Fourlimbs

Fourlimbs are the paired forelimbs and hindlimbs of tetrapod vertebrates. In most species, forelimbs attach to the pectoral girdle and hindlimbs attach to the pelvic girdle. The term is used in comparative anatomy and evolution to refer to the common two-pair limb plan shared by amphibians, reptiles, birds, and mammals, including humans.

Fourlimbs evolved from lobe-finned fishes two or more hundred million years ago, enabling a transition from aquatic to terrestrial life. The forelimbs and hindlimbs are built on a shared skeletal pattern, with bones that are homologous across groups, though their proportions and shapes vary to suit different modes of locomotion such as walking, climbing, swimming, or flight.

Anatomy: Each limb comprises a proximal girdle attachment, a series of long bones, and distal digits. Forelimbs typically include the humerus, radius and ulna, carpals, metacarpals, and phalanges; hindlimbs include the femur, tibia and fibula, tarsals, metatarsals, and phalanges. Joints at the shoulder/hip and elbow/knee, along with the wrist or ankle, allow a range of movements.

Variation among tetrapods is extensive. Fourlimbs are well developed in many terrestrial mammals and birds, while some species retain very specialized forms for propulsion or grasping. In snakes, hindlimbs are reduced or absent. Birds and bats have wings formed from remodeling of forelimbs. In aquatic mammals, hindlimbs may be reduced or vestigial. Limb development in embryos is governed by genetic networks that pattern the limb along proximal–distal, anterior–posterior, and dorsal–ventral axes.