In its roughly 4.5 billion year history, the Earth has seen five major mass extinction events when at least 75 percent of the species that inhabited it disappeared. The last, between the Cretaceous and Peleogene periods 65 million years ago, killed the dinosaurs .

A growing number of scientists warn were on the cusp of, or witnessing the early stages of, the sixth.

In a study published Monday in the Proceedings of the National Academy of Sciences, biologists from Stanford University and the Universidad Nacional Autnoma de Mxico investigated the health of Earths vertebrate population and described their findings as biological annihilation.

What is at stake is really the state of humanity, Gerardo Ceballos, one of the studys authors and an ecology professor at the Universidad Nacional Autnoma de Mxico, told CNN .

Ceballos based that alarming conclusion on a sample of 27,600 vertebrate species (roughly half of all that exist) and a more thorough assessment of 177 mammal species, whose populations he and his co-authors tracked from 1900 to 2015.

A full 32 percent of the vertebrate species (8,851) they studied are decreasing in both population and geographic range. And more than 40 percent of the mammals are undergoing severe population declines as defined by the loss of more than 80 percent of their range.

Fortunately, most of those species have not gone extinct. However, the study said, their populations even among species classified as low concern are dropping dramatically, and at a fast enough rate to justify sounding the alarm.

When public mention is made of the extinction crisis, it usually focuses on a few animal species (hundreds out of millions) known to have gone extinct, and projecting many more extinctions in the future, the authors said. But a glance at our maps presents a much more realistic picture: They suggest that as much as 50% of the number of animal individuals that once shared Earth with us are already gone.

Given how short a human lifespan is relative to the length of a mass extinction event, its difficult to comprehend broad ecological change, the study cautioned. But these changes can cascade throughout ecosystems, adding up to fundamental and potentially catastrophic ecological, economic and social shifts, they said:

When considering this frightening assault on the foundations of human civilization, one must never forget that Earths capacity to support life, including human life, has been shaped by life itself.

While major threats to the health of the global ecosystem include habitat conversion, climate disruption, overexploitation, toxification, species invasions, and disease, the authors argued the two biggest drivers of population loss are often much less frequently mentioned: human overpopulation and overconsumption, especially by the rich.

These drivers, all of which trace to the fiction that perpetual growth can occur on a finite planet, are themselves increasing rapidly, the study concluded. Thus, we emphasize that the sixth mass extinction is already here and the window for effective action is very short, probably two or three decades at most.

Dr. Stuart Pimm, an ecologist at Duke University who was not involved with the study, disputed the authors assertion, classifying it more as a highly likely future outcome.

It is something that hasnt happened yet. he told The Guardian . We are on the edge of it.