Life on Earth has experienced at least five major events we call “mass extinctions,” during which a huge number of species have gone extinct in a short period of time. In this Arnold Arboretum talk, paleontologist Phoebe Cohen will explore how scientists decide which extinctions get to be considered “mass,” the ways in which these events have reshaped life as we know it, and how a deep understanding of past extinctions can help us see the future. The lecture takes place in the Hunnewell Building on Thursday, February 26, from 7 – 8:15, and the fee to attend is $5 for Arboretum members, $10 for nonmembers. Register online at http://my.arboretum.harvard.edu/Info.aspx?EventID=1. Fee $5 member, $10 nonmember.  Dr. Cohen is Assistant Professor of Geosciences at Williams College.