Source: BBC

By Niki Rust

There might be as many as 10 million species of complex life on this planet today – a huge number. But add up all of the complex species that ever lived and some biologists think the grand total would be about five billion.

The estimate leads to an astonishing conclusion: a staggering 99% of species are not around any more. They have been driven to extinction.

More species are joining the ranks of the extinct every year. Many scientists believe we are living through an episode of remarkably rapid extinction, on a scale that has been seen only five times in the last half a billion years.

They call this current episode the sixth mass extinction – a large, global decline in a wide variety of species over a relatively short period of time. And they tend to agree that humans are the main cause.

Overhunting, overfishing, and human-driven habitat loss are pushing many species to the brink. In fact, we have changed the planet so much that some geologists are now suggesting that we have entered a new phase in Earth’s history; an epoch they call the “Anthropocene“.  By 2100, it is expected that humans will have caused the extinction of up to half of the world’s current species.

Because we are living through this extinction, it is relatively easy for us to study the driving forces behind it. But how do we determine what caused other mass die-offs that happened long ago? To do so we have to look at what archaeologists, palaeontologists, geologists and other scientists have concluded from the evidence they have gathered.

The trouble is, those scientists do not always agree with one another – even about the most recent extinction event.

As well as the five – or six – mass extinctions, there have also been many smaller extinctions.

One of these mini extinction events happened towards the end of the Pleistocene, a few tens of thousands of years ago. It is sometimes called the “megafaunal” extinction because many of the species it claimed were particularly large animals, weighing more than 97lb (44kg). However, its cause remains a bone of contention amongst scientists – pun intended.

Global temperatures are thought to have soared by about 6C

The problem in trying to untangle the cause of the Pleistocene megafaunal extinction is that the evidence is scanty, so there has been a protracted debate amongst scientists about how best to interpret it.

To complicate things further, the Pleistocene extinctions that happened in some areas of the world seemed to have occurred at a much slower rate than those in other areas, and the environmental conditions and human activity levels also differed.

One popular argument to explain the extinctions is that they were due to climate change. Our planet was beginning to emerge from the last ice age as the extinctions began. Global temperatures are thought to have soared by about 6C – a change that would have affected larger animals more as they cannot lose heat as fast as smaller animals.

On top of this, the climate is thought to have been more changeable at the time, with swings from very wet to very dry conditions. This could have exacerbated megafaunal extinctions. Because mammals from the ice age would have likely had thick fur coats, they would have found it difficult to adapt to the changing climate.

Overhunting could have been the culprit for the American and Australian megafaunal extinctions

The other main school of thought blames humans for the demise of the ice age megafauna. This is the hunting hypothesis, which first emerged way back in the 1870s after it was discovered that humans had lived alongside mammoths.

However, later evidence showed that the extinctions in Eurasia took place over too long a time for overhunting to be the main cause. At that point, the disagreement over the cause of the extinction began to emerge.

To muddy the waters further, the possible causes for the extinctions do not stop at climate change and overhunting. Others think contagious and deadly diseases – carried by migrating humans or their animals – may have been the main culprit. So who is correct?

Today, there are some researchers who believe that overhunting could have been the culprit for the American and Australian megafaunal extinctions. This “overkill” theory rose to fame in the late 1960s through the work of the late Paul S. Martin, a geoscientist from the University of Arizona.

It is far from clear that the overhunting argument can explain the megafaunal extinctions

It is generally accepted that overhunting was the main cause of extinctions in Australia and New Zealand. The climate in this region at the time of the extinctions was roughly the same as it is now and the species living at that time were arid-adapted.

Evidence shows that, as humans migrated into the area, they hunted the native fauna with ease. The indigenous animals had never seen humans before and were naïve to human hunting tactics. The arid environment was also very combustible and, with their impressive fire-starting skills, the first humans in the region could burn vast areas of habitat, contributing to the decline of the native species.

However, in other parts of the world it is far from clear that the overhunting argument can explain the megafaunal extinctions.