Sixty-six million years ago, an asteroid roughly ten kilometers wide struck what is now the Yucatán Peninsula in Mexico. The impact ended the age of dinosaurs and wiped out an estimated three quarters of the plant and animal species on Earth. For a long time, the popular picture of what came next was a blank page, a slow and empty recovery before mammals eventually took over.
The fossil record tells a faster story. In the forests of what is now northeastern Montana, a small tree-climbing mammal was already living in the aftermath. It is called Purgatorius, and it may be one of the earliest known relatives of every primate alive today, including us.
A tiny animal with a big family tree
Purgatorius was a small animal, weighing roughly 3.5 ounces, about as much as a deck of playing cards. It lived on insects and fruit, and its teeth are the main reason scientists know it exists. For decades it was known almost entirely from isolated teeth and jaw fragments, which made it hard to say much about how it lived.
That began to change in 2015, when a study in the Proceedings of the National Academy of Sciences described the first Purgatorius bones from outside the skull, ankle bones found in the Garbani Channel area of Montana. Their shape suggests an animal suited to moving through trees, which supports the idea that the earliest relatives of primates were climbers from the very start.
In 2021, a team including paleontologists from the University of Washington pinned down its age in the journal Royal Society Open Science. They described Purgatorius teeth and jaws from the Hell Creek area of northeastern Montana and dated the rocks around them to about 65.9 million years ago. That places the animal roughly 105,000 to 139,000 years after the asteroid impact, a short span in geologic terms.
Why the survivors were small
Mammals did not appear after the dinosaurs. They had been around for well over 100 million years by the time the asteroid hit, living alongside them through most of the Mesozoic. During that long stretch, most mammals stayed small and many were probably active at night, feeding on insects and seeds while large dinosaurs dominated the daytime landscape.
That way of life turned out to be useful when the world collapsed. After the impact, dust and soot in the atmosphere blocked sunlight for months, plants died back, and food chains that depended on large plant-eaters fell apart. Animals that were small, ate a varied diet and could shelter in burrows or trees had a much better chance of making it through. Large animals that needed a lot of food did not.
Purgatorius fits that pattern. A small body, a flexible diet and a life spent off the forest floor all helped a lineage like this one hang on while the giants disappeared.
The fastest part of the recovery
Purgatorius matters because of how quickly its group spread out. Within a few million years of the extinction, mammals had branched into a wide range of new shapes. Hoofed plant-eaters, early meat-eaters and, eventually, very large animals all appear in the fossil record of the Paleocene and Eocene epochs.
That burst of new forms is sometimes called an adaptive radiation. With the large dinosaurs gone, roles that had been filled for tens of millions of years were suddenly open, from treetop fruit-eaters to big grazers on river floodplains. Mammals moved into those roles quickly, and some of the results look unusual to modern eyes.
The After the Dinosaurs episode of The Story of Life on Earth opens on this moment, following Purgatorius through the ruined forest and then tracing how the survivors became the mammals we know today, from whales to early apes.
How the story fits into the longer history of life
Big turning points in the history of life are often visible only in small fossils. A few teeth and ankle bones from Montana say a great deal about how quickly life recovered after the asteroid impact.
That theme runs through good natural history writing for younger readers too. Martin Jenkins, a Cambridge-trained conservation biologist who writes nonfiction for children, takes the same long view in Life: The First Four Billion Years, his 2019 collaboration with artist Grahame Baker-Smith. The book does not stop when the dinosaurs do. It follows the rise of mammals after the extinction and ends with early primates, close to the point where the human and chimpanzee lineages split. The Walker Studio edition of Life presents the dinosaurs as one long chapter in that family story rather than the whole of it.
Two very different audiences have noticed that structure. Dino Dad Reviews, a blog written with a paleontologist’s eye, praised the book’s attention to life before the Mesozoic, while readers on Goodreads include librarians and homeschooling parents who use it to show children what came before and after the most famous animals in prehistory.
What Purgatorius still tells us
Scientists continue to debate exactly where Purgatorius sits on the family tree. Some place it among the plesiadapiforms, an early group closely related to true primates, rather than as a direct ancestor. More fossils, especially more complete skeletons, will help settle the question.
What is clear is that the first chapter of the age of mammals started very quickly. Within roughly a hundred thousand years of the asteroid impact, a small climber was living in the trees of Montana, eating fruit and insects in a world that had just lost its largest animals. From that modest beginning came a lineage that would eventually include monkeys, apes and people.
For anyone following the series on the Martin Jenkins YouTube channel, Purgatorius is where the final chapter of the story begins.
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