Squirrels are a familiar sight in many parts of the world, observed scampering up trees or foraging on the ground. Their agility and distinctive bushy tails make them easily recognizable. These widespread creatures have a much longer history on Earth than many might realize, with their evolutionary journey spanning tens of millions of years and connecting them to some of the earliest mammals. Understanding their deep past reveals how these adaptable rodents came to inhabit such diverse environments globally.
The Earliest Rodent Ancestors
The evolutionary lineage leading to modern squirrels traces back to the very first rodents, a group of mammals whose fossil record emerges during the Paleocene epoch. These ancient rodent ancestors appeared approximately 60 million years ago, primarily in regions that are now North America and Asia. Early forms were small, likely insectivorous creatures. They were part of a larger mammalian group called Glires, which also includes the lagomorphs, such as rabbits and hares.
Following their initial appearance, rodents underwent significant diversification during the Eocene epoch, spreading across various continents. Fossil evidence provides insights into these early forms. For instance, Paramys, a genus of early rodents, lived around 50 million years ago during the Eocene. Another notable fossil, Ischyromys, found from the Oligocene epoch around 30 million years ago, is considered a potential early relative within the broader rodent family that includes squirrels.
The Emergence of True Squirrels
The specific family of “true” squirrels, known as Sciuridae, first appeared in the late Eocene epoch, with fossil evidence suggesting their origin in North America around 36 to 41.3 million years ago. One of the earliest identified squirrel fossils is Douglassciurus jeffersoni, dating back approximately 37.5 to 35 million years ago. This ancient squirrel possessed skeletal and dental characteristics similar to those of present-day squirrels.
However, Douglassciurus jeffersoni lacked a specific jaw muscle arrangement, the sciuromorphous zygomasseteric system, which is a defining feature of later squirrels. The earliest squirrel fossil exhibiting this characteristic is Palaeosciurus, discovered in Europe and dating to the early Oligocene epoch, approximately 33.7 to 23.8 million years ago. Molecular studies further indicate that the five primary groups of squirrels, including flying squirrels, diverged during the late Eocene and Oligocene, roughly 34 to 23 million years ago.
Global Spread and Modern Diversity
From their ancient origins, squirrels expanded and diversified, leading to the wide array of species observed today. The family Sciuridae encompasses approximately 279 species across 51 genera, broadly categorized into five subfamilies:
- Ratufinae
- Sciurillinae
- Sciurinae
- Xerinae
- Callosciurinae
This diverse group includes familiar tree squirrels, ground squirrels such as marmots, chipmunks, and prairie dogs, and the unique flying squirrels.
Squirrels subsequently spread from North America into Asia and South America about 3.4 million years ago. They are now found on every continent except Antarctica, having been introduced to Australia by humans. Among their specialized forms, flying squirrels (tribe Pteromyini) evolved from tree squirrels, with molecular evidence pointing to their divergence around 18 to 20 million years ago. Ground squirrels (subfamily Xerinae) originated in Eurasia during the early Oligocene, later diversifying into various forms, including marmots and chipmunks. Tree squirrels (tribe Sciurini) also underwent significant diversification, colonizing forested areas across Europe, Asia, and the Americas during the Miocene and Pliocene epochs.