Relationship between trackmakers of the Laetoli footprints from gait synchronization

The parallel trails of footprints at Laetoli site G are important fossils for studying the characteristics of <i>Australopithecus afarensis</i>. However, the relationship between the trackmakers - i.e. whether it was that of an adult male-female pair or of parent-offspring - remains unclear. The footprints show that the two individuals walked side by side with a narrow and constant distance between them and synchronized their leg movements and step lengths (gait synchronization), although they had a large height difference. In this study, live camera videos were collected to obtain data on gait synchronization in <i>Homo sapiens</i>, the closest extant species to <i>A. afarensis</i>. The data showed that when two humans with a large height difference walked alongside each other, with (at least) one of the pair having their arm around the other's shoulder or back, adult male-female pairs (couples) frequently synchronized their gait, but parent-offspring pairs did not, whereas both couples and parent-offspring seldom synchronized when they walked side by side without connection or with handholding. Two individuals only maintained a narrow and constant distance like that between the Laetoli footprints when they walked with an arm-around connection. Therefore, assuming that <i>A. afarensis</i> had the same gait synchronization tendency as <i>H. sapiens</i>, the trackmakers were more likely to be an adult male-female pair than a parent-offspring one.