A recent survey of the DNA of over 100,000 of the Earth’s animal species, including modern humans, has yielded a shocking result: 90 percent of all extant species arose at the same time, between 100,000 and 200,000 years ago, upending the assumption that most creatures would have reached their modern forms at different point throughout the planet’s history. The survey also found that genetic diversity between different species doesn’t increase over time–meaning modern humans haven’t diverged genetically over the course of our history from other species at all.

The survey was conducted by Rockefeller University’s Mark Stoeckle, and David Thaler, from Switzerland’s University of Basel. Their research employed a novel process called "DNA barcoding": the COI gene, one of the 37 genes contained in mitochondrial DNA (mDNA), can be used to identify the genes of different species, including those of species that are otherwise closely related, due to the gene’s typically fast mutation rate; because of the recordable individuality of this gene, it can be used to identify a species in a manner similar to how a carton of milk can be identified at the checkout of a grocery store by reading its barcode.

Unlike the genes contained in nuclear DNA, of which can display massive differences from species to species, mitochondrial DNA remains comparatively constant across different species, giving researchers a common basis for when they need to compare the genes of different organisms. "The mitochondrial sequence has proved perfect for this all-animal approach because it has just the right balance of two conflicting properties," remarks Thaler.

Stoeckle and Thaler used this DNA barcoding method to analyze over five million genetic samples representing 100,000 species, and found that most animal species– 90 percent of them–emerged at about the same time as modern humans, somewhere between 100,000 and 200,000 years ago.

But what caused the sudden emergence of over 90,000 species at the same time, nearly 200 millennia ago? Jesse Ausubel, director of the Program for the Human Environment at The Rockefeller University, says that environmental trauma might be the cause. "Viruses, ice ages, successful new competitors, loss of prey—all these may cause periods when the population of an animal drops sharply," Ausubel explains "In these periods, it is easier for a genetic innovation to sweep the population and contribute to the emergence of a new species."

The last great extinction event occurred 65 million years ago at the end of the Cretaceous period, when three-quarters of the planet’s species disappeared following the impact of a massive asteroid; the time period illustrated in Stoeckle and Thaler’s survey, however, falls right in the middle of the Pleistocene, a period that didn’t see any major extinction events until the start of the modern Holocene, when the climate warmed significantly–an extinction event that didn’t begin until nearly 100,000 years after the survey’s projected period for the emergence for modern species.

The Pleistocene was, however, a period that experienced a series of repeated glacial cycles, when ice sheets made numerous advances and retreats across the Earth’s surface. Perhaps it was this steady climatic pressure that affected so many of the world’s species, en masse?

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