Around 56 million years ago, Earth experienced one of the most significant warming events in its 4.5-billion-year history. Temperatures rose by up to 8°C (14.4°F), and there was little to no ice anywhere on the planet.
During this period of sustained warming, the first 'true' primates emerged, and set off the trajectory that would eventually lead to the evolution of humans. And now geologists say they've found evidence that a mystery comet impact could have triggered this crucial event.
Around 56 million years ago - roughly 10 million years after the mass extinction of the non-avian dinosaurs - something triggered a period of skyrocketing global temperatures.
Many experts consider the PETM to be the best ancient analogue of modern climate change, because it saw atmospheric carbon dioxide levels escalate so rapidly, temperatures around the globe increased by 5°C to 8°C.
Sea surface temperatures in the tropics rose as high as 35°C, and seafloor temperatures were 10°C higher than they are today. Oceans acidified, permafrost melted, wildfires tore through the forests, and life evolved to cope with the new conditions.
Many land mammals were forced out of the forests and into the open plains, allowing them to grow larger, and these groups formed the basis of modern-day rhinoceroses, horses, pigs, camels, and hippopotamuses.
The ancient ancestors of whales appear to have taken to the sea around this time, and placental mammals with larger bodies and bigger brains than ever before began to appear in the fossil record.
The first 'true' primates also evolved during this time, with an ability to grasp onto objects and branches with their hands and feet - something that would define their monkey relatives many millions of years later.
But as crucial as the PETM is to Earth's history, what actually triggered it has remained one of the biggest mysteries in geology.
Now, a team a geologists suggest that the impact of a small comet could have set off this global warming event, and say they have evidence of microscopic glassy spheres from up and down the US East Coast to back them up.
Yep, just as an impact from a space rock put an end to the lives of so many dinosaurs, 10 million years later, a similar event could have sparked the evolution of countless species of heat-loving plants and animals.
At the annual meeting for the Geological Society of America in Colorado last week, Schaller and his team presented two papers describing the discovery of strange silica 'beads' in eight sediment cores tied to the beginning of the PETM.
They say these glassy spheres - found in three Atlantic Coastal Plain sites - are often associated with extraterrestrial strikes, and they've been hiding in plain sight for decades.
"The spheres looked like microtektites, the debris created and tossed aside when comets or asteroids strike Earth at high speeds.
This was a surprise to the team: these sediments had been studied many times before. The spheres may have blended against the background of the black trays that are commonly used to hunt for light-coloured forams [fossils of microscopic organisms]."
But the team says the water content of these microtektites is less than 0.03 percent, which is much lower than what's been found in volcanic microtektites, and they contain inclusions of quartz glass that are characteristic of a hot impact.
They also found that three of the PETM sediment cores contained large spikes of charcoal immediately above the microtektite layers, which suggests that right after their proposed impact came the spread of wildfires.
The team detailed how this charcoal also contains preserved remnants of charred plants, which they say further strengthens their case.
"We infer that the thermal anomalies resulting from the impact and ejecta fallout probably ignited widespread wildfires, evidence for which has been documented previously from other Paleocene-Eocene sections globally," they conclude in one of the papers.
The discovery, which has yet to be peer-reviewed, was met with much discussion at the conference, with some geologists open to the idea that this really is evidence of a life-altering comet impact, while others remain entirely skeptical.
"It is a really amazing discovery," Birger Schmitz, a geologist from Lund University in Sweden, told Voosen. "The data look sound."
Meanwhile, Jerry Dickens - an oceanographer at Rice University in Houston, Texas - said he doesn't doubt that the spheres originated from an impact, or that the charcoal stemmed from forest fires, but suggests that these spheres and charcoal can be found throughout the PETM sediment layers - not just at the beginning.
The team now has to explain how such a small impact - estimated to be around a couple kilometres across - could have sparked such a planet-wide warming event, but one possibility is that it happened to hit a massive carbon-filled area, such as natural oil reservoir.
The jury's still out on this one, but if the researchers can find more evidence to back up their claim, it could mean that not one, but two impacts set off the chain of events that would give rise to our early ancestors.