Laetoli was first recognized by western science in 1935 through a man named Sanimu, who convinced archeologist Louis Leakey to investigate the area. Several mammalian fossils were collected with a left lower canine tooth originally identified as that of a non-human primate, but later was revealed (in 1979, by P. Andrews and T. White) as the site's first fossil hominin.
In 1938 and 1939, German archaeologist Ludwig Kohl-Larsen studied the site extensively. Several hominin remains, including premolars, molars, and incisors, were identified. A later excavation in 1959 revealed no new hominins, and Laetoli went relatively unexplored until 1974—when the discovery of a hominin premolar by George Dove revived interest in the site. Mary Leakey returned and almost immediately discovered the well-preserved remains of hominins. In 1978, Leakey's 1976 discovery of hominin tracks—"The Laetoli Footprints"—provided convincing evidence of bipedalism in Pliocene hominins and gained significant recognition by both scientists and laymen.
Although much debated, researchers have tentatively concluded that Australopithecus afarensis is the species of the three hominins who made the footprints at Laetoli. This conclusion is based on the reconstruction of the foot skeleton of a female A. afarensis hominin by anthropologists Tim D. White and Gen Suwa of the University of California, as well as detailed footprint analysis by Russel Tuttle of the University of Chicago; he compared human and other bi-pedal animals such as bears and primates, including gaits and foot structure, and taking into account the use of footwear. For gait Tuttle looked at the step length, stride length, stride width, and foot angle, and determined that A. afarensis was more human-like in gait than ape-like.
A. afarensis is an obligate bipedal hominin with the beginnings of sexual dimorphism attributed to its species, and brain size very similar to that of modern chimpanzees and gorillas. Analysis of the Laetoli footprints indicated the characteristics of obligate bipedalism: pronounced heel strike from deep impressions, lateral transmission of force from the heel to the base of the lateral metatarsal, a well-developed medial longitudinal arch, adducted big toe, and a deep impression for the big toe commensurate with toe-off.
Two dating techniques were used to arrive at the approximate age of the beds that make up the ground layers at Laetoli: potassium-argon dating and analysis of stratigraphy. Based on these methods, the layers have been named as follows, starting with the deepest: Lower Laetolil Beds, Upper Laetolil Beds, Lower Ndolanya Beds, Upper Ndolanya Beds, Ogol lavas, Naibadad Beds, Olpiro Beds, and Ngaloba Beds; it is the ancient Laetolil Beds that contain the footprints trackway. The upper unit of the Laetolil Beds dated back 3.6 to 3.8 million years ago. The beds are dominantly tuffs and have a maximum thickness of 130 meters. No mammalian fauna were found in the lower unit of the Laetolil Beds, and no date could be assigned to this layer.
The Ndolanya Beds, which are located above the Laetolil Beds and underlie the Ogol lavas, are clearly divisible into upper and lower units separated by a widespread deposit of calcrete up to one meter thick. However, like the Lower Laetolil Beds, no date can be assigned to the Ndolanya Beds. The Ogol lavas date back 2.4 million years. No fauna or artifacts are known from the Naibadad Beds, but they are correlated with a bed layer at Olduvai Gorge based on mineral content. Pleistocene fauna and Acheulean artifacts have been found in the Olpiro Beds. Based on a trachytic tuff which occurs within the beds, the Ngaloba Beds may therefore be dated between 120,000 and 150,000 years BP.
The principal discovery, made by Mary Leakey and her team in 1976 (and fully excavated by 1978), is an 75-foot (24-meter) line of hominin fossil footprints, preserved in powdery volcanic ash originally thought to have been from an eruption of the nearby (20 km) Sadiman volcano. However, recent study of the Sadiman volcano has shown that it is not a source for the Laetoli Footprints Tuff (Zaitsev et al. 2011). Soft rain cemented the ash-layer (15 cm thick) to tuff without destroying the prints. In time, they were covered by other ash deposits.
The hominin prints were produced by three individuals, one walking in the footprints of the other, making the preceding footprints difficult to recover. As the tracks lead in the same direction, they might have been produced by a group visiting a waterhole together, but there is nothing—or very little (see below, Interpretation and significance)—to support the common assumption of a nuclear family.
The footprints demonstrate that the hominins habitually walked upright as there are no knuckle-impressions. The feet do not have the mobile big toe of apes; instead, they have an arch (the bending of the sole of the foot) typical of modern humans. The hominins seem to have moved in a leisurely stroll.
Computer simulations based on information from A. afarensis fossil skeletons and the spacing of the footprints indicate that the hominins were walking at 1.0 m/s or above, which matches human walking speeds. The results of other studies have also supported the theory of a human-like gait.
In 2015 footprints of the same age as the first reported footprints were unearted at a site approximately 150 meters south of the original site G footprints. This site is called site S, and the 2 individuals who made the prints are named S1 and S2. S2 is represented by only 1 print, but S1 left a track of prints, the first 4 of which are shown in the composite image, along with an analysis of step and stride lengths. Furter analysis indicated that individual S1 was considerably larger than any of the three indivduals from site G.
Other prints show the presence of twenty different animal species besides the hominin A. afarensis, among them hyenas, wild cats (Machairodus), baboons, wild boars, giraffes, gazelles, rhinos, several kinds of antelope, Hipparion, buffaloes, elephant relatives (of the extinct Deinotherium genus), hares and birds. Rain-prints can be seen as well. Few footprints are superimposed, which indicates that they were rapidly covered up. Most of the animals are represented by skeletal remains discovered in the area.
No artifacts have been found in the vicinity, at least within the ancient Laetolil Beds that contain the trackway. However, artifacts from the younger Olpiro and Ngaloba Beds, also preserved at Laetoli, have been found.
Before the discovery of the Laetoli footprints, there was much debate as to which developed first in the human evolutionary time line: a larger brain or bipedalism. The discovery of these footprints settled the issue, proving that the Laetoli hominins were fully bipedal long before the evolution of the modern human brain, and were bipedal close to a million years before the earliest known stone tools were made. The footprints were classified as possibly belonging to Australopithecus afarensis.
Some analysts have noted in their interpretations that the smaller trail bears "telltale signs that suggest whoever left the prints was burdened on one side." This may suggest that a female was carrying an infant on her hip but this cannot be proven for certain.
The footprints themselves were an unlikely discovery because they closely resemble modern human footprints, despite being almost 4 million years old. It is noted that the toe pattern is much the same as the human foot, which is much different than the feet of chimpanzees and other non-bipedal beings. The footprint impression has been interpreted as the same as the modern human stride, with the heel striking first and then a weight transfer to the ball of the foot before pushing off the toes.
Based on stratigraphic analysis, the findings also provide insight into the climate at the time of the making of the footprints. Pliocene sediments show that the environment was more moist and productive than now. Climate changes that caused a shift from forest to grassland environments have a strong correlation with upright posture and bipedalism in hominins. This could have initiated the evolution to bipedalism of the hominins found at Laetoli.
In 1979, after the Laetoli footprints were recorded, they were re-buried as a then-novel way of preservation. The site was re-vegetated by acacia trees, which later gave rise to fears over root growth. In mid-1992, a GCI-Tanzanian team investigated this by opening a three-by-three meter trench, which showed that roots had damaged the footprints. However, the part of the trackway unaffected by root growth showed exceptional preservation. The success of the experiment led to an increased practice in reburials for preserving excavated sites.
In 1993, measures were taken to prevent erosion. The original trackway was remolded and new casts were made. As the trackway is very fragile, the new replica cast was used to guide re-excavation in the field. A team of specialists, one being Fiona Marshall, re-excavated half of the trackway to record its condition, stabilize the surface, extract dead roots and rebury it with synthetic geotextile materials. This allows the trackway surface to breathe, and protects it against root growth.
Proposals for lifting the track and moving it to an enclosed site have been suggested, but the cost is viewed as outweighing the benefits: the process would require much research, a large amount of money, and there is a risk of loss or damage. Thus, burial seems to be the most effective method of preservation.