Laetoli is the name of an archaeological site in northern Tanzania, where by some very odd twist of fate, the footprints of three Australopithecines have have been preserved in the ash fall of a volcanic eruption some 3.5-3.8 million years.
The two 27.5 meter long trails of footprints were created in moist volcanic ash which later hardened because of desiccation and chemical change. Three hominin individuals are represented, called G1, G2, and G3. G1, represented by 38 footprints, was a short individual, 1.22 meter or less in height. Individuals G2 and G3 were larger, G3 following almost but not quite, in the 31 footprints of G2. The footprints were discovered in the late 1970s by Mary Leakey, and they are believed to represent Australopithecus afarensis.
The footprints have been definitely linked to A. afarensis, because, like the fossils of afarensis, the Laetoli footprints do not indicate an opposable great toe. Many researchers believe that the Laetoli footprints prove that our Australopithecine ancestors were bipedal, and walked in a modern manner, heel first, then toe. However, a recent study suggests that the speed at which the footprints were made might effect the kind of gait required to make the marks.
The Sadiman Volcano and Laetoli
The source of the volcanic tuff in which the footprints were pressed has long been believed to be the Sadiman volcano. Sadiman, located about 20 kilometers southeast of Laetoli, is now dormant, but was active between 4.8 and 3.3 million years ago. A recent examination of outflows from Sadiman (described in Zaitsev et al below) has shown that the geology of Sadiman does not fit with the tuff at Laetoli. Zaitsev et al. believe that another, as yet unknown source for the tuff must have been responsible.
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Zaitsev AN, Wenzel T, Spratt J, Williams TC, Strekopytov S, Sharygin VV, Petrov SV, Golovina TA, Zaitseva EO, and Markl G. 2011. Was Sadiman volcano a source for the Laetoli Footprint Tuff? Journal of Human Evolution 61(1):121-124.