In this issue:
"Mystery Human": An astonishing trove of fossils has scientists, and the media, in a tizzy over our origins.
"The Puzzle of Dark Energy": Why is the expansion of the universe accelerating? After two decades of study, the answer is as mysterious as ever, but the questions have become clearer.
"Brain Drain": An internal plumbing system rids the brain of toxic wastes. Sleep is when this cleanup ritual occurs.
"Syria's Climate Refugees": Farmers who have escaped the embattled nation explain how drought and government abuse have driven social violence.
©2015 Scientific American