Humans are driving the loss of entire branches of the "Tree of Life," according to a new study published on Monday which warns of the threat of a sixth mass extinction. "It is a really significant contribution, I think the first time anyone has attempted to assess modern extinction rates at a level above the species," Robert Cowie, a biologist at the University of Hawaii who was not involved in the study, told AFP.  "As such it really demonstrates the loss of entire branches of the Tree of Life,