-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

Imagine a broad swath of flat, wet sand along a beach. Two sets of footprints extend away from you and disappear into the dry powdery sand above the wave line about 70 feet away. One set is large; the other is small and tracks close and parallel to the first. You might wonder who made those prints. Were they a young man and woman walking hip-to-hip, embraced? Were they an adult and child, chatting merrily as they walked along?

Now imagine similar footprints, not in today's wet sand, but preserved in hardened volcanic ash that is 3.6 million years old. What do you wonder now?

Such footprints do indeed exist, and are known as the Laetoli footprints. They were discovered in 1976, not far from the village of Laetoli in a remote part of Tanzania. We tend to think that major scientific discoveries are made by dull, plodding scientists with narrowly-focused eyes and minds, but the Laetoli discovery happened far differently. Two paleoanthropologists in by a party led by Mary Leakey were horsing around, throwing elephant dung at each other one day. When Andrew Hill dodged one well-aimed fecal projectile, he found himself face-down on the ground and staring at animal footprints fossilized in a layer of hardened volcanic mud that no one had noticed before. Later excavation revealed something astonishing, what came to be known as the Laetoli footprints.

What makes these prints an almost incredible discovery is that not only are they clearly made by fully bipedal creatures, but they are almost indistinguishable from modern human footprints, even though they are dated millions of years earlier than the earliest known human prints. The footprint trail extends about 80 feet over level ground. One set of prints was made by a larger, heavier creature, the other was made by a smaller, lighter one. The smaller set parallels the larger closely, and shows that the two were walking in step and side-by-side most of the time. It appears that a third individual, much smaller than the first two, was following behind by stepping in the footprints left by the larger individual, playfully one might imagine.

The individual footprints are sufficiently well-formed and well-preserved to provide information on the soft tissues, but it is the skeletal information that is most interesting. The toe pattern is much the same as the modern human foot. The toes are relatively short and the big toe is in line with the other toes. This is much different than in the feet of gorillas and chimpanzees (pongids). The feet of pongids are more like human hands than human feet, with the long great toe sticking out at about 45 degrees. The fossil footprint impression clearly shows that the heel strikes first, weight is transferred forward to the ball of the foot, and then the toes push off. This is the same as the modern human stride. Also, the spacing of the footprints shows that the leg bone structure of this creature must also have been similar to that of humans, particularly in that the upper bone (femur) slanted inward so that the feet could fall near the body center line, thus allowing upright bipedal walking.

These footprint fossils have generated great controversy, a lot of it within the relevant fields of science and even more between scientists and creationists. In fact, the creation scientists had a field day, declaring this to be a clear fossil anomaly that destroys some of the basis for evolution and current anthropological theories and strongly impugns paleoanthropologist's methods and interpretation of the fossil record. About a third to half of the Web hits for "Laetoli footprints" lead to creation science sites.

The first question anyone might ask is how do we know how old these footprints are? They were dated by the potassium-argon dating potassium-argon method. The stratum below the footprints was determined to be 3.7 million years before present and the layer above the prints was determined to be 3.5 million years BP. With a measurement error of plus or minus 10%, it would be very conservative to say that the footprints are between three and four million years old.

The next question might be, what humanoid fossils have been found for that time period? The only hominid known to have lived at that time was Australopithecus afarensis. The famous Lucy skeleton, found in 1975, was complete enough to show that A. afarensis, while not much brighter than a chimp, could well have been bipedal.

Creationists want to see the Laetoli footprints as evidence that man existed much earlier than evolutionists or paleoanthropologists will admit. Scientists are surprised at how early bipedalism developed, but want to see the footprints as evidence that A. afarensis was fully bipedal. If it in fact was A. afarensis that made the Laetoli footprints, they may have walked like humans, but they most surely didn't talk like humans. Their brain cavity held a mere 400 cc or so, just one-fourth the brain volume of a modern Homo sapiens sapiens. Nor would you likely want to date one. Their heads were quite ape-like, with no chin, no forehead and a protruding jaw.

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