The question of whether humans are more closely related to cats or dogs is a fascinating puzzle often asked by those observing the animals that share our homes. Superficial appearances and behaviors can be misleading, and the true answer requires a deep dive into the evolutionary history of mammals. By employing the tools of modern evolutionary science, specifically phylogenetics, we can trace the ancient branches of the tree of life to identify the true biological relationship. The scientific verdict is not based on shared habitat or domestication, but on the precise timing of when our respective ancestral lineages last shared a single common forebear.
Defining Genetic Relatedness
Genetic relatedness in a scientific context is determined by the recency of the Last Common Ancestor (LCA), not by how many traits two species share today. The more recently two species shared an LCA, the more closely related they are considered to be because less time has passed for evolutionary changes to accumulate. This ancestral connection is visually represented through a phylogenetic tree, a diagram that maps the branching pattern of evolution over geological time. Scientists estimate these deep divergence times by analyzing the shared genetic material between species using the molecular clock. By comparing the number of genetic differences between two species, researchers can infer the time elapsed since their lineages diverged, establishing a closer genetic relationship if the distance is smaller.
Shared Ancestry of Mammals
Tracing the lineage of humans, cats, and dogs begins with the Class Mammalia, characterized by traits like hair, live birth, and the nursing of young with milk. All three species belong to the infraclass Eutheria, encompassing placental mammals whose young develop fully within the uterus. This shared ancestry points to a tiny, shrew-like creature that lived approximately 66 million years ago as the earliest common placental ancestor. Humans, cats, and dogs are also members of the magnorder Boreoeutheria, a vast group including nearly all placental mammals native to the Northern Hemisphere. Understanding this ancient, shared history is necessary to properly contextualize the final, more recent splits that define the relationship between the three species today.
The Scientific Verdict: Comparing Divergence Times
The core of the matter is identifying the point where the human lineage separated from the ancestral line of cats and dogs. Humans belong to the Order Primates, while both cats (Family Felidae) and dogs (Family Canidae) are nested within the Order Carnivora. The evolutionary split between the Primate order and the Carnivora order occurred roughly 90 to 100 million years ago, marking the initial divergence of these major groups. Following this ancient separation, the Carnivora line divided into the suborders Feliformia (cats) and Caniformia (dogs) approximately 50 million years ago. Because the ancestors of cats and dogs remained together on the Carnivora branch for millions of years after diverging from the Primate line, they share the exact same distant Last Common Ancestor with humans, meaning humans are equally related to both cats and dogs.