Environmental changes, not the often-blamed ancestors of modern humans, led to the several-million-year decline of east African megaherbivores -- large-bodied mammals like elephants, rhinos and hippos-- a new study finds. The results suggest that anthropogenic impacts on natural systems are unique to modern Homo sapiens. Africa is home to many of Earth's modern megaherbivores; however, despite this diversity, the region has witnessed a decline in the diversity of these creatures over time. For decades, research has suggested that the ancient precursors of modern humans, hominins like Homo erectus, drove ecological shifts that led to extinction in large-animal communities in Africa. While the details differ, most competing hypotheses agree that tool-bearing pre-modern hominin hunters were an important culprit. Despite decades of research is this space, there have been few attempts to test the hypothesis that ancient hominins shaped African ecosystems, or to explore alternatives. In this study, Tyler Faith and colleagues challenge the traditional "ancient impacts" hypothesis. They analyze megaherbivore diversity in eastern Africa -- which features the longest, most well-documented history of hominin-mammal interaction in the world -- over the last 7 million years using present-day and fossil animal data. Faith et al.'s analysis revealed that the decline of megaherbivores began nearly 4.6 million years ago - more than a million years before the first evidence of meat-eating hominins and about 1.8 million years before the rise of Homo sapiens. What's more, the long-term decline of megaherbivores closely tracks with changes in atmospheric carbon dioxide and with an associated expansion of tropical grasslands, the authors say. The grassland expansion came at the cost of a diminished number of plant types that larger-bodied species depended on, according to the authors. In a related Perspective, René Bobe and Susana Carvalho critique the results and argue that the role of hominins is still open to question given the limitations of current archaeological and paleontological data.