The passenger pigeon. The Tasmanian tiger. The Baiji, or Yangtze river dolphin. These rank among the best-known recent victims of what many scientists have declared the sixth mass extinction, as human actions are wiping out vertebrate animal species hundreds of times faster than they would otherwise disappear.

Yet, an analysis from Stanford University and the National Autonomous University of Mexico, published in the Proceedings of the National Academy of Sciences, shows the crisis may run even deeper. Each of the three species above was also the last member of its genus, the higher category into which taxonomists sort species. And they aren't alone.

Up to now, public and scientific interest has focused on extinctions of species. But in their new study, Gerardo Ceballos, senior researcher at the Institute of Ecology at the National Autonomous University of Mexico, and Paul Ehrlich, Bing Professor of Population Studies, Emeritus, in the Stanford School of Humanities and Sciences, have found that entire genera (the plural of "genus" ) are vanishing as well, in what they call a "mutilation of the tree of life."

"In the long term, we're putting a big dent in the evolution of life on the planet," Ceballos said. "But also, in this century, what we're doing to the tree of life will cause a lot of suffering for humanity."

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