Imagine a broad swathe of flat, wet sand along a beach with two sets of footprints extending away and disappearing into the dry, powdery sand above the wave line about 70 feet away. One set large, the other small, parallel, close 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, holding hands and merrily chatting as they walked?
Now imagine similar footprints, not in today's wet sand, preserved in hardened volcanic ash mud that is almost four million years old. What might you wonder now? Such footprints indeed exist, and are known as the Laetoli footprints.
The Laetoli footprints 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 in laboratories by dull, plodding scientists with narrowly-focused minds and eyes, but the Laetoli discovery happened far differently. Two paleoanthropologists, in a group led by the famous anthropologist Mary Leakey, were horsing around, throwing elephant dung at each other while walking a familiar path back from the dig one day. After Andrew Hill dodged one well-aimed faecal projectile, he found himself face-down on the ground and staring at footprints fossilised in a layer of hardened volcanic mud. No one had noticed them before. Later excavation revealed an astonishing find that came to be known as the Laetoli footprints.
The footprint trails are set in cement-like hardened volcanic mud and extend about 25 metres, 80 feet, over level ground. One set of prints was made by a larger, heavier creature; the other by a smaller, lighter one. Both sets are very similar and show that the two were walking in step - side-by-side most of the time. It appears that a third individual, much smaller than the first two, was following behind, stepping playfully in the footprints left by the larger individual.
Strikingly, the stride and gait are quite human-like in appearance, entirely different from those of modern apes when they walk upright. Looking more closely at the individual prints, they are considerably smaller than the prints of modern adult humans. Still, a non-expert observer would probably mistake them for human footprints, although the big toes angle off towards the outside and all of the toes are a bit longer than usual. One thing to keep in mind is that our idea of a 'normal' human footprint is that of feet that have been shaped by growing up in shoes. The footprints of people that traditionally do not wear shoes feature longer toes that spread more widely than are usually seen.
An unlikely but, for science, fortunate combination of events created and preserved these footprints. The ground in the area where the footprints were found was occasionally covered with ash from nearby volcanoes, much like how ground is covered with snow during a snowfall. On the day these three beings took their stroll, rain had turned the fallen ash into a fine mud that captured the footprints like a plaster casting. Before the prints were lost, they were buried beneath another heavy fall of ash. The layers of ash hardened, preserving the Laetoli footprints.
What makes these prints an almost unbelievable discovery are that not only are they clearly made by fully bipedal1 creatures, but are also almost indistinguishable from modern human footprints, despite being formed millions of years earlier than the earliest known fossilised human footprints.
The individual footprints are sufficiently well-formed and well-preserved to provide information on the soft tissues (skin and muscle) of their creators, yet even more interesting is the information determined about the skeletons of these upright-walking creatures. 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 to the feet of chimpanzees and the other pongid apes2. The feet of pongids are more like human hands than feet, with the long great toe sticking out at about 45°, like the human thumb.
The footprint impression clearly shows that the heel strikes first with weight then transferred forward to the ball of the foot before finally the toes push off. This is the same as the modern human stride. The spacing of the footprints also shows that the leg bone structure of this creature must have been similar to that of humans - particularly in that the upper bone, the femur, slanted inward so that the feet could fall near the body centre line. This allowed upright bipedal walking in the manner of humans.
A good sceptical question to ask is: 'How do we know that these footprints are really so old?' The answer is convincing - they were dated by the potassium-argon radiometric method. The stratum (layer) below the footprints was determined to be 3.7 million years before present (BP) and the layer above the prints at 3.5 million years BP. There is 10% error using this technique so it is fairly certain that the footprints are three to four million years old.
These fossilised footprints have generated much controversy, a lot of it within the relevant fields of science and even more between evolutionists and creationists. The creationists were delighted with the find, gleefully declaring this to be a clear fossil anomaly that destroys some of the basis for evolution and current anthropological theories as well as strongly challenging the methods of paleoanthropologists and their interpretation of the entire fossil record. Indeed, at the time of writing, a third to a half of web search results for 'Laetoli footprints' are for creationist sites.
Although scientists admit to being surprised that such an early hominid (proto-human) resembles modern humans closely in stride and standing posture, many scientists do not accept the idea that these footprints were made by humans. To begin with, there is no evidence of human culture from that time period. The artefacts that have been found do not indicate a human level of intelligence or of society.
Furthermore, the fossilised hominid skeletons from that time period, which include skulls, are clearly not human. The only hominids known to have lived at that time are Australopithecus afarensis, found in Ethiopia, and Australopithecus africanus, found in southern Africa.3 The famous Lucy skeleton, found in 1975, was complete enough to show that A afarensis, while appearing to be about as intelligent as a chimp4, could easily have been bipedal. Specifically, the shape of the pelvis was very humanlike and designed for upright walking. The femurs angled inward, a configuration that is necessary to allow the feet to catch the weight of the body near the midline allowing efficient and effective walking and running.
Although creationists want to see the Laetoli footprints as evidence that humans existed much earlier than evolutionists or paleoanthropologists admit, most scientists see the footprints as evidence supporting the theory that a hominid of that time was fully bipedal.
If it was indeed A afarensis/africanus that made the Laetoli footprints, they may have walked 'the walk', but they didn't talk 'the talk' - skull casts of A afarensis do not show significant development of the brain areas that support language abilities in modern humans. They also looked different to modern humans having ape-like heads with no chin, no forehead and a protruding jaw.