Snakes are reptiles defined by their lack of limbs and elongated, flexible bodies. This distinctive body plan has evolved multiple times, creating confusion about which animals truly belong to the snake lineage. Distinguishing a true snake from a snake-like animal requires understanding their evolutionary history and specific anatomical adaptations. The resemblance between species is often due to convergent evolution, where unrelated organisms develop similar traits to solve a common environmental challenge.
Closest Evolutionary Relatives
Snakes belong to the order Squamata, which includes all lizards, confirming that lizards are their closest living relatives. Phylogenetic studies suggest snakes evolved from small, terrestrial lizards that adopted a burrowing lifestyle. This ancestral connection explains why some snakes, such as pythons and boas, still retain vestigial, non-functional hind limb remnants.
The loss of limbs is a recurring phenomenon across the lizard family tree, having occurred dozens of times independently. In snakes, this loss is genetically traced to mutations in the Zone of Polarizing Activity Regulatory Sequence (ZRS), a regulatory region of the sonic hedgehog gene. Since this gene is crucial for limb development, its disruption resulted in the anatomical change seen in the earliest snakes.
Historically, there was debate about whether snakes evolved from land-dwelling or marine lizards, such as the extinct Mosasaurs. Modern genetic evidence does not support a close relationship between snakes and monitor lizards (the Mosasaur’s closest living kin). This suggests the ancestral snake lost its limbs while on land, likely adapting for movement through tight spaces like burrows.
Animals That Mimic the Snake Form
Many animals have independently evolved a serpentine shape through convergent evolution, making them superficially resemble true snakes. Legless lizards, such as the glass snake, are the most common cause of confusion because they are fellow members of the order Squamata. Unlike true snakes, which have fused, transparent eyelids (spectacles), legless lizards retain movable eyelids and can blink.
Legless lizards also possess external ear openings, which are absent in true snakes. Their skull structure is far more rigid, lacking the extreme flexibility and specialized jaw mechanisms that allow snakes to swallow prey much larger than their head. This difference reflects their distinct evolutionary paths, even within the same biological order.
Other unrelated groups have adopted the elongated form, including caecilians and eels. Caecilians are amphibians, possessing moist, slimy skin without protective scales, and often have visible grooves encircling their bodies. These burrowing creatures feature unique chemosensory tentacles on their head, which snakes lack, and a robust, fused skull structure used for pushing through soil. Eels, as fish, are fundamentally different, possessing gills for breathing underwater and often having dorsal and anal fins, features never found on a snake.
Specialized Body Structure
The serpentine body form relies on a highly specialized internal anatomy. Snakes possess a high number of vertebrae, ranging from 200 to over 400, and sometimes up to 700. Nearly every vertebra is connected to a pair of ribs, providing both the flexibility to contort and the structural support needed for locomotion.
Unlike most vertebrates, snakes lack a sternum (breastbone), allowing their ribs to move independently and expand widely when consuming large prey. This skeletal arrangement enables specialized movements like sidewinding or rectilinear motion, which involves lifting and pulling segments of the body forward. The internal organs are elongated and arranged sequentially to fit the narrow body cavity, often featuring a reduced or entirely absent left lung.
The ventral scales on the underside of a snake play a mechanical role by gripping surfaces and reducing backward slippage during forward motion. These broad, specialized scales work with underlying muscles to provide friction and allow the snake to propel itself across various terrains. This combination of hyper-flexible skeleton, specialized musculature, and unique external scales defines the physical mechanism of the snake’s limbless existence.