Sixty-six million years ago, a meteor smashed into the coast of Mexico's Yucatan Peninsula, marking the beginning of the end for most dinosaurs.
"The climate just went nuts," said Robert Cowie, a professor at the University of Hawaii at Manoa. Within the time span of anywhere from a few decades to a few thousand years, these prehistoric reptiles vanished.
The dinos met their demise during a mass extinction event, one of five to hit Earth. Each of these events wiped out the majority of species living at that time. Now some researchers believe we're in the midst of a sixth such extinction, and it's getting worse.
"Fifty years ago, the species who were endangered were very specific, the large animals or the ones who competed with humans and so on," said Gerardo Ceballos, a professor at the National Autonomous University of Mexico. "But now everything is maybe at risk -- the small and big, conspicuous and inconspicuous, the bad and not so bad."
Past mass extinctions had natural causes, but researchers pin the sixth solely on humans. Still, that also means we have the power to do something about it. Here's a guide to the sixth mass extinction, which some researchers say will have dire consequences for all life -- including us.
A mass extinction event involves the disappearance of most species on Earth because of a natural catastrophe, according to Ceballos, who works at UNAM's Institute of Ecology. They can take place over a few million years, which is fast in geologic terms.
More than 500 million years ago, almost all modern animal groups first appeared during what's known as the Cambrian Explosion. In the time since, five catastrophic extinction events have occurred that wiped out around 70% or more of all plants and animals. These events included natural disasters such as changes in the gases of the atmosphere or, in the case of the mass extinction that erased most dinosaurs 66 million years ago, an asteroid impact.
From oldest to most recent, the five mass extinction events are: the Ordovician-Silurian, the Late Devonian, the Permian-Triassic, Triassic-Jurassic and the Cretaceous-Paleogene. The Permian-Triassic extinction wais the most deadly, resulting in the loss of about 90% of species. The Cretaceous-Paleogene extinction decimated the dinosaurs.
The sixth extinction is sometimes called the Holocene extinction, which refers to the geologic epoch beginning around 10,000 years ago, or the Anthropocene extinction, referring to the proposed epoch when humans started significantly impacting the planet's ecosystems and climate.
While mass extinctions are rare, extinction itself isn't unusual, according to conservationist Anthony Giordano. Species go extinct, but they also evolve, leaving a genetic legacy. One species could evolve into another, for instance, or populations of a species could split, no longer looking like the original, and become different species over time.
Life recovers after a mass extinction event, but in the past it's taken 10 million, 15 million or 20 million years, Ceballos said.
Ceballos and Giordano believe the sixth extinction is underway. They point to the high rate at which species are going extinct.
In a headline-making 2015 study, Ceballos and other researchers compared conservative estimates of the background and current rate of vertebrate extinctions (background refers to the natural rate before human activity) and still found the current rate exceeded the background by a large margin.
"What we've lost in 100 years would have been lost in 10,000 years in normal times," Ceballos said.
The sixth extinction is occurring rapidly from a geologic perspective, easily as fast as many prior mass extinction events, Ceballos said. The extinction rate has increased over the past several decades and will likely continue to climb as human population growth increases, exacerbating human-caused extinctions. The sixth extinction has the potential to be as deadly as the past five.
While Cowie didn't conclude that we're unequivocally in a mass extinction, he said this is probably the beginning of one.
In a 2022 study, he and his team also found an alarming disparity between extinction rates when factoring in an estimation of the likely number of invertebrate extinctions.
Scientists are grappling with the problem of not knowing about every species that's been lost. Cowie and his team extrapolated boldly from land snail data to compensate for this unknown. If those estimations are on target, the number of species (snail and otherwise) lost since 1500 could be much higher than originally thought -- 150,000 to 260,000 versus 897, a number reported in 2021 by the International Union for Conservation of Nature.
"We came up with this number that is ginormous," Cowie said. It would mean the current extinction rate is some 100 times greater than the background.
Not everyone is in agreement that the sixth mass extinction is definitely or probably underway. According to Cowie's paper, some researchers say extinction rates aren't dramatically heightened. Others say that extinction numbers are counterbalanced by the generation of new species, Cowie said.
In a 2017 paper, John Briggs of the University of South Florida found that losses from human-caused extinctions "have probably been equalled or surpassed by species born during that time." Briggs has since passed away.
Still, researchers like Cowie don't think evolution will make up for the extinction rate.
Unlike mass extinctions in the past, the sixth is caused by people.
"It's the way we're modifying and impacting the planet, which is leading to extinctions of species so rapid that they are not able to leave some kind of genetic legacy," Giordano said.
Humans are causing the sixth mass extinction both directly and indirectly, he said. Major causes include habitat destruction, illegal trade and overexploitation of species, the spread of invasive species, pollution, emerging diseases and climate change, Ceballos said.
Biodiversity hotspots -- places most associated with the global south and the tropics -- are under attack. There are disproportionately high numbers of species in some of the most vulnerable habitats, such as rainforests, mangrove swamps and coral reefs. Corals are suffering from climate change, warming oceans and acidification, Giordano said. Rainforests, where more than half of Earth's animal and plant species are found, make up 6% of Earth's land cover, but they once covered 14%.
Extinction is irreversible.
"Once you lose a species, it's gone forever," Ceballos said.
Humans are noted to have caused the extinctions of species like the dodo, Steller's sea cow, Tasmanian wolf and great auk. Those species are likely not the last to disappear by human hands.
Giordano focuses on carnivores at his organization SPECIES, and points to ways they've been impacted by humans. Jaguars, a threatened species, have gone extinct from two countries where they used to be present, and their range has declined by about half of what it was 150 years ago. Wolves in the US occupy less than 10% of their original range.
Rhinos and elephants are under threat, targeted by illegal wildlife traffickers for their horns and ivory. Giordano also singles out North Atlantic right whales, a species that's been reduced to between 300 and 400 individuals, largely because of fishing practices that cause them to get entangled and drown. "All of these species have stories," he said.
In a 2017 paper, Ceballos and others concluded Earth is experiencing population declines and local extinctions on a scale that amounts to "biological annihilation."
Ceballos warns this could have dire consequences for humans.
"We're losing so many species that civilization is facing a possibility of global collapse in the next two or three decades," he said.
Animals and plants create the conditions that are required for life on Earth. They play a crucial part in providing what scientists call ecosystem services, which are "all the benefits that we get from the proper function of nature for free," Ceballos said. These benefits span from the correct combination of gases in the atmosphere to potable water to nutrient cycling.
Another example involves crops. Seventy percent of all crops today are pollinated totally or partially by animals, including bees, butterflies and birds. "By losing those species, we're losing, for example, the capability of the planet to sustain us in terms of our food," Ceballos said. If nothing were to be done, loss of species would add up over time and lead to collapse, he said.
Cowie and his colleagues note that there is a lack of political, economic and social will to act on the high extinction rate. While they support what efforts are being made to prevent extinctions, they feel that those efforts can't save everything. He said a biodiversity priority should be documenting what we have for our descendants.
Giordano, on the other hand, supports policies that hold people, companies and other governments accountable. The average person can even make an impact by volunteering for a conservation organization, or paying attention to how food and other products they purchase are made, he said.
For example, when consumers decide to buy fair trade and biodiversity-friendly coffee, and workers on the ground make a fair wage, it incentivizes local communities to make ecologically responsible choices, he said.
Ceballos shared more examples. Those in more affluent countries can reduce their huge appetite for consumption. They can eat less meat and stop keeping exotic animals as pets, which can invade new ecosystems and destroy them.
He thinks there's still time to change the course of the sixth mass extinction.
"If it wasn't caused by us, we could do almost nothing to stop it," Ceballos said. "We can do something to reduce or to stop the process."