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 Australopithecus afarensis. 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 Homo sapiens, the closest extant species to A. afarensis. 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 A. afarensis had the same gait synchronization tendency as H. sapiens, the trackmakers were more likely to be an adult male-female pair than a parent-offspring one.