Humans have had such a profound impact on the planet’s ecosystems and climate that Earth might be defined by a new geological epoch: the Anthropocene (where “anthropo” means “human”).
Some think this new epoch should start during the Industrial Revolution, and some at the advent of agriculture 10,000 to 15,000 years ago. This feeds into the popular notion that environmental destruction is a recent phenomenon.
The lives of our ancestors are often romanticized. Many think they lived in balance with nature, unlike modern society where we fight against it. But when we look at the evidence of human impacts over millennia, it’s hard to see how this was true.
Our ancient ancestors contributed to the extinction of many of the world’s largest mammals (‘megafauna’). This was during an event known as the Quaternary megafauna extinction (QME).
The extent of these extinctions across continents is shown in the chart. Between 52,000 and 9,000 BCE, more than 178 species of the world’s largest mammals were killed off. These were mammals heavier than 44 kilograms, ranging from mammals the size of sheep to mammoths.
There is strong evidence to suggest that these were largely driven by humans – we look at this in more detail later.
Africa was the least hard-hit, losing only 21% of its megafauna. Humans evolved in Africa, and hominins had already interacted with mammals for a long time. The same is also likely to be true across Eurasia, where 35% of megafauna were lost. But Australia, North America, and South America were particularly hard-hit; very soon after humans arrived, most large mammals were gone. Australia lost 88%; North America lost 83%; and South America, 72%.
Far from being in balance with ecosystems, tiny populations of hunter-gatherers changed them forever. By 8,000 BCE – almost at the end of the QME – there were only around 5 million people in the world.
The driver of the QME has been debated for centuries. Climate and human impacts have been proposed as potential drivers.
One possibility is that both played some role in the downfall of the mammals. This could be through overhunting, the reshaping of landscapes through fire, or the introduction of invasive species.
There are several reasons why we think our ancestors were at least partly responsible.
Extinction timings closely match the timing of human arrival. The timing of megafauna extinctions was not consistent across the world; instead, the timing of their demise coincided closely with the arrival of humans on each continent. The timing of human arrivals and extinction events is shown on the map below.
Humans reached Australia somewhere between 65 to 44,000 years ago.1 Between 50 and 40,000 years ago, 82% of megafauna had been wiped out. It was tens of thousands of years before the extinctions in North and South America occurred. And several more before these occurred in Madagascar and the Caribbean islands. Elephant birds in Madagascar were still present eight millennia after the mammoth and mastodon were killed off in America. Extinction events followed in man’s footsteps.
QME selectively impacted large mammals. There have been many extinction events in Earth’s history. There have been five big mass extinction events and several smaller ones. These events don’t usually target specific groups of animals. Large ecological changes tend to impact everything from large to small mammals, reptiles, birds, and fish. During times of high climate variability over the past 66 million years (the ‘Cenozoic period’), neither small nor large mammals were more vulnerable to extinction.2
The QME was different and unique in the fossil record: it selectively killed off large mammals. Now there are obvious reasons why larger mammals are at a greater risk of extinction from any cause: they are slower to reproduce, so declines or crashes in their populations take longer to recover from.
But there is also a strong bias to human pressures: humans selectively hunt the larger ones.
Islands were more heavily impacted than Africa. As we saw previously, Africa was less heavily impacted than other continents during this period. We might expect this since hominids had been interacting with mammals for a long time before this. These interactions between species would have impacted mammal populations more gradually and to a lesser extent. They may have already reached some form of equilibrium. When humans arrived on other continents – such as Australia or the Americas – these interactions were new and represented a step-change in the dynamics of the ecosystem. Humans were efficient new predators.
There have now been many studies focused on the question of whether humans were a key driver of the QME. Many suggest that the answer is yes. Climatic changes might have driven an initial decline in large mammal populations – small population crashes – but human pressures are likely to have thwarted their recovery. Large mammals survived previous periods of climatic change, but the arrival of humans put pressure on already-depleted populations.
Human impact on ecosystems dates back tens of thousands of years, despite the Anthropocene paradigm implying this is a recent phenomenon. We’ve not only been in direct competition with other mammals, but we’ve also reshaped the landscape beyond recognition.