Mary Leakey in the context of "Louis Leakey"

The Olduvai Gorge or Oldupai Gorge in Tanzania is one of the most important paleoanthropological localities in the world; the many sites exposed by the gorge have proven invaluable in furthering understanding of early human evolution. A steep-sided ravine in the Great Rift Valley that stretches across East Africa, it is about 48 km (30 mi) long, and is located in the eastern Serengeti Plains within the Ngorongoro Conservation Area in the Olbalbal ward located in Ngorongoro District of Arusha Region, about 45 kilometres (28 miles) from Laetoli, another important archaeological locality of early human occupation. The British/Kenyan paleoanthropologist-archeologist team of Mary and Louis Leakey established excavation and research programs at Olduvai Gorge that achieved great advances in human knowledge. The site is registered as one of the National Historic Sites of Tanzania.

The gorge takes its name from the Maasai word oldupai which means "the place of the wild sisal" as the East African wild sisal (Sansevieria ehrenbergii) grows abundantly throughout the gorge area. Twenty-five kilometers downstream of Lake Ndutu and Lake Masek, the gorge is the result of up to 90 m (300 ft) erosion cutting into the sediments of a Pleistocene lake bed. A side gorge, originating from Lemagrut Mountain, joins the main gorge 8 km (5.0 mi) from the mouth. This side gorge follows the shoreline of a prehistoric lake, rich in fossils and early hominin sites. Periodic flows of volcanic ash from Olmoti and Kerimasi helped to ensure preservation of the fossils in the gorge.

Australopithecus afarensis is an extinct species of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first fossils were discovered in the 1930s, but major fossil finds would not take place until the 1970s. From 1972 to 1977, the International Afar Research Expedition—led by anthropologists Maurice Taieb, Donald Johanson and Yves Coppens—unearthed several hundreds of hominin specimens in Hadar, Ethiopia, the most significant being the exceedingly well-preserved skeleton AL 288-1 ("Lucy") and the site AL 333 ("the First Family"). Beginning in 1974, Mary Leakey led an expedition into Laetoli, Tanzania, and notably recovered fossil trackways. In 1978, the species was first described, but this was followed by arguments for splitting the wealth of specimens into different species given the wide range of variation which had been attributed to sexual dimorphism (normal differences between males and females). A. afarensis probably descended from A. anamensis and is hypothesised to have given rise to Homo, though the latter is debated.

A. afarensis had a tall face, a delicate brow ridge, and prognathism (the jaw jutted outwards). The jawbone was quite robust, similar to that of gorillas. The living size of A. afarensis is debated, with arguments for and against marked size differences between males and females. Lucy measured perhaps 105 cm (3 ft 5 in) in height and 25–37 kg (55–82 lb), but she was rather small for her species. In contrast, a presumed male was estimated at 165 cm (5 ft 5 in) and 45 kg (99 lb). A perceived difference in male and female size may simply be sampling bias. The leg bones as well as the Laetoli fossil trackways suggest A. afarensis was a competent biped, though somewhat less efficient at walking and slower at running than humans. The arm and shoulder bones have some similar aspects to those of orangutans and gorillas, which has variously been interpreted as either evidence of partial tree-dwelling (arboreality), or basal traits inherited from the chimpanzee–human last common ancestor with no adaptive functionality.

Paranthropus boisei is a species of australopithecine from the Early Pleistocene of East Africa about 2.5 to 1.15 million years ago. The holotype specimen, OH 5, was discovered by palaeoanthropologist Mary Leakey in 1959 at Olduvai Gorge, Tanzania and described by her husband Louis a month later. It was originally placed into its own genus as "Zinjanthropus boisei", but is now relegated to Paranthropus along with other robust australopithecines. However, it is also argued that Paranthropus is an invalid grouping and synonymous with Australopithecus, so the species is also often classified as Australopithecus boisei.

Robust australopithecines are characterised by heavily built skulls capable of producing high stresses and bite forces, and some of the largest molars with the thickest enamel of any known ape. P. boisei is the most robust of this group. Brain size was about 450–550 cc (27–34 cu in), similar to other australopithecines. Some skulls are markedly smaller than others, which is taken as evidence of sexual dimorphism where females are much smaller than males, though body size is difficult to estimate given only one specimen, OH 80, definitely provides any bodily elements. The presumed male OH 80 may have been 156 cm (5 ft 1 in) tall and 61.7 kg (136 lb) in weight, and the presumed female KNM-ER 1500 124 cm (4 ft 1 in) tall (though its species designation is unclear). The arm and hand bones of OH 80 and KNM-ER 47000 suggest P. boisei was arboreal to a degree.

Laetoli is a pre-historic site located in Enduleni ward of Ngorongoro District in Arusha Region, Tanzania. The site is dated to the Plio-Pleistocene and famous for its Hominina footprints, preserved in volcanic ash. The site of the Laetoli footprints (Site G) is located 45 km south of Olduvai Gorge. The location and tracks were discovered by archaeologist Mary Leakey and her team in 1976, and were excavated by 1978. Based on analysis of the footfall impressions "The Laetoli Footprints" provided convincing evidence for the theory of bipedalism in Pliocene Hominina and received significant recognition by scientists and the public. Since 1998, paleontological expeditions have continued under the leadership of Amandus Kwekason of the National Museum of Tanzania and Terry Harrison of New York University, leading to the recovery of more than a dozen new Hominina finds, as well as a comprehensive reconstruction of the paleoecology. The site is a registered National Historic Sites of Tanzania.

Dated to 3.7 million years ago, they were the oldest known evidence of hominin bipedalism at that time. Subsequently, older Ardipithecus ramidus fossils were found with features that suggest bipedalism. With the footprints there were other discoveries excavated at Laetoli including Hominina and animal skeletal remains. Analysis of the footprints and skeletal structure showed clear evidence that bipedalism preceded enlarged brains in Hominina. At a species level, the identity of the Hominina who made the trace is difficult to construe precisely; Australopithecus afarensis is the species most commonly proposed.