Paleoanthropologists have had difficulty calculating the brachial index (radius length / humerus length X 100) of Australopithecus afarensis A.L. 288‐1, commonly known as “Lucy,” because bilateral segments of radial and ulnar diaphyses are missing. Conventional approaches to reconstruction have produced models of the upper limb ranging from “ape‐like” to “human‐like.” The aim of this study was to quantitatively estimate the brachial index of A. afarensis by aligning, scaling, and morphing a 3D surface model of a complete ulna from a larger A. afarensis individual (A.L. 438) to the A.L. 288‐1 ulna using a thin plate spline technique. To evaluate the potential effects of allometry and choice of “template” (i.e., the complete bone specimen used to reconstruct the incomplete specimen), the method was tested by simulating ulnar reconstructions in a sample of modern adult hominids exhibiting varying degrees of sexual dimorphism (Gorilla gorilla n=13, Pan troglodytes n=17, and Homo sapien n=14). The thin plate spline algorithm in Viewbox4 (dHAL) was used to transform models using 207 homologous landmarks on the proximal ulnae. Radius length was then estimated from the reconstructed ulna length. The resulting model of Lucy's ulna based on A.L. 438 measured 218 mm, and her radius was calculated as 206 mm. With an established humerus length of 237 mm, Lucy's brachial index was thus estimated to be 87. Although the simulation experiments show that error in the method could result in broad range of estimates depending on the template individual used, results are sufficiently constrained to indicate that Lucy's brachial index was probably unlike that of modern humans; rather, she most likely had a relatively long forearm giving a brachial index intermediate between that of chimpanzees and gorillas. This result has important implications for furthering discussion regarding A. afarensis locomotion and habitat use in the context of hominin evolution.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.