The reason for this is human activity, which has always been detrimental in nature that changed it and made it uninhabitable, reports the Proceedings of the National Academy of Sciences.
If conservation efforts are not vastly improved, mammal species will die off so quickly in the next 50 years that it will take nature 3-5 million years to recover, researchers from Aarhus University, Denmark, and the University of Gothenburg, Sweden, find.
Another study published in the Proceedings of the National Academy of Sciences, predicts it will take at least another two million years to be restored to the level it was before the arrival of modern humans - that's five to seven million in total.
What is happening today with the biosphere of the Earth, many scientists boldly called the "sixth mass extinction".
And, it would take 3 to 5 million years to reach the levels they're at now, even if humans cease destructive practices. When habitats and climates change, species which die out are replaced by newly-emerging species. Succeeding each mass extinction evolution has developed anew to diffuse the gap created.
The analysis, published Monday in the Proceedings of the National Academy of Sciences, specifically focused on mammals that now exist as well as those which went extinct as humans spread across the globe, but it provides insight on the broader biodiversity crisis.
They used computers, evolutionary simulations and their huge data sets to calculate the amount of evolutionary time that would be lost from past and future extinctions and to assess how long recovery would take. But even in that best-case scenario, the timeline depends on how quickly mammals start recovering. There are hundreds of species of shrew, so if half of them should become extinct, it wouldn't kill out the entire shrew population on Earth. Since then, humans have helped erase another 500 million years ― and an additional 1.8 billion years could be lost in the next five decades if the high rate of mammal extinctions continues.
Mr Davis said: "Large mammals, or megafauna, such as giant sloths and sabre-toothed tigers, which became extinct about 10,000 years ago, were highly evolutionarily distinct".
'Since they had few close relatives, their extinctions meant that entire branches of Earth's evolutionary tree were chopped off. So when they all went extinct, many years of evolutionary history disappeared with them. "There were only four species of a saber-toothed tiger; they all went extinct", said paleontologist Matt Davis from Aarhus University. Scientists now argue that we're the brink of a sixth mass extinction - but while previous events were triggered by calamities, such as massive volcanic eruptions or an asteroid impact, this time the vector is a single species: Homo sapiens.
Large mammal species, the researchers warn, are disproportionately at risk of dying out altogether.
The findings make a case for strengthening conservation efforts now because simple math dictates that it's much easier to save biodiversity than to wait for it to evolve later.
"The few remaining giants, such as rhinos and elephants, are in danger of being wiped out very rapidly", he continued.
Though their calculations regarding extinction on Earth are still dire, the scientists think their work could be used to figure out which endangered species are evolutionarily unique and represent the most important parts of evolutionary history on Earth.