During the past 500 million years, there were five "mass extinctions" during which many species rapidly died.

Now scientists say we've entered a sixth mass extinction, and humans are the primary cause, according to a new study.

“This is the case of a biological annihilation occurring globally,” said co-author Rodolfo Dirzo, a professor of biology at Stanford University.

Previous mass extinctions were due to natural climate changes, huge volcanic eruptions or catastrophic meteor strikes. But this one is due to human activities such as deforestation, overpopulation, pollution, poaching and extreme weather events tied to man-caused global warming, the study said.

"The massive loss of populations and species reflects our lack of empathy to all the wild species that have been our companions since our origins," said the new study's lead author, Gerardo Ceballos of the National Autonomous University of Mexico. "It is a prelude to the disappearance of many more species and the decline of natural systems that make civilization possible."

The study suggests that as much as 50% of the number of animal individuals that once shared Earth have disappeared. Researchers determined that billions of mammals, birds, reptiles and amphibian populations have been lost worldwide.

This amounts to "a massive erosion of the greatest biological diversity in the history of Earth," the authors said.

Particularly hard hit have been the mammals of south and southeast Asia, where all the large-bodied species of mammals analyzed lost more than 80% of their geographic ranges.

Even if it's far from extinction, a species in decline can cause "cascading effects on vegetation and habitat" in ecological networks that depend on balance between animals, plants and microorganisms, scientist Robin Naidoo of the World Wildlife Fund told CBS News.

At the conclusion of the study, the authors write that "the resulting biological annihilation obviously will have serious ecological, economic and social consequences. Humanity will eventually pay a very high price for the decimation of the only assemblage of life that we know of in the universe.”

"All signs point to ever more powerful assaults on biodiversity in the next two decades, painting a dismal picture of the future of life, including human life," the researchers said.

The study appeared Monday in the peer-reviewed journal Proceedings of the National Academy of Sciences