We had been working really hard that day and were heading back toward camp when one of our team decided to liven things up by slinging elephant dung at the rest of us. He aimed one at me, and I had to dive out of the way. I ended up flat on my face. I started to rise and saw marks in the ground. I realized they were fossilized raindrops. Then I looked around and saw ancient animal footprints all over the place. We had passed over that ground so many times before that evening, but none of us had noticed a thing. But once we saw the first prints, we could see them everywhere: fossilized tracks of rhino, elephants, antelopes, all sorts of animals.
(Recollection of Andrew Hill, a paleontologist on Mary Leakey's team at Laetoli, Tanzania in 1976. From McKie, Robin. Dawn of Man: The Story of Human Evolution. [New York City: Dorling Kindersley Publishing, Inc., 2000] pp.10-11)
As Leakey's team studied the Laetoli site over the next two years, more prints were uncovered in the ash -- hominid footprints that looked incredibly similar to those that people today make as they walk barefoot along a beach. But, these tracks were at least 3.6 million years old. It was the first time paleontologists had actually found behavioral evidence of bipedalism in early hominids. As Ian Tattersall, curator of physical anthropology at the American Museum of Natural History, has said, "Usually behavior has to be inferred indirectly from the evidence of bones and teeth, and there is almost always argument over inferences of this kind. But at Laetoli, through these footprints, behavior itself is fossilized."