The question of whether humans are more closely related to cats or dogs is an inquiry into our deep evolutionary history. Understanding the answer requires looking beyond superficial physical traits and delving into the rigorous methods scientists use to map the vast, branching tree of life. This involves examining the structure of life’s classification system and pinpointing the ancient moments when our lineages diverged.
Defining Evolutionary Relatedness
Determining the precise relationship between any two species relies on taxonomy, the science of naming and classifying organisms. This system organizes life into a hierarchy of increasingly inclusive groupings, starting from the most specific (species) and moving up through genus, family, order, class, phylum, and kingdom. The closer two species are in this hierarchy, the more recently they shared a common ancestor.
The most accurate depiction of relatedness is shown through a phylogenetic tree, which illustrates the branching patterns of evolution. The point where two branches meet represents their most recent common ancestor (MRCA). Evolutionary relatedness is measured by the time elapsed since that MRCA existed, not by how much genetic material is shared in total. Species that share an MRCA more recently are considered more closely related. To determine if humans are closer to cats or dogs, the focus must be on identifying the point in time when our lineage split from theirs.
The Shared Mammalian Heritage
All three species—humans, cats, and dogs—are members of the Class Mammalia. This means the common ancestor of all three was a mammal, possessing defining mammalian traits such as hair, being warm-blooded, and feeding young with milk. They also share the characteristic of giving live birth, placing them within the infraclass Eutheria, or placental mammals.
The shared history extends to the Magnorder Boreoeutheria, which encompasses almost all placental mammals that originated in the northern continents. This expansive group includes rodents, bats, and whales. This ancient commonality serves as the baseline for comparison. The definitive answer to the question lies in the divergence of the major Superorders within Boreoeutheria.
Mapping the Major Divergences
The critical split occurred between two major Superorders of placental mammals: Euarchontoglires and Laurasiatheria. Humans belong to Euarchontoglires, which includes primates, rodents, and rabbits. Cats and dogs both belong to Laurasiatheria, a vast group that also contains bats, whales, horses, and the Order Carnivora. The divergence between the Euarchontoglires and Laurasiatheria lineages is estimated to have taken place approximately 85 to 95 million years ago.
Because both the cat and dog lineages were grouped within Laurasiatheria when the split from the human lineage occurred, both are equally distant from humans. The common ancestor shared by humans and cats lived at the same time as the common ancestor shared by humans and dogs. After this ancient divergence, the Laurasiatheria lineage continued to evolve, giving rise to the Order Carnivora. The subsequent split between the dog-like caniforms and the cat-like feliforms occurred much later, roughly 43 to 53 million years ago. This later split confirms that cats and dogs are far more closely related to each other than either is to a human.
Genetic Confirmation
Modern molecular biology provides conclusive evidence that validates the timeline established by fossil records and morphological classification. Scientists use comparative genomics and DNA sequencing to measure the precise genetic distance between species. By comparing entire genomes, researchers confirm the timing of the major splits in the phylogenetic tree. The genetic distance between a human and a cat is statistically identical to the genetic distance between a human and a dog, confirming that our last common ancestor with both pets is the same.
While some studies comparing specific conserved coding regions suggest a slightly higher percentage of shared DNA between cats and humans, the overall phylogenetic relationship is determined by whole-genome analysis and the timing of the evolutionary branches. The structure and organization of genes on the chromosomes of cats is considered more conserved and similar to the ancestral placental mammal arrangement than the structure found in dogs. This structural similarity does not alter the fact that the genetic distance confirms that humans are equally distant from both cats and dogs.