A photo provided by Rob Harbert shows part of an old pack rat’s nest, or midden. Since the 1960s, scientists have examined thousands of these middens to learn about regional changes in climate and ecosystems over time. Now, with advanced molecular technology, they have been able to work out what lived in part of the Western U.S. tens of thousands of years ago by studying DNA found in the nests. (Rob Harbert via The New York Times)