As long as there has been life on Earth, there has been extinction. In fact, nearly every life form that has called Earth home has gone extinct.

“Of the 50 billion or so species that have [lived] during our planet’s 4.5 billion year history, more than 99 percent have disappeared,” says Jessica Whiteside, a planetary paleontologist at University of Southampton.

In particular, mass extinction events have shaped the course of evolution and Earth itself. This refers to relatively short spans of time when a majority of species go extinct.

Of course, short means something a bit different in geological time. These cataclysmic events – mere “blips” in Earth’s history – can encompass hundreds of thousands or even millions of years, making it incredibly difficult to say just how long they last. 

Yet, understanding how scientists study mass extinctions reveals the incredible insights we do have about the length of mass extinctions and their impact on the planet. 

Read More: The 5 Mass Extinction Events That Have Swept Our Planet

What Is a Mass Extinction?

Extinction happens. Under typical circumstances, scientists believe the “background extinction rate” is 0.1 to 1 species per 10,000 species per 100 years.

In other words, extinction is typically slow, which allows newly evolved species to fill the ecological gap left behind when another species disappears.

During a mass extinction, conditions on Earth are transformed – by rapidly freezing and melting ice sheets, volcanic eruptions or even an asteroid impact. As a result, species are lost much faster than they are replaced.

Events that qualify as a mass extinction generate the loss of at least three of every four species, dealing a plummeting blow to diversity on Earth.

Worldwide Loss of Species

This accelerated extinction rate doesn’t happen at one or two locations, but around the world, which contributes to the complexity of studying these calamitous events. 

Today, with vertebrate species going extinct at a rate up to 100 times higher than pre-industrial times, many experts argue that humans are causing the sixth mass extinction.

However, scientists likely wouldn’t know for sure until long after our biodiversity crisis spiraled out of control. “Mass extinction and death are kind of similar,” explains Hull, “you only know when it happened after it happened.”

Read More: Why We May Be in the Midst of a Sixth Mass Extinction

Looking into the Fossil Record 

Absent time machines, unraveling the mysteries of mass extinctions is a tricky task. However, fossils offer invaluable glimpses into the distant past. 

Interpreting the fossil record is like doing a puzzle without having all the pieces or knowing what the end result should look like.

Fossils are assigned a relative age based on other fossils found below or above them in rock layers. In general, the deeper a fossil is, the farther back in time the organism lived and died.  

Gaps in the Record 

Missing pieces further complicate the fossil record.

Not all organisms fossilize equally well; creatures with hard shells or bones become fossils much more readily than soft, squishy animals such as jellyfish.   

“When we look at the fossil record on a global scale and from the perspective of mass extinctions, we’re normally looking at the fossil record of the ocean on the scale of several million years,” says Pincelli Hull, a paleontologist at Yale University. 

Looking at such huge swaths of geologic time gives scientists a very coarse idea of when creatures lived and went extinct. For example, the giant ground sloth, which disappeared about 10,000 years ago, would be a contemporary of modern domesticated animals in the fossil record, even if the latter emerged hundreds of years later.    

“Five million years from now,” says Hull, “we would cluster the loss of giant sloths with the explosion of cows and chickens.”

The Consequences of Mass Extinctions 

Although catastrophic, the mass extinctions of the past have paved the way for new ecosystems and species – including us.

“Without the asteroid that caused the mass extinction that wiped out the non-flying dinosaurs,” adds Whiteside, “mammals might never have taken over and we would not be here having this conversation right now.”

Science will likely never provide precise estimates of how long mass extinctions take. But new data can continue to improve estimates and further illuminate the causes and consequences of mass extinction.

Read More: How Humans Survived the Ice Age