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.
"Significant climate changes were happening at the very time the first humans were arriving in the continents, which makes it difficult to determine which factor made the most difference in megafaunal survival," explains overkill proponent Gary Haynes of the University of Nevada in Reno.
Some scientists are quick to point out that there are problems with applying the overkill hypothesis to an area like North America. For instance, there have been very few kill sites found in the Americas that demonstrate that humans were responsible for the death of large numbers of mammals. In fact, some studies suggest only two megafaunal species were hunted extensively in North America: mammoths and mastodons.
Why would humans have overhunted 23 species but not the other nine?
What's more, during the period of ice age extinctions in North America, the human population is thought to have been too low to have caused widespread extinctions, and these humans did not have the tools available to kill huge mammals in great numbers.
One of the greatest critics of the overkill hypothesis, David Meltzer of the Southern Methodist University in Dallas, Texas, feels there is not enough evidence to show that humans were to blame for the American ice age extinctions.
In a 2015 review, Meltzer spends 25 pages trying to destroy the overkill theory by poking holes in the data. For instance, he points out that, out of 32 genera of megafaunal mammals that were present in North America during the last ice age, nine survived – and there is no clear reason why this should be the case under the overkill hypothesis.
Why would humans have overhunted 23 species but not the other nine? And why would they have hunted certain species to extinction when other, more abundant species would have surely been easier to find?
Overkill proponents are quick to defend their theory against these attacks.
Haynes, writing in 2007, argues that arguments against the overkill hypothesis are "from ignorance, based on either incomplete knowledge, prejudiced prior opinions, or misrepresentations". He says that, just because we do not have strong evidence for the overkill hypothesis, that does not mean it is not true. His idea invokes a common saying in science: an absence of evidence should not be confused with evidence of absence.
As a rather surprising analogy, he uses the Bigfoot mystery. There is an absence of evidence in terms of preserved remains of the mythological giant ape. But strictly speaking that is not the same as saying there is evidence that Bigfoot does not exist.
The main factor in the die-outs was the presence of humans
Likewise, there might be an absence of evidence that humans hunted a wide variety of large mammals. But that should not be confused with evidence that they did not, and that such hunting was a bigger impact than climate change in the extinction of those animals.
"I give more weight to humans," Haynes says, although he admits that climate change could well have played a part.
"It can't be doubted that changes in climate and vegetation were hard on megafauna populations, both regionally and continentally," he says. "But the inexorable disappearance of so many large mammals in different continents, in synchrony with the expansive spread of modern Homo sapiens to those continents, is enough circumstantial evidence to convince me that the main factor in the die-outs was the presence of humans."
Haynes is not the only firm believer in the overkill theory. Todd Surovell, an archaeologist at the University of Wyoming in Laramie, is also a proponent. He points to the strong correlation between the arrival of humans into areas and subsequent waves of extinctions.
Surovell also says there is not enough evidence to support the climate change theory.
"We have no clear cases of large-scale extinctions of animals in the absence of humans. Mammoths, for example, survived into the Holocene in oceanic islands not reached by humans," he says. "Unless climate change perfectly tracked human global colonisation, it cannot explain extinctions in different places at different times."
Merely pointing out the problems with the climate argument does not prove that it must have been humans
Haynes also wonders how climate change could be the sole culprit when "so many of the extinct genera survived numerous climatic reversals over the last two to three million years that don't seem so very different from the last one when the extinctions occurred".
"Another problem with climate change is that some extinctions seem to have happened when no major climate changes were occurring," he says.
Ross Barnett, a Pleistocene extinction expert at the University of Copenhagen in Denmark, agrees with Haynes. "The Pleistocene/Holocene transition was not fundamentally different from many of the previous glacial/interglacial changes - except that modern humans were around," he says.
But other researchers disagree. "Merely pointing out the problems with the climate argument does not prove that it must have been humans," says Tony Stuart, a megafaunal extinction expert from Durham University, UK. Stuart does, however, believe that the climate change theory is not clear-cut and that "much more work is needed to determine if there was something unique about the Last Glacial".
Other experts think neither the overkill nor climate change argument is correct.
The disease theory, proposed by Ross MacPhee and Preston Marx, suggests that a "hyperdisease" was introduced to native mammals by migrating humans, or possibly their dogs. It particularly affected larger mammals, proponents suggest, because smaller-bodied species are more resilient as a result of their larger population sizes and shorter gestation times.
There's a strong philosophical resistance to the idea that pre-industrial hunter-gatherers were not conscious conservationists
For the disease to have wiped out so many species, it would have to have been: 1) able to exist outside the host, 2) highly contagious, 3) able to infect many species and 4) able to kill at least 50% of those infected.
However, this idea does not appear to have much evidence to support it, so it is regarded by many as being an unlikely main cause of the ice age extinctions.
Unpicking the arguments for and against the different theories to explain the megafaunal extinctions, one is left wondering how much of the debate is due to differences of opinions and how much is due to opposing ideologies.
"There's a strong philosophical resistance to the idea that pre-industrial hunter-gatherers were not conscious conservationists," says Haynes.
Other researchers feel the disagreement has turned into a catfight. "Those loudly claiming that humans could never be the cause seem as ideologically driven as some on the overkill side," says Barnett.
We all see the same evidence, but we interpret it in different ways
Stuart also believes that the different arguments are fuelling conflict between researchers. "It is especially wrong to move on as if the question of cause has been solved," he says. He feels this is "stifling further investigation and discouraging alternative views".
Could it be that the extinctions were caused by both a changing climate and overhunting by humans? This does seem to be the case for the late Pleistocene megafaunal extinctions in some parts of the world – particularly South East Asia – so it is possible that the combination may also have been the downfall of megafauna in other areas.
"It is possible there is no single explanation for every extinction event," says Surovell.
Stuart thinks that the issue is far from solved. "These are very complex issues and the evidence is inadequate for most parts of the world, including North America," he says. "So it comes down to how you interpret the available evidence. My view is that it is entirely premature to come to definite conclusions given our present state of knowledge."
Surovell agrees: "We all see the same evidence, but we interpret it in different ways."
The mystery surrounding the cause of the last ice age extinction might not have been solved yet, but the current mass extinction is unquestionably due to us.
Given that the climate is now changing rapidly and our destructive activities on the environment continue unabated, perhaps Pleistocene scientists will finally come to a consensus and agree that these combined factors are what drives multiple groups of species to global extinction.
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