Mammalia is the class of species to which mammals—and by association, we humans—belong. While we take their abundance for granted, it actually took millions of years for the first mammals to appear, along with a bit of luck for them to thrive. Let’s dive into their story.
Around 275 million years ago, the world was dominated by cold-blooded amniotes, descendants of fish. Their major weakness was their reliance on external heat, which forced them to remain immobile at night. However, evolution had a trick up its sleeve: therapsids, a new group of animals, evolved warm-bloodedness, allowing them to stay active at night. This adaptation gave them an advantage, enabling them to hunt the immobile reptiles that were asleep during the night.
But warm-bloodedness also had its downsides. Around 250 million years ago, the Permian-Triassic mass extinction nearly wiped out therapsids, as maintaining body heat required more energy—a disadvantage in a time of extreme food scarcity. Only a small subgroup, cynodonts, survived by being smaller and requiring less food. Meanwhile, reptiles evolved into dinosaurs, dominating the planet and forcing cynodonts and early mammals into hiding, living mostly at night.
To thrive in this dinosaur-dominated world, mammals developed the neocortex, a brain region unique to them that enables planning and complex thought. This adaptation helped them survive the next mass extinction, which wiped out the dinosaurs, and eventually led to mammals becoming the dominant class of animals on Earth.
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