Every species alive on Earth today is the descendant of survivors.
Every plant, every insect, every mammal, every bird, every fungus, every microbe — every living thing you have ever seen, or eaten, or been — came from a lineage that got through what most of the lineages around it did not. Five times in the past 450 million years, a large fraction of everything alive on the planet stopped being alive within a geologically short window. Around 75 percent of species is the standard threshold. Some of the events went considerably higher.
The most extreme was the End-Permian extinction, roughly 252 million years ago, in which approximately 96 percent of all species then in existence disappeared. Life very nearly ended. What survived spent the next several million years slowly rebuilding, from a base so small that the entire subsequent architecture of complex life — including the eventual evolution of mammals, and eventually of humans — depended on those specific few percent of lineages that happened to make it through.
The five mass extinctions have distinct causes. Massive volcanic eruptions. Rapid sea-level and climate shifts. In one case, an asteroid hitting the coast of what is now Mexico. What they share is that they were all triggered by forces outside the biological system itself.
Many biologists now argue that a sixth mass extinction is underway. What makes it different from the previous five is not primarily the scale — which has not yet reached the 75 percent threshold — but the fact that this time the cause is not external. It is a single biological species doing what asteroids and supervolcanoes have historically done. And it is doing it substantially faster.
The specific quality of the current event
The five past mass extinctions unfolded over what geologists call “short” periods. In geology, “short” typically means less than 2.8 million years, and sometimes considerably longer. The End-Permian took perhaps 60,000 years to complete its worst phase. The end-Cretaceous asteroid impact triggered its cascade over months to a few thousand years, but the full extinction sequence extended over hundreds of thousands of years afterward.
What is happening now, biologists like the Mexican ecologist Gerardo Ceballos have documented, is unfolding at a rate roughly 100 to 1,000 times higher than the natural background rate of species loss. The Natural History Museum in London puts the multiplier at 100 to 1,000. Ceballos and his colleagues at Stanford, in a series of high-profile papers in Science Advances and the Proceedings of the National Academy of Sciences over the last decade, have argued that even conservative estimates place the current rate at multiple orders of magnitude above baseline.
The specific quality that separates this from the previous five is time. The End-Permian event was the fastest of the previous mass extinctions, and it played out over tens of thousands of years. The current event, if it continues on its present trajectory, may play out over hundreds of years — a compression of a factor of roughly a hundred.
For most species on Earth, the difference between “50,000 years” and “500 years” is not a difference of degree. It is a difference of kind. Fifty thousand years is enough time for many species to adapt genetically to changing conditions. Five hundred years is not. Evolutionary adaptation has a specific speed limit set by generation time, mutation rate, and selection pressure. Anything faster than that is not adaptation. It is being outrun.
The debate biologists are actually having
There is a genuine scientific dispute within the field about whether the current situation technically meets the definition of a mass extinction. This is worth reporting honestly, because the popular framing sometimes overstates the settled nature of the science.
A 2025 paper in PLOS Biology by Wiens and Saban, drawing on species-loss data across multiple taxonomic groups, argued that the current rate of documented extinction, while alarmingly high compared to the background rate, does not yet meet the technical criteria for a mass extinction event. They noted that most recent extinctions have been concentrated in island habitats, and that overall extinction rates for mammals and birds may actually be decelerating due to conservation efforts.
The Wiens and Saban critique does not deny that biodiversity is declining sharply, or that human activity is the cause. The point is more narrow — whether the current decline meets the technical 75-percent-loss definition of a mass extinction as it is understood in palaeontology. On the underlying observations of accelerating biodiversity loss and human causation, there is broad scientific agreement.
The honest state of the science is roughly this. Extinction rates are undisputedly far above the geological background. The cause is undisputedly anthropogenic. Whether we have technically entered a sixth mass extinction, or are on a trajectory toward one that could still be diverted, is a live scientific dispute.
What one species is doing that the others couldn’t
The mechanism this time is worth being specific about, because it is genuinely unlike anything that has happened in the previous 450 million years.
Human activity has converted approximately half of the planet’s habitable land surface to agricultural and urban use. It has raised atmospheric CO₂ concentrations by roughly 50 percent above pre-industrial levels. It has acidified the surface oceans by about 30 percent. It has introduced invasive species, novel chemical pollutants, and direct hunting pressure across essentially every habitat on Earth. It has warmed the planet’s surface by more than one degree Celsius in a century.
Any one of these would be a significant perturbation. In combination, they represent something the planet has not experienced before — a synchronous, planet-wide alteration of atmosphere, oceans, temperature, and ecosystems, all driven by a single biological species over a period of roughly 200 years.
The Cretaceous-Paleogene extinction was caused by an event lasting hours. The Permian extinction was caused by volcanic activity lasting several thousand years. What is happening now is caused by an event that has been ongoing for approximately two centuries and shows no clear sign of stopping. And unlike the asteroid or the volcanic activity, it is not an event Earth is going through passively. It is something being done, actively and continuously, by roughly eight billion members of one particular primate species.
What the framing actually means
The question of whether to call this a sixth mass extinction is, in a sense, a question about which time scale you are willing to take seriously.
If you look at the current data through the lens of the past few decades, the situation is alarming but does not yet reach the technical thresholds of the previous five events. If you look at the same data through the lens of geological time — where “recent” means “within the last few million years” and where trends of the current magnitude, if extended, would produce mass-extinction-level losses well within the next few thousand years — the situation looks different.
Most biologists working on the question, when pressed, would say something like this. We may not technically be in a mass extinction yet. We are demonstrably on a trajectory toward one. Whether we complete that trajectory is a question about human decisions in the coming decades, not a question about biology.
The distinction between “in progress” and “at the beginning of” is the whole space in which human action still matters. If the sixth mass extinction is already underway, we are its cause. If it is not yet underway, but is trajectory-wise inevitable without change, we are its architects. Either framing places the outcome, unusually, within our own capacity to alter.
That is the qualitative difference from the previous five. The asteroid did not choose. The volcanoes did not choose.
We are choosing.
