We can all be picky at times, but you probably think of this planet as reasonably bearable. In a new model for what constitutes a habitable zone, however, the Earth barely scrapes over the bar.
Erratic changes in the East African climate around two million years ago may have been the driving force behind human evolution, say researchers at Penn State and Rutgers University.
Despite being separate species, polar bears and brown bears interbred during periods of global warming in the past - and appear to be doing the same today.
