It is postulated that the specimens in the region were deposited by way of a large river system with associated crevasse channels/splays, deltas, and distributary channels, as well as periodic transgressions of paleolake Hadar located east of the research area (Aronson and Taieb, 1981, Tiercelin, 1986, Campisano and Feibel, in press) possibly related to geological activity or climatic cycles in at least the Kada Hadar Member (Yemane et al., 1996, Yemane, 1997, Campisano and Feibel, in press)."

In October 1973, 16 individuals with the International Afar Research Expedition (IARE) arrived at Hadar and camped there for two months during which the first hominin fossil was found.

)[4] The IARE party examined a series of sedimentary layers called the Hadar Formation, which was dated to the late Pliocene to early Pleistocene epochs (3.5 to 2.3 million years ago).

[5] The anthropologist Donald Johanson, a member of the 1973 expedition to Hadar, returned the next year and discovered the fossil hominin "Lucy" in the late fall of 1974.

In 1975, Donald Johanson made another discovery at a nearby site in Hadar: 216 specimens from approximately 17 individuals, most likely related and varying in age, called AL 333 (colloquially referred to as the "First Family").

[3] Although Australopithecus is well-known for its discovery since the 1970s, other specimens from the Hadar Formation contain several fossil remains of artiodactyls, perissodactyls, carnivorans, proboscideans, and other African species that are well preserved.

[8][9][10] Artiodactyls outside the bovid family were present within the formation as well, namely the giraffids (Giraffa and Sivatherium), Hippopotamidae (Hippopotamus), and suids (Kolpochoerus, Notochoerus, and Nyanzachoerus).

Mammals within the formation outside the artiodactyl and carnivoran families include a bat (indeterminate), the leporid (Lepus), the equid (Eurygnathohippus),[15] rhinoceroses (Ceratotherium and Diceros),[16] old world primates (Parapapio, Theropithecus, and Cercopithecoides), proboscideans (the deinothere Deinotherium and elephants Elephas, Loxodonta, and Mammuthus) [17][18][19] old world porcupines (Hystrix and Xenohystrix),[20] murid rodents (Gerbilliscus, Acomys, Golunda, Oenomys, Praomys, Saidomys, Millardia, and Mus),[21] the spalacid Tachyoryctes, a squirrel indet., and an aardvark species.

Taxons within other classes are present within the Hadar Formation as well, such as birds (Plectropterus, Balearica, Anhinga, and Struthio) and reptiles (Crocodylus, Python, Varanus, and Bitis).