Laetoli Footprints

Resource for Grades 6-12

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Running Time: 3m 20s
Size: 5.2 MB

Source: NOVA: "In Search of Human Origins"

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Laetoli Trackways Diagram
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The Laetoli footprints were formed and preserved by a chance combination of events -- a volcanic eruption, a rainstorm, and another ashfall. When they were found in 1976, these hominid tracks, at least 3.6 million years old, were some of the oldest evidence then known for upright bipedal walking, a major milestone in human evolution. In this Evolution video segment, Paleoanthropologist and consultant forensic scientist Owen Lovejoy compares the ancient biped prints with those of modern humans and chimpanzees.

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Background Essay

Thank goodness for the irrepressible urge of humans (and other animals) to joke and play around in nearly any situation. Sometimes, it pays big dividends. It certainly did in 1976, when paleoanthropologist Andrew Hill and a colleague were tossing elephant dung at each other in Laetoli, a hominid archeological site in Tanzania. As Hill dived out of the way, he stumbled on what turned out to be one of the wonders of prehistoric finds: a trail of hominid footprints about 3.6 million years old.

The majority of the Laetoli footprint site was excavated in 1978. Until then, the oldest known footprints of human ancestors were tens of thousands of years old. But this trail, some 80 feet long and preserved in cementlike volcanic ash, had been made by some of the first upright-walking hominids. An almost unimaginable sequence of events preserved what paleontologist Ian Tattersall calls a fossil of human behavior -- prehistoric walking.

Initially, a nearby volcano called Sadiman erupted a cloud of fine ash, like beach sand, that left a layer on the landscape. Then a light rain fell onto the ash to create something like wet cement -- an ideal material for trapping footprints. Birds and mammals left a great number of prints, but, spectacularly, so did a pair of hominids, one large and one small, trekking across the ash. (Some analysts conclude that it is possible to detect the trail of a third, smaller individual whose tracks overlap the footprints left by one of the others.) A subsequent eruption from Sadiman dropped more ash, sealing the footprints like a laminated driver's license. Finally, erosion over millions of years unveiled the prints for Hill and other researchers in Mary Leakey's group to discover.

The prints, say experts on hominid body structure, are strikingly different from those of a chimpanzee, and in fact are hardly distinguishable from those of modern humans. The only known hominid fossils of that age in that location are those of Lucy and her kind, the small-brained but upright-walking hominids classified as Australopithecus afarensis. Some analysts have noted that the smaller of the two clearest trails bears telltale signs that suggest whoever left the prints was burdened on one side -- perhaps a female carrying an infant on her hip. While the detailed interpretation of the prints remains a matter of debate, they remain an extraordinary and fascinating fossil find, preserving a moment in prehistoric time.

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Discussion Questions

Describe the unusual series of circumstances that caused the Laetoli footprints to be preserved. Does this combination of events say anything about why such footprints are rare finds?

Discuss why these footprints are so important to paleoanthropologists.

The existence of an arch in the foot is described as an important advantage for a primate that always walks erect on two legs. Discuss and explain this line of reasoning.

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Transcript

NARRATOR: A volcanic eruption devastated the landscape near Laetoli, Tanzania, some three and a half million years ago...releasing tons of ash into the atmosphere.

Multiple blasts covered the countryside with sooty sediment. By chance, the eruptions coincided with the rainy season. Rain turned the ash to mud. As the rain let up, animals ventured across the smooth, wet surface, leaving prints. Here, a bird meandered through the muck. And then, an African hare. And eventually, other footprints were left, ones that look as familiar as our own.

Before another rain shower could wash them away, more ash fell, covering the prints and capturing them forever, as the ash hardened into rock. Three and a half million years later, anthropologist Mary Leakey found this path of footprints exposed once more by erosion.

Two individuals clearly walked here. And details of the prints suggest that another followed them... stepping in the prints left by one of those who walked ahead. Footprints can reveal other secrets, too, as forensic scientist Owen Lovejoy knows. (siren) They're often crucial pieces of evidence at crime scenes.

OWEN LOVEJOY, Forensic Scientist: There's no better evidence than that provided by a footprint. That's what makes the Laetoli prints so exciting, because they give us a direct record of how our ancestors walked almost four million years ago.

When we compare the Laetoli print to that of a chimpanzee, the difference is immediately obvious. The chimpanzee, which is a quadruped--but occasionally a biped--still has a free great toe, and that great toe extends out away from the foot and leaves a very distinct mark. On the other hand, when we compare the Laetoli print to that of a crime scene human print, they're virtually indistinguishable.

The great toe is in line with the rest of the toes, and what this has done, in the human and the Laetoli print, is to create an arch, and that's a hallmark of typical modern upright locomotion, because the arch is an energy absorber and that's the kind of fine tuning that you would expect in a biped that had been that way for a very long period of time.