Gaby
Scientists have long debated when placental mammals — like humans, dogs, and bats — first appeared. A recent fossil analysis finds something striking: human ancestors were around during the age of dinosaurs. Their overlap with those giants was short, but the evidence suggests it happened. The dinosaurs died out, and our ancestors kept evolving.
Some researchers have argued that placental mammals existed before the catastrophic asteroid impact that wiped out nearly 75% of life on Earth. They say dinosaurs perished in the disaster while early mammals survived and later diversified. Now, paleontologists who supported that view have new evidence: an in-depth fossil analysis that points to a brief period of coexistence between our ancestors and non-avian dinosaurs.
As reported by the Daily Mail, fossils of placental mammals have mostly been found in rock layers younger than 66 million years. That has led many scientists to think this group evolved after the extinction event. A distinctive worldwide sediment layer rich in iridium — a metal rare on Earth but common in asteroids and comets — is a key piece of evidence for the impact hypothesis. Fossils also show a sharp drop in biodiversity below that layer.
Most non-avian dinosaurs disappeared in the catastrophe; birds, lizards, and frogs survived. But molecular data long hinted that placental mammals, including the line that led to humans, are older than the fossil record alone suggested. A team of paleobiologists used statistical analysis of fossils to test whether early placental mammals existed before the mass extinction. Their results indicate that primates (which include humans), along with groups that led to rabbits, dogs, and cats, appeared just before the catastrophe — meaning some early placental mammals lived in the Cretaceous alongside non-flying dinosaurs.
Researchers think placental mammals survived the impact and that the extinction of dinosaurs removed competitors, opening ecological opportunities that let mammals diversify. Lead researcher Emily Carlisle says her team collected thousands of placental mammal fossils and tracked patterns of emergence and extinction across groups. Small early ancestors appear to have been among the survivors.
Carlisle says the data allow estimates for when early human ancestors emerged, even though their exact appearance remains unclear. The first placental mammals were tiny — one early primate ancestor was about the size of a modern shrew — so many of those Cretaceous mammals were probably very small.
Recovery after the asteroid impact took millions of years. The creatures that survived set the stage for today’s global biodiversity. Small size helped early placental mammals find refuge and survive on limited food in the harsh post-impact world. That survival allowed mammals to expand and evolve after the non-avian dinosaurs vanished. If the asteroid hadn’t struck, the world — and the path of life that led to humans — might look very different today.
