Prof Douglas Futuyma at Stony Brook University in the USA, who was not part of the research team, said: "They have made a dramatic and convincing statement of how much evolutionary diversity has already been lost".
Sadly, the team's estimation of a three to five million year recovery time is a best-case scenario. If mammals expand at their normal rate it will still take them 5-7 million years to establish biodiversity to its levels prior to evolution of modern humans and 3-5 million years just to reach present biodiversity levels as per analysis.
In the past half-billion years, there have been five major mass extinction events that annihilated most of life on Earth. The worldwide Union for conservation of nature predicts that 99.9% of species under threat of extinction, and 67% of endangered species will be lost over the next 100 years. As habitats and climates change, species that can't survive die, and new species slowly emerge. Succeeding each mass extinction evolution has developed anew to diffuse the gap created.
For their calculations, the Aarhus University researchers used a database containing existing mammal species and mammals that already went extinct as humans spread across the planet. Then they calculated diversity loss since the Last Interglacial about 130,000 years ago, a preanthropogenic baseline, and determined how likely it is now threatened species will go extinct in the near future.
In order to establish the time required for mammalian species to recover to biodiversity levels seen before humans entered the big picture, the researchers performed advanced evolutionary simulations on powerful computers. If the extinction rate doesn't start falling for another 20-100 years, more species will likely disappear, causing greater diversity loss, the study said. This rate of extinction is increasing over the course of the last few years.
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". On the other hand, scientists say there were only four species of saber-toothed tigers, so when they became extinct, many years of evolution disappeared with them. The authors note that the disappearance of different species may have different consequences from the point of view of biodiversity: for example, the death of one dog is not much affected even in this family, but the death of the South American macrauchenia, exotic ungulates, closes the whole window of opportunities for further evolution and a potential branch on the "tree" of mammals.
"If you look on a more local scale, many species aren't extinct, they have just had their ranges reduced by humans", said Davis. The upcoming sixth mass extinction, however, is largely the work of humans.
"We once lived in a world of giants: giant beavers, giant armadillos, giant deer, etc., we now live in a world that is becoming increasingly impoverished of large wild mammalian species". "It is much easier to protect biodiversity than to fix it later", adds Matt Davis.
"The few remaining giants, such as rhinos and elephants, are in danger of being wiped out very rapidly", he continued.
While the findings are sobering, researchers hope that their work can be used to help prioritize conservation work to protect evolutionarily unique species most at risk but it's still going to be a massive undertaking on our part.