Sloths are most closely related to anteaters, and their next closest living relatives are armadillos. All three groups belong to an ancient mammalian superorder called Xenarthra, a lineage that split from other mammals tens of millions of years ago and evolved in near-isolation in South America. Beyond that shared ancestry, sloths have a surprisingly complicated family tree, including massive extinct ground sloths and even an aquatic species that foraged in the ocean.
Anteaters Are Their Closest Living Relatives
Sloths and anteaters together form the order Pilosa, meaning they share a more recent common ancestor with each other than either does with armadillos. The two groups split apart roughly 55 to 60 million years ago, during the Paleocene epoch, not long after the extinction of the dinosaurs. That may sound like a long time, but in evolutionary terms, it’s a relatively tight bond: for comparison, the last common ancestor of humans and lemurs lived around 70 million years ago.
At first glance, sloths and anteaters look nothing alike. Anteaters are ground-dwelling insect specialists with long snouts and no teeth at all. Sloths hang upside down in trees eating leaves. But under the surface, the two share telltale skeletal features that mark them as family, especially a set of unusual spinal joints found nowhere else in the animal kingdom.
The Xenarthra Superorder
Zoom out one level on the family tree and you reach Xenarthra, the superorder that unites sloths, anteaters, and armadillos. The name comes from Greek: “xenos” (strange) and “arthros” (joint), referring to extra articulations between the vertebrae in the lower back. These bony connections stiffen the spine and are thought to have originally evolved to help with digging. No other mammals have them.
The three groups share other traits too. All xenarthrans have reduced teeth compared to most mammals. Sloths and armadillos have simple, peg-like teeth that grow continuously throughout life and lack enamel. Anteaters took tooth reduction to its logical extreme and lost theirs entirely. Armadillos were the first to branch off from the xenarthran family tree, diverging around 65 to 70 million years ago. The sloth-anteater split followed about 10 million years later.
Six Living Species in Two Unrelated Families
There are six living sloth species, divided into two groups: four species of three-toed sloths (genus Bradypus) and two species of two-toed sloths (genus Choloepus). Here’s where the family tree gets genuinely surprising. Despite looking and behaving almost identically, two-toed and three-toed sloths are not each other’s closest relatives. Their two lineages diverged 30 to 40 million years ago.
Two-toed sloths actually belong to a family called Megalonychidae, which includes many extinct ground sloths. Three-toed sloths sit on their own branch, Bradypodidae, as the sister group to all other sloths. The tree-hanging lifestyle, the slow metabolism, the algae-covered fur, the upside-down posture: both groups evolved these traits independently, arriving at nearly identical solutions to the same environmental challenges. Biologists call this convergent evolution, and sloths are one of the most striking examples among mammals. Even their bone structure converged. Most terrestrial mammals, including ground-dwelling sloths, have dense, compact bone. Both Bradypus and Choloepus independently evolved lighter, less compact bones suited to a life spent suspended from branches.
Giant Ground Sloths and Other Extinct Relatives
Today’s six species represent a tiny fraction of sloth diversity. The fossil record includes dozens of extinct species, many of them ground-dwellers that dwarfed their modern tree-climbing cousins. The most famous is Megatherium, a ground sloth that stood as tall as a giraffe and weighed several tons. Megatherium specimens date back to the late Pleistocene, with some as recent as 12,500 years ago. These massive animals vanished near the boundary between the Pleistocene and Holocene epochs, likely due to a combination of climate change and human hunting.
Collagen analysis of Megatherium bones has helped clarify its position on the sloth family tree. Rather than being a direct ancestor of modern sloths, Megatherium sits on a separate branch (Megatheriidae). Another extinct genus, Lestodon, appears to be more closely related to living sloths than Megatherium is, though both are more distant cousins than siblings.
Perhaps the most unexpected extinct relative is Thalassocnus, a sloth that adapted to life in the ocean along the coast of what is now Peru and Chile during the Neogene period (roughly 5 to 10 million years ago). Over the course of several species in the genus, Thalassocnus progressively developed denser, heavier bones for buoyancy control, much like manatees have today. Its nasal structures thickened dramatically in later species, with bone occupying about 40% of the nasal cavity compared to 10 to 19% in earlier forms. These changes suggest a step-by-step transition from wading in shallow water to fully aquatic foraging.
How Sloths Fit Into the Broader Mammal Family Tree
Xenarthra is one of the most ancient lineages of placental mammals. It branched off very early from the group that would eventually produce everything from rodents to primates to whales. This means sloths are only distantly related to most of the animals people encounter in daily life. Their closest relatives outside Xenarthra are part of a group called Afrotheria, which includes elephants, manatees, and aardvarks. But that connection dates back to near the base of the placental mammal family tree, over 90 million years ago.
The practical upshot is that sloths occupy a genuinely unique branch of mammalian evolution. Their lineage spent tens of millions of years evolving in South America when the continent was an island, isolated from North America and the rest of the world. That long isolation is part of why xenarthrans look so different from almost every other group of mammals alive today.