One Blueprint, Many Forms
Every tetrapod forelimb — from the wing of a bat to the flipper of a whale — is built from the same set of bones: one humerus, two forearm bones (radius and ulna), a cluster of carpals, a fan of metacarpals, and jointed phalanges. Richard Owen first formalized this observation in 1843, coining the term 'homology' to describe structural correspondence independent of function. Darwin later explained homology as inheritance from a common ancestor, making it one of the strongest lines of evidence for evolution.
Morphing Between Species
This simulator lets you interpolate the skeletal outline of one species into another, watching bones stretch, compress, and fuse in real time. As you drag the morph slider from human to horse, you see metacarpals III elongate dramatically while digits I, II, IV, V regress to vestigial splints — a visual record of 55 million years of equine evolution compressed into a single animation.
Bone-by-Bone Comparison
Highlight individual bone groups to see exactly how each element transforms across taxa. The humerus stays relatively conservative; the real action is in the digits. Bats elongated digits II–V to support a membrane wing, whales shortened and flattened all elements into a hydrodynamic paddle, and humans retained moderate proportions for dexterous manipulation. Same parts, endlessly remodeled.
Quantifying Homology
Modern comparative anatomy uses geometric morphometrics to move beyond qualitative 'same bone' statements. Landmark coordinates on each bone are superimposed using Procrustes analysis, removing size and orientation differences, then principal component analysis reveals the major axes of shape variation across species. The similarity index in this simulator approximates that pipeline, giving a single number that captures how structurally close two forelimbs really are.