ST. LOUIS, MISSOURI—An international team of anthropologists has discovered a well-preserved fossil of the hominin species Paranthropus robustus in South Africa's Drimolen cave system, according to a statement released by Washington University in St. Louis. Dating to about 2 million years ago, the cranium belonged to a male member of the small-brained species, which appeared during a dry climatic period between the disappearance of the more primitive Australopithecus and the emergence of early Homo species. The new fossil is markedly smaller than other P. robustus specimens unearthed at the nearby site of Swartkrans, which was inhabited some 200,000 years later than Drimolen. The researchers suggest that the anatomical differences between the populations, including changes in cranial and tooth size, are evidence of a rapid evolution within the species in response to environmental stressors. To read about Homo naledi, a transitional species between austraolpithecines and early humans, go to "A New Human Relative," one of ARCHAEOLOGY's Top 10 Discoveries of 2015.