The ancestors of all mammals were once the dominant land animals on Earth.

Nature · Published 3 October 2026 Swipe more like this →

In short: Long before dinosaurs ruled the planet, a group of synapsids called therapsids rose to prominence. These creatures bridged the gap between reptilian ancestors and modern mammals, developing upright limbs, specialized teeth, and eventually, the internal heat regulation that defines our own biology today. They are our deep evolutionary history.

Therapsids emerged during the Late Carboniferous or earliest Permian, splitting from the sphenacodontid synapsids—a group that includes the famous, sail-backed Dimetrodon. While their early history remains obscured by a significant gap in the fossil record, they successfully replaced pelycosaurs to become the dominant large land animals by the Middle Permian. This group was remarkably diverse, encompassing six major lineages including the saber-toothed Gorgonopsia and the tusked, herbivorous dicynodonts.

The transition toward mammalian traits was gradual and profound. Unlike the sprawling, lizard-like gait of their ancestors, therapsids evolved a more vertical limb posture that positioned their legs directly beneath their bodies. Their teeth became increasingly complex, differentiating into incisors, canines, and postcanines. Perhaps most significantly, the group began a long, complex shift toward endothermy. While early Permian therapsids likely relied on a parietal eye for thermoregulation, later cynodonts—the lineage that would eventually give rise to mammals—showed evidence of internal heat production and the eventual loss of the pineal foramen.

The Permian–Triassic extinction event devastated the group, leaving only a few lineages to persist into the Triassic. While archosaurian reptiles began to diversify and eventually dominate the landscape, the cynodonts continued to evolve. By the Late Triassic, around 225 million years ago, the first Mammaliaformes appeared. Today, mammals remain the sole surviving branch of this once-mighty group, carrying forward the legacy of skeletal and physiological innovations that began hundreds of millions of years ago.

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