Humans are really bad for the environment. But just how long we've been bad for it can be surprising. As far back as 50,000 years ago, humans arriving in a region has been linked with a lot of the local animals going extinct. However, human arrivals are also correlated with environmental change (perhaps explaining how we were able to reach the region in the first place). This has led to ongoing debate over which factor was more to blame for this wave of extinctions. Us, or the planet.
Don't hold your breath we'll be redeemed. It was us all along. A growing body of evidence is showing human arrival is much more strongly correlated with species' extinction than the environment. Throw away that mental image of the hunter-gatherer living in balance with their surroundings. We all screwed up.
But I suppose admitting you have a problem is the first step to fixing it. Hello, I'm Homo sapiens and I haven't killed off a species for 422 days.
We're to blame for extinctions
100,000 years ago the planet was full of megafauna. Large mammals that roamed the earth making for cool looking backdrops. You can still get a feel for how impressive these must have been in Africa. This continent is one of the few that retains a large and diverse megafauna population (although our species is trying our hardest to ruin even that).
It has been hypothesised that the survival of such megafauna in Africa is because they evolved alongside us. Thus, they were adapted to us and knew to avoid our point spears. But as we migrated out of Africa we encountered unfamiliar species. By the time they learned the danger it was too late. We ploughed through the world, leaving behind a wave of extinctions. By the time we'd finished colonising the world at least 90 species of megafauna were extinct. In some regions, this amounted to 95% of the locals. In Australia, everything weighing more than 100 kg went extinct when we arrived.
Given the strong connection between these extinctions and our arrival, it's not exactly a new idea that we're to blame. In fact, I've written about it before. People have tried to implicate the environment, but the charges just don't stick. For more than a decade a new research paper comes out. The data is combed through with an even finer, more reliable toothcomb. More resolution and accuracy is introduced into the results. But the conclusion is always the same. Humans are so strongly correlated with most of these extinctions we're definitely to blame.
The last such paper came out in 2015, examining the link between megafana extinction and human arrival across 19 regions in the world.
A note on timing
As this latest work shows, the correlation between human arrival and megafauna extinction is strong. Sometimes environment played a role. Sometimes we both did. But for the most part, we were the ones driving things extinct.
When discussing such anthropogenic disasters it's easy to imagine it happening quite quickly. Humans rock up in a region and start burning everything down, murdering every animal they see, and just generally making the Lorax sad. But in reality, these changes took a long time. In most places, the time between humans arriving and the megafauna going extinct was close to 5,000 years.
Now, when you're looking at things over millions of years of evolution that is impossibly fast. It's easy to see the link between us and the extinction. But from a human perspective, it's unimaginably ancient. There would be more time between humans arriving in Australia and their giant wombat going extinct than there are between us and the pyramids. Entire cultures could have developed and died in the time it took us to drive these animals to extinction.
In this context its easy to see why they didn't stop. They just might not have noticed a major change in their lifetime. Until one day, there was one.
It's a bit more complicated than that
The idea that humans drove so many species extinct so long ago is hardly new. But that doesn't mean there isn't more to find out. Notably, the way the earth was hit unequally by our spread. As I mentioned, Africa emerged relatively unscathed. And whilst Eurasia had its fair share of extinctions, few can be clearly linked to human arrival. Meanwhile, the rest of the world was hit hard.
Some point to adaptation as the answer. These animals evolved alongside humans in Africa (and Neanderthals in Europe). They had a chance to evolve defences and fears before we got to be truly dangerous. As such, they were able to survive our attack. However, this isn't the only idea out there. Another posits that humans were having too much trouble surviving in Europe to have much of an impact. We were travelling further north than we ever had before. We were competing with the Neanderthals for resources. All of this hardship kept our impact low until animals had either begun to adapt to us or died off to other factors. But then we mastered the north, so there was nothing stopping us hitting North America with everything we had.
Yet another idea suggests we might not even have been directly involved too much. Large herbivores need a lot of territory, so if we cut them off from large swathes of it (by hunting them if they came close) we could have made them suffer indirectly. In regions with a lot of territory, like Eurasia and Africa, this would have been less of a problem. But in places like Australia where there's less habitable land we could have had a huge indirect impact.
There's still a lot of mystery, and that's just examining extinctions were involved in. Around 1/3 of megafauna species dying out during this period aren't linked to us or the environment. What's going on there?
Conclusion
There's still a lot we don't know about the last pre-modern wave of extinctions. Why were some regions more severely impacted than others? Why did some animals die out well after human/environmental change occurred? On-going research is helping narrow down these mysteries. But one thing remains clear: we played a big role in most of these species' disappearance. Can we learn from our past mistakes?
References
Abramson, G., Laguna, M.F., Kuperman, M.N., Monjeau, A. and Lanata, J.L., 2015. On the roles of hunting and habitat size on the extinction of megafauna. Quaternary International.
Araujo, B.B., Oliveira-Santos, L.G.R., Lima-Ribeiro, M.S., Diniz-Filho, J.A.F. and Fernandez, F.A., 2015. Bigger kill than chill: The uneven roles of humans and climate on late Quaternary megafaunal extinctions. Quaternary International.
Koch, P.L. and Barnosky, A.D., 2006. Late Quaternary extinctions: state of the debate. Annu. Rev. Ecol. Evol. Syst., 37, pp.215-250.
Sandom, C., Faurby, S., Sandel, B. and Svenning, J.C., 2014, July. Global late Quaternary megafauna extinctions linked to humans, not climate change. In Proc. R. Soc. B (Vol. 281, No. 1787, p. 20133254). The Royal Society.
