Human footprints, preserved by some geological twist of fate for hundreds of thousands or even millions of years after some human ancestor walked that way, are an amazing source of information about evolution.
In particular, the footprints of our human ancestors allow a fascinating glimpse into the evolution of bipedal locomotion—walking up-right on two legs. The study of footprints can tell us about the height, weight, and gait of an individual, how well she or she walked, and, occasionally, behavioral clues about where and how he or she lived.
The most famous set of foot prints are also the oldest ones found to date, from Laetoli, in Tanzania. Those prints were of three different Australopithecus, and date to 3.5-3.7 million years ago.
The Devil's Footprints, made about 345,000 years ago on the volcanically active Campanian plain of Italy, were probably made by H. heidelbergensis.
About a decade ago, a trackway of footprints along the shore of Langebaan Lagoon in South Africa proved to be anatomically modern humans, about 117,000 years ago.
GaJi10 is a site in the Koobi Fora region of Kenya where a trail of seven hominin footprints were discovered in the 1970s. These tracks, dated to 1.435 million years ago, were made in a lake margin environment, and were found near other tracks made by wading birds and hippopotamus. Recent research by Bennett et al. cited below leads them to suggest these were made by Homo erectus.
The most recent discovery set of ancient human footprints is the Ileret footprints, from the FwJj13E site in Kenya, dated to about 1.5 million years ago and likely also representing Homo erectus.
Bennett, Matthew R., et al. 2009 Early Hominin Foot Morphology Based on 1.5-Million-Year-Old Footprints from Ileret, Kenya. Science 323:1197-1201.
Feibel, Craig S., et al. 1995-1996 The Laetoli Hominid footprints--A preliminary report on the conservation and scientific restudy. Evolutionary Anthropology 4(5):149-154.
Leakey, M. D. and R. L. Hay. 1979. Pliocene footprints in the Laetolil Beds at Laetoli, northern Tanzania. Nature 278:317-323.
Mietto, Paolo, Marco Avanzini, and Giuseppe Rolandi. 2003. Palaeontology: Human footprints in Pleistocene volcanic ash. Nature 422:133
Raichlen, David A., Herman Pontzer, and Michael D. Sockol. 2008. The Laetoli footprints and early hominin locomotor kinematics. Journal of Human Evolution 54(1):112-117.
Roberts, D. and L.R. Berger 1997. Last interglacial (c. 117 kyr) human footprints from South Africa, South African Journal of Science 93(8):349-350
Tuttle, R. H., D. M. Webb, and M. Baksh 1991 Laetoli toes and Australopithecus afarensis. Human Evolution 6(3):193-200.
Scaillet, Stéphane, Grazia Vita-Scailleta, and Hervé Guillou 2008 Oldest human footprints dated by Ar/Ar. Earth and Planetary Science Letters 275(3-4):320-325.