AL 333

Topographic map showing the Afar Triangle

In the late 1960s, the French paleoanthropologist Maurice Taieb started geological exploration of the relatively unexplored area of Ethiopia known as the Afar Triangle, located in the north of the country. Also known as the Danakil depression or Afar depression, this triangle is the lowest point in Ethiopia and one of the lowest in Africa. In 1972, Taieb invited Yves Coppens, a French paleontologist, Jon Kalb, an American geologist, and Donald Johanson, an American anthropologist, to survey the region in order to appraise the area's field exploration potential. They soon settled on working in the Hadar Formation, a sedimentary geological formation within the region. The four men established the International Afar Research Expedition (IARE), with Johanson in charge of the paleoanthropology aspect of the expedition.[2]^{: 466–468}

Historically, the Afar Triangle had been unexplored because it was remote and inhospitable. However, the IARE chose to explore the region for other reasons. The geological sequence of the Hadar Formation consists of nearly 200 meters of strata, or rock layers, which span a significant geological time. The sediment was also fossil-rich and often preserved partial skeletons of animals, implying that the researchers could potentially recover well-preserved and more complete fossils from the environment. Furthermore, the area had feldspars and volcanic glass that would be valuable for chronometric dating.[2]^: 467

From 1973 to 1977, the IARE campaigns resulted in the discovery of about 250 hominid fossils.[2]^{: 466–468} The most famous of the Hadar discoveries is Lucy, the most complete A. afarensis skeleton that has been discovered. However, in 1975, this same formation also witnessed the discovery of numerous remains from another site, AL 333. These remains became known as the "First Family," and represent at least thirteen different individuals, both adults and children. The recovery of these 216 hominid specimens is unique in African paleoanthropology, since the close proximity of the different fossils suggests that these were individuals who might have lived in a group or been part of the same family.[2]^{: 471–472}

Of the 216 specimens, 197 were surface finds, and 19 were found within 80 cm in the ground, suggesting a common time of death. Further visits to AL 333 resulted in the discovery of 23 additional postcranial and 3 mandibular and dental specimens. This increased the estimate from 13 to at least 17 individuals (9 adults, 3 adolescents, and 5 young children). The fossils lacked extensive weathering.[2]^{: 471–472}

In 2000, a complete fossil of the fourth metatarsal was recovered from AL 333. The morphology of this bone suggests that A. afarensis had transverse and longitudinal foot arches and therefore also had a very human-like bipedal gait.[3]

The discovery of all of the fossils at AL 333 aligned close together in one geological stratum is an obvious sign that they died at about the same time. But absolute dating had to be used to ascertain that time. Because the specimens were found between two layers of volcanic ash, potassium-argon dating was used. Potassium-argon dating measures the ratio of radioactive potassium and the argon it decays into. It is ideal for dating volcanic material. In the case of AL 333, this method yielded an age of 3.18-3.21 million years.[4]