-The discovery-
In 1976 Mary Leakey, accompanied by her husband Louis Leakey, led a research team to Olduvai Gorge to study and hunt for hominid fossils. Along the way they decided to stop just south of their destination. It was here that an important discovery in human evolution took place.

Credit generally goes to Mary Leakey and her husband for the find; they were the anthropologists leading the team and certainly the most widely known. However, it was paleoanthropologist Andrew Hill who stumbled upon a 3.6 million-year-old trail of footprints while tossing elephant dung at a colleague.

Mary took samples from, and made molds of, the seventy footprints that ran in two parallel lines for thirty meters, then covered the site. This last bit was important to prevent the further wearing away that had already begun on the exposed prints. Later the site was preserved against the elements and encroaching vegetation and any saplings that spring up are removed.

-The prints-
The footprints are an important find because they are "a fossil of human behavior--prehistoric walking." They showed that 3.6 million-years-ago our ancesters were bipedal hominids, something that at the time hadn't been proven.

Scientists can look at the impressions, measure the depth of them, and deduce that, like modern man, these hominids walked upright. There are two primary characterizations of the prints that give this away. One is that unlike the foot of a chimpanzee or orangutan, the big toe of this hominid isn't diverged from the rest of the foot. Rather, it is set close to the remaining toes much the same as the one in your shoe right now.

The other characteristic can be seen in the gait of the footprints. When chimpanzees walk on their legs alone, they put their weight on the outside edges of their feet. A hominid that primarily walks upright and doesn't spend much (if any) time in the trees, however, initially puts their weight on the heel then transfers it along the side, across the ball of the foot and pushes off with the big toe.

-Placing the prints-
That we found such a great specimen of footprints after millions of years was lucky. It required an excellent set of circumstances to occur. In the case of the Laetoli footprints this is what we believe happened:

Millions of years ago in what is now known as Laetoli, Tanzania a volcano erupted. This volcano, now known as Sadiman, sent great clouds of ash into the air. The ash coated the earth in a fine layer as the inhabitants of the area fled for safety. Among the animals and birds that scampered in the sand were two Australopithecus afarensis travellers. Since one set of footprints is slightly smaller than the other it is believed that it was a male and female who transversed Laetoli. This couple, possibly followed or accompanied by a smaller hominid, stepped in the ash as it began to rain lightly. Their feet sank into the moist sand leaving impressions. The volcano erupted a cloud of ash again and sealed the footprints, drying in a cement-like way and preserving them for later discovery.

Human Ancestors Hall, http://www.mnh.si.edu/anthro/humanorigins/ha/laetoli.htm
Laetoli: Footprints in the Past, http://emuseum.mnsu.edu/archaeology/sites/africa/laetoli2.html
PBS: A Science Odyssey, http://www.pbs.org/wgbh/aso/tryit/evolution/footprints.html
PBS: Evolution, http://www.pbs.org/wgbh/evolution/library/07/1/1_071_03.html