The Last Common Ancestor Between Grey Squirrels and Humans

The study of evolutionary biology offers fascinating insights into the origins of life. One intriguing question is: what is the last common ancestor between the common grey squirrel (Sciurus carolinensis) and humans (Homo sapiens)?

While the specific identity of this ancestor is not definitively known, it is estimated to have lived around 80 to 100 million years ago. This ancient creature would have been a small, tree-dwelling mammal, resembling modern-day shrews or small rodents. Despite the vast evolutionary journey, these two species share a common point of origin, underscoring the complexity and beauty of life's journey.

Evolutionary Paths of Grey Squirrels and Humans

Both humans and grey squirrels belong to the class Mammalia, but they diverged on different evolutionary paths. Humans are part of the order Primates, while grey squirrels belong to the order Rodentia. This divergence occurred during the evolutionary history of mammals, following the split from their common ancestor with reptiles and birds.

Evolutionary Fossils and Their Significance

The earliest known fossil mammal, Spinolestes xenarthrosus, is a 125-million-year-old species found in Spain. This triconodont mammal, nearly 24 cm in length, resembles a rat and is preserved with intense detail including guard hairs, underfur, and spines. This fossil provides invaluable information about the early evolutionary stages of mammals.

Compared to Spinolestes xenarthrosus, the earliest fossil treeshrew, Archonta (around 34 million years old), is strikingly similar to the modern Pen-tailed Treeshrew (Ptilocercus lowii). This suggests that certain species, like the treeshrew, have undergone little evolutionary change in their morphology since the early Oligocene. This supports the hypothesis that Ptilocercus treeshrews represent a living fossil, much like Spinolestes xenarthrosus for mammals.

Mammalian Lineages and Their Evolution

With no fossils of placental mammals before they split into their major clades, the divergence is near the root of their evolutionary tree. Most researchers suggest that the common ancestor was shrew-like. However, the basal form of placental mammals, Xenarthra, has maintained its ancestral morphology for over 50 million years. Armadillos, for instance, are nearly unchanged from their ancient ancestors.

This evolutionary continuity underscores the remarkable resilience of these species. The ancestral morphology provided a framework that could withstand various environmental challenges, including the end-Cretaceous extinction event. However, as niches emerged, the need to adapt and exploit new opportunities led to significant evolutionary transformations.

The journey back in time to the last common ancestor between grey squirrels and humans highlights the interconnectedness of life and the incredible diversity that has evolved over millions of years. Understanding these connections not only deepens our appreciation of nature but also informs our approach to conservation and ecological studies.

In conclusion, while the specific identity of the last common ancestor remains a subject of scientific inquiry, the shared heritage of these species serves as a reminder of the incredible journey of life on Earth.