As more and more species go extinct, biologists wonder whether we are on the verge of the earth's sixth mass extinction. A new study, by Marc Carrasco and Tony Barnosky of the UCMP and Russell Graham of Pennsylvania State University, uses the fossil record to examine mammal biodiversity in North America over the past 30 million years. Carrasco and his collaborators used data from two fossil databases, MioMap and Faunmap, to determine the baseline of mammal diversity before humans arrived in North America. Their results, published in the journal PLoS ONE, show that the arrival of humans 13 thousand years ago coincided with a 15 to 42% decline in mammal diversity. These data show that humans had a negative impact on mammals long before we factor in the effects of current industrialization and global climate change. Now that a pre-human baseline of North American mammal diversity has been established, we can compare current diversity to the continent's "normal" diversity level — an important comparison as we plan and evaluate conservation efforts in the future.