Cats and rats appear to be polar opposites, yet they are related in a distant, ancient sense. Their kinship reveals a shared ancestry that stretches back millions of years within the vast mammalian family tree. Despite obvious differences in size, diet, and behavior, their relationship is a testament to the common evolutionary path shared by all life on Earth. The true answer lies in how scientists classify the depth of their evolutionary split.
Understanding Orders of Divergence
The first layer of separation between cats and rats is the taxonomic level of Order. Cats belong to the Order Carnivora, a group characterized by adaptations for a predatory lifestyle, including dogs, bears, and seals. A distinguishing feature of this order is the specialized dental arrangement, specifically prominent canine teeth and blade-like carnassial teeth used for shearing meat. Cats, as obligate carnivores, have skulls and jaws built for a strong, vertical bite necessary for consuming their meat diet.
Rats, in contrast, belong to the Order Rodentia, the largest order of mammals, encompassing mice, squirrels, and beavers. Rodents are defined by a singular, specialized dental trait: a pair of continuously growing incisor teeth in both the upper and lower jaws. These incisors lack roots and are constantly worn down by gnawing, a behavior essential to their primarily herbivorous or omnivorous diet. The presence of these unique incisors and the absence of canine teeth places rats on a different evolutionary path from cats.
The Superorder Divide
The true depth of the separation is revealed by examining the classification level above Order, known as the Superorder. Cats are members of the Superorder Laurasiatheria, a vast group that includes animals like horses, bats, shrews, and whales. This grouping is based on molecular and genetic evidence, with the last common ancestor of this lineage estimated to have lived 76 to 90 million years ago. The Laurasiatheria group is thought to have originated on the northern supercontinent of Laurasia.
Rats belong to the Superorder Euarchontoglires, which includes primates, rabbits, and tree shrews. This classification also relies heavily on genetic analysis, as members share few obvious anatomical features. The divergence of Euarchontoglires from other placental mammals occurred around 85 to 95 million years ago, representing a major, ancient split. The existence of these two distinct Superorders shows that the lineages leading to cats and rodents separated long before the evolution of many modern mammal groups.
Connecting the Mammalian Tree
Despite the ancient separation at the Superorder level, cats and rats are still connected by their shared membership in the Class Mammalia. Both species share the fundamental, defining traits of all mammals, including being warm-blooded, having hair or fur, and possessing mammary glands to produce milk. Both also have a vertebral column, placing them within the subphylum Vertebrata.
The last common ancestor of the cat’s Superorder (Laurasiatheria) and the rat’s Superorder (Euarchontoglires) was a placental mammal from the late Cretaceous period, approximately 90 to 100 million years ago. This ancestral creature was likely a small, generalized, insectivorous animal, perhaps resembling a modern shrew. This shared ancestry is incredibly ancient, dating to the time non-avian dinosaurs went extinct.