dawkins Foundation for Reason and Science, and Jane Goodall Institute of Canada. I am also exploring science popularization in new mediums in collaboration with PBS Digital Studios with an animated YouTube channel. You can contact me on Twitter (@cadelllast) or via email: cadell.last@gmail.com

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