Squirrels are a familiar sight in many landscapes. Their widespread presence today stems from a deep evolutionary history. Tracing the appearance of these rodents reveals a journey through millions of years.
Ancient Rodent Ancestors
The lineage of squirrels extends back to older, generalized rodent-like mammals. These early ancestors emerged approximately 60 million years ago in North America and Asia. They were small, insect-eating creatures, part of a group called Glires, which includes rabbits and hares.
The Paleocene-Eocene Thermal Maximum (PETM), around 55.8 million years ago, was a significant period for mammalian diversification. This era’s rapid global temperature increase spurred evolutionary changes, allowing these small mammals to spread and adapt. Fossil evidence from this time indicates early protorodents, such as Ischyromyidae, exhibited diverse lifestyles from burrowing to climbing.
The First True Squirrels
The first animals recognizable as squirrels appeared during the Late Eocene Epoch, roughly 41.3 to 33.7 million years ago. Fossil discoveries in western North America provide the earliest evidence. One of the oldest identified squirrel fossils, Hesperopetes, dates back approximately 40 to 35 million years ago and shows similarities to modern flying squirrels, suggesting early adaptations for gliding.
Another early squirrel fossil is Protosciurus, found in early Oligocene deposits in Wyoming. This nearly complete skeleton reveals that early squirrels possessed key anatomical features for an arboreal lifestyle, similar to modern tree squirrels. These adaptations included a flexible spine, strong claws, and hind limbs longer than forelimbs, all beneficial for climbing and jumping. These specialized features distinguish early squirrels from their more generalized rodent predecessors.
Global Diversification and Modern Forms
Once established, squirrels underwent significant diversification, leading to the wide array of species found across the globe. They expanded into various ecological niches, resulting in distinct groups like tree squirrels, ground squirrels, and flying squirrels. This adaptive radiation allowed them to thrive in diverse environments, from dense forests to open grasslands.
Squirrels are now indigenous to the Americas, Eurasia, and Africa, having spread across continents over millions of years. Fossil records indicate their presence in Europe by about 30 million years ago and in Africa around 18-20 million years ago. Their ability to adapt to different habitats and exploit various food sources has contributed to their success and global distribution, with over 270 species recognized today.