Hadar is a palaeontological site in the Mille district of the Afar Region in Ethiopia about 700 kilometers from Addis Ababa. It is located along the left bank of the Awash River, between two minor tributaries, the Kada Hadar and the Kada Gona, on the southern edge of the Afar Triangle [4][6]. This site has produced a number of famous hominin fossils, the most notable of which is “Lucy” a partial skeleton of an Australopithecus afarensis who lived 3.2 million years ago. The age of the hominin fossils in Hadar range approximately from 3.40 to 2.40 million years ago [4][7][6][2]. 

Stratigraphy and Chronology 

The Hadar formation is primarily composed of fluvial-lacustrine-related mudstones, siltstones, fine-grained sandstones, and volcanic tuffs. The 150-meter thick formation is divided into four geologic members, which are temporarily calibrated by palaeomagnetic records and ⁴⁰Ar/³⁹Ar of tephra beds. These members are the Basal Member (~3.80- 3.42 Ma), the Sidi Hakoma Member (~3.42-3.24 Ma), the Denan Dora Member (~3.24-3.20 Ma), and the Kada Hadar Member (~3.20-2.94 Ma) [8][9][10][11][6][2][4]. 

The four members are separated by three tufts: the Sidi Hakoma Tuff (SHT), the Triple Tuff (TT), and the Kada Hadar Tuff (KHT) [6][2][4]. The Hadar tuff horizons were dated using both Single Crystal Laser Fusion ⁴⁰Ar/³⁹Ar dating techniques [9][10], and paleomagnetic dating [12], making Hadar one of the best-dated hominin-bearing geological formations in eastern Africa [2]. The Sidi Hakoma Tuff is geochemically identical to the Tulu Bor Tuff in Kenya’s Koobi Fora Formation, with both having been dated to 3.42 Ma. The Triple Tuff was dated to 3.24 Ma, the Bouroukie Tuff-2 (BKT-2), at 2.96 Ma, and the BKT-3 at 2.33±0.07 [13][2]. 

The top of the Hadar Formation is defined by the Bouroukie unconformity surface (BUS), which separates it from the overlying Budisima Formation. The Bouroukie Tuff 2 (BKT-2) complex (referring to three tephras: Green Marker Bed (GMB), BKT-2L, and BKT-2U) [14][15] lies in the youngest Kadar Hadar Member strata. BKT-2L and BKT-2U are dated to 2.96 and 2.94 Ma, respectively [16][17][8] and provided the previous upper age estimate for the Hadar Formation [6][2]. There is a notable absence of A. afarensis in the Budisima Formation, but A.L. 666-1, a 2.4 Ma Homo maxilla was found in the Makaamitalu Basin [7][18][2].
 

History and Key Discoveries

In the late 1960s, Maurice Taieb initiated a geological survey in a relatively remote and unexplored but highly promising region in Ethiopia. He discovered previously unknown Plio/Pleistocene sedimentary deposits rich in well-preserved and diverse mammalian fossils during his geological fieldwork in the Afar region [19][20][2].  

Following his initial discovery of extensive fossil-and-archaeology-rich sedimentary sequences in the Afar in 1972, Taieb established the International Afar Research Expedition (IARE), a multinational, transdisciplinary team dedicated to exploring these geological exposures in greater depth with the goal of recovering hominin fossils [19][2].

Field explorations focused on Hadar in 1973, where fossil-rich sedimentary exposures could be found just north of the Awash River. The first hominin discovered was a 3.4 Ma fossil knee joint. The knee and associated proximal femoral elements from Afar localities 128 and 129 (A.L. 128 and A.L. 129) provided undeniable evidence for bipedalism [21][2].

Donald Johanson, a member of the 1973 expedition returned the following year and discovered a 40% complete fossil hominin skeleton “Lucy” on November 24, 1974. Lucy (A.L. 288-1), named after the Beatles’ song “Lucy in the Sky with Diamonds”, is the most well-known fossil discovered at Hadar and one of the oldest hominin fossils ever discovered. It was later given the taxonomic classification Australopithecus afarensis [4][19][1][22][2].

In the hominin fossil record, associated partial skeletons are uncommon, and Lucy is one of the most complete hominin skeletons discovered. With some bilateral preservation, she is represented by elements of the skull, upper limb, nine vertebrae, pelvis, lower limb, and foot [9][23].

In 1975 over 200 hominin bones were recovered at A.L. 333. These finds dubbed the “First Family” most likely died in a catastrophic event and were subsequently buried in a shallow depression or swale in the upper reaches of an abandoned channel [24][25]. Lucy and the First Family both dates to 3.2 Ma and are thus crucial for understanding anatomical and developmental variation in the Hadar hominins. The estimated number of minimum individuals at A.L. 333 is at least 17 (nine adults, three adolescents, and five juveniles) [2]. 

Other notable Hadar discoveries include A.L. 444-2 (male) and A.L. 822-1 (female) skulls, and A.L. 438-1, a partial skeleton from the upper layers of the Hadar Formation. A complete skull was a crucial element in understanding the Australopithecus afarensis cranial-facial anatomy, however, this key element was missing until A.L. 444-2 was found in 1990. This skull has large canines and is the largest Australopithecus skull thus far discovered. Based on the heavy dental wear, it appears to be an elderly individual [26][2]. 

In 2002, a slightly older at 3.1 Ma, but nearly complete second skull was found at A.L. 822 [27]. This small, presumably female skull, displays typical A. afarensis cranial anatomy with expected minor anatomical variations. Despite her small stature, A.L. 822-1 is larger than Lucy, confirming that Lucy is one of the smallest members of her species [2].

The majority of the hominin finds in Hadar since 1990 have been dental and cranial remains, which makes the discovery of A.L. 438-1, a partial upper limb skeleton from middle KH Member, significant. This massive skeleton dated to 3.0 Ma offers insight into both locomotion and size variation in A. afarensis. This specimen includes the right half of an edentulous mandible, a frontal bone fragment, a complete left ulna, two second metacarpals, one third metacarpal, and portions of a humerus, radius right ulna, and clavicle [26].    

Based on the associated mandible and frontal fragment, A.L. 438-1 is most certainly a male. Although the slightly curved A.L. 438 ulna is nearly twice the size of Lucy’s ulna, the two ulnae are otherwise anatomically identical, further confirming a high level of sexual dimorphism in A. afarensis [26][2].  

Beginning in 1990, a strategically focused field survey confirmed the scarcity of fossils in the Kada Hadar Member. However, a significant number of new hominin localities, many above the KHT have doubled the temporal range for A. afarensis at Hadar from 200,000 to 400,000 years ago [28][26]. Between 1990 and 2012, a total of 187 additional fossil hominin specimens were discovered bringing the total to 427 [2].

At 800,000 years, Australopithecus afarensis is the most geographically widespread and longest-living Australopithecus species. And, with over 400 specimens, the Hadar sample represents more than 90% of the species’ hypodigm [2].
 

Genus Homo and Lithic finds at Hadar

A.L. 666-1, a 2.4 Ma Homo maxilla, was discovered at A.L. 666 in the Makaamitalu Basin in the “upper” Kada Hadar Member in November 1994. The maxilla and associated stone tools were eroded from a siltstone horizon 80 cm below the BKT-3, giving the locality a minimum age of 2.33 ± 0.07 Ma preserving the oldest known co-occurrence of Homo and Oldowan stone tools [7][2].

Interestingly, the presumably male (large canine) A.L. 666 maxilla lacks Australopithecus morphology and instead presents classic Homo anatomy, such as a deep, short anterio-posteriorly, and broad deep palate with a parabolic dental arcade. Other Homo features include mild subnasal prognathism, a broad flat subnasal plane that is angled relative to the nasal cavity floor, and a square anterior maxillary profile [18][2].

Excavations at A.L. 894, yielded in situ bone and lithic artefacts. The excavated lithics consist of small flakes and flake fragments as well as some larger flakes and a core. Results of an experiment that involved the refitting of angular flakes onto cores suggest that the lithic artefacts were produced “on the spot”[29].