If so, walking may have evolved in leaps and bounds, rather than gradually, among ancient hominids.
The discovery comes from the famed trackway site in Laetoli, Tanzania, where more than 30 years ago researchers discovered footprint trails from two, and possibly three, human ancestors who had walked across a wet field of volcanic ash.
The new analysis shows that the Laetoli hominids made equally deep heel and toe impressions while walking across a soft surface, say anthropologist David Raichlen of the University of Arizona in Tucson and his colleagues.
That pattern is a cardinal sign of a humanlike gait, and suggests that an energetically efficient, extended-leg stride appeared surprisingly early in hominid evolution.
Until now, many researchers suspected that such a gait did not appear at least until the appearance of early Homo species around 2.5 million years ago.