The Hadar Formation (Fig. 5), as defined by Taieb et al. (1972) consists primarily of fluvio-lacustrine related mudstones, siltstones, fine-grained sandstones and volcanic tuffs. The 150-meter thick formation is divided into four members: the Basal Member, the Sidi Hakoma Member, the Denan Dora Member and the Kada Hadar Member. Three tuffs, the Sidi Hakoma Tuff (SHT), the Triple Tuff (TT) and the Kada Hadar Tuff (KHT) separate the four members.

Reed (2008) identifies fluctuating paleoenvironments over the 400,000 years of time in the Hadar Formation. Habitat reconstructions were based on careful analysis of the faunal remains within the Hadar Formation. The Sidi Hakoma Member was characterized by a more closed habitat with high rainfall and low seasonality. The overlying Denan Dora Member was an edaphic grassland habitat. Finally, the Kada Hadar Member was even a more open and arid habitat as seen in the high abundance of antilopins; open area bovids. Australopithecus afarensis was a euryotropic species found throughout the Hadar Formation, able to tolerate a wide range of habitats.

The Hadar Formation is separated from the overlying Busidima Formation just above the 2.95 Mya Bouroukie Tuff-2 (BKT-2) by a geological unconformity. Reed (2008) notes that above this datum, in significantly younger deposits, is a recognizable faunal turnover. She interprets this change as a response to a wooded grassland habitat with floodplain environments. Notable is the absence of A. afarensis in the Busidima Formation, but a 2.4 Mya maxilla assigned to Homo (Kimbel et al., 1996 and Kimbel et al., 1997) was found in the Makaamitalu Basin.

Fossil pollen recovered from the Hadar Formation (Bonnefille et al., 2004) confirmed a forested habitat in the Sidi Hakoma Member. Pollen from the Denen Dora Member reflects a drier habitat in contrast to the Kada Hadar Member, which was more wooded and humid.

Further evidence from studies of paleosols and carbon and oxygen isotopes of mollusks (Hailemichael, 2000 and Hailemichael et al., 2002) agrees with the habitat reconstructions reported by Reed (2008). The fluctuations in paleo-habitats at Hadar do not support the notion that there was a trend toward more arid conditions as is the case for the Shungura Formation (Bobe and Eck, 2001). Confirmation of the paleoenvironmental picture for the Hadar Formation comes from detailed geological studies of depositional conditions (Campisano and Feibel, 2008a).

The recovery of more than 240 fossil hominin specimens by the end of the 1977 (Fig. 6) IARE field season offered an incredible storehouse clarifying the nature of pre-3 Mya hominins (for full descriptions see Johanson et al., 1982). In 1973 and 1974, when the first significant fossils were recovered, interpretation of anatomical and size variation pointed to the possibility of more than one taxon (Johanson and Taieb, 1976, Taieb et al., 1974 and Taieb et al., 1975). However, by late 1977, following extensive study of the fossil collection on temporary loan to the Cleveland Museum of Natural History (CMNH), a team decision concluded that only one hominin taxon was represented in the Hadar Formation. Currently all Hadar hominin fossils are curated in the National Museum of Ethiopia, Addis Ababa.

All of the fossil hominins recovered from the Hadar Formation, representing over 400,000 years of time were assigned to A. afarensis, as was announced at a Nobel Symposium in Karlskoga, Sweden, in May of 1978 (Johanson, 1978). The formal publication of A. afarensis appeared later that year in the CMNH's in-house journal, Kirtlandia (Johanson et al., 1978).

The type specimen, LH-4, from Laetoli, a 3.5-Mya mandible, was chosen because it was fully described and published (Fig. 7). Most importantly, it embodied a plethora of anatomical features found in abundance in the Hadar sample, indicating that the hominins found at these two sites were the same species. The Hadar and Laetoli specimens were placed in the genus Australopithecus not Homo, largely because they lack facial reduction and cranial enlargement.

Several primitive cranial (Fig. 8) and postcranial features makes A. afarensis distinct from other species of Australopithecus: small cranial capacity, palate similar to African apes (parallel tooth rows, shallow, long from antero-posteriorly, narrow from side to side), primitive occipital and basal cranium anatomy, high frequency of unicuspid third premolars, prognathic face, and primitive mandibular anatomy. Postcranially, the pelvis, knee, ankle and foot indicate habitual, terrestrial bipedalism, but ape-like curved finger and foot bones are retained ancestral ape-like features. The high level of sexual dimorphism in A. afarensis (Fig. 9), with significantly larger males, is also characteristic of apes (except for the gibbon). For a detailed review of A. afarensis anatomy see Kimbel and Delezene, 2009 and Kimbel et al., 2004.

There was initial opposition to the veracity of the newly named species, especially from the late Phillip Tobias (Tobias, 1980), who saw the Hadar and Laetoli hominins as two subspecies of A. africanus. Other scholars (Falk et al., 1995, Leakey and Walker, 1980, Olson, 1981, Senut, 1983 and Tardieu, 1981) favored taxonomic diversity within the Hadar collection, but were unable to separate convincingly the hominin material into distinctive morphs.

Extensive argument centered on the nature of bipedalism in A. afarensis. The rich collection of postcranial material, including a pelvis, a nearly complete foot and numerous lower limb bones, provided solid evidence that A. afarensis was a fully-bipedal creature (Lovejoy, 1988 and Lovejoy, 1981). In contrast, others suggested that these creatures were highly arboreal, and that perhaps males and females walked differently (Stern and Susman, 1983 and Susman et al., 1984). They further suggested that during terrestrial bipedal locomotion, A. afarensis was not capable of full extension at the hip and knee. However, the detailed study of the biomechanics of the postcranial bones does not support this observation.

The excavation of a 3.6-Mya hominin footprint trail at Laetoli, Tanzania (Leakey and Harris, 1987) revealed that the prints impressed and preserved in a volcanic ash were identical to modern human footprints (Fig. 10). The presence of a large, adducted, great toe, used as a propulsive organ, the presence of longitudinal and transverse plantar arches and the alignment of lateral toes provide indisputable evidence for bipedalism in A. afarensis that is essentially equivalent to modern humans.

Following an extensive review of all Plio/Pleistocene hominin taxa (Johanson, 1978, Johanson and White, 1979 and White et al., 1981), it was concluded that A. afarensis is characterized by a large number of primitive (plesiomorphic) features in the face, cranial vault and dentition. Based on the generalized nature of the anatomy of A. afarensis, it was suggested that this species be entertained as the basal taxon to both the Homo lineage and the later Australopithecus (Fig. 11).

The cranio-dental anatomy of A. africanus, seen in the light of A. afarensis, is derived in the direction of robust A. robustus. In order for A. africanus to be an ancestor to Homo, a high number of evolutionary reversals would be necessary; an unlikely evolutionary event.

The novel phylogeny stimulated by A. afarensis implied that A. africanus evolved into A. robustus in South Africa, while a parallel set of evolutionary adaptations to a heavily masticated diet occurred in eastern Africa, culminating in A. boisei. With 1.2 million years separating the last occurrence of A. afarensis and the appearance of A. boisei and this scenario seemed unlikely to some investigators (Skelton et al., 1986).

This hypothesis was tested with the discovery of the Black Skull (Walker and Leakey, 1988 and Walker et al., 1986). The cranium KNM-WT 17000, assigned to A. aethiopicus, was dated at 2.5 Mya (Fig. 12), chronologically intermediate between the disappearance of A. afarensis and the appearance of A. boisei.

Reflecting its ancestry, the Black Skull retained a number of features characteristic of A. afarensis, such as a highly prognathic face and cranial cresting. Yet other features of the skull, such as strong sagittal cresting, extremely enlarged cheek teeth, and so on, forecast the derived anatomy of the later A. boisei. The mosaic of ancient and derived features in A. aethiopicus, bridged the more than one million year gap between A. afarensis and A. boisei, and provided convincing evidence for an evolving lineage (Kimbel et al., 1988).

A possible interpretation here is that robust Australopithecus species evolved independently in eastern and South Africa. This suggests parallel evolution in these two geographical regions, most likely as adaptation to a heavily masticated diet. Furthermore, no species of Australopithecus is found in both regions, supporting the notion of independent evolutionary arenas.

Paleoanthropological field research at Hadar resumed in 1990. The HRP developed a strategic research program to address a number of outstanding but crucial questions concerning the geology, paleontology, and anthropology of the Hadar Formation.

There were multiple primary research goals. First, to resolve the geochronology of the Hadar Formation, with the objective of providing a precise chronological framework in which to assess faunal and geological evolution at the site. Second, to refine the paleoenvironmental settings in which the Hadar Formation accumulated, and the nature of the ecological settings in which vertebrates and hominids lived and interacted (see discussion above). Third, to assess the depositional history of the “First Family” locality and potential for further excavation. Fourth, to survey the upper Kada Hadar Member with the goal of extending the stratigraphic range of hominids at Hadar. Fifth, to test the hypotheses of A. afarensis systematics and paleobiology based on the recovery of additional (especially cranial) specimens.

In order to date more precisely the Hadar tuff horizons, both Single Crystal Laser Fusion ⁴⁰Ar/³⁹Ar dating techniques (Walter, 1994 and Walter and Aronson, 1993), and paleomagnetic dating (Renne et al., 1993) were employed.

These efforts led to one of the best-dated hominin-bearing geological formations in eastern Africa. The Sidi Hakoma Tuff was determined to be 3.42 Mya, and geochemically identical to the Tulu Bor tuff in the Koobi Fora Formation in Kenya, also dated to 3.42 Mya. The Triple Tuff was dated at 3.24 Mya, the Kada Damum Basalt at 3.30 Mya, the Kada Hadar Tuff at 3.20 Mya and the Bouroukie BKT) Tuff-2, at 2.96 Mya and the BKT-3 at 2.33 ± 0.07 (Fig. 5). See Campisano and Feibel (2008b) for more details of the tephrochronology.

The A.L. 333, or “First Family Site” presents an intriguing dilemma since we have no reasonable explanation for this catastrophic death assemblage. The vast majority of the more than 200 specimens collected at A.L. 333 during Phase I were surface finds; however, 18 specimens were excavated in situ from an 80-cm thick horizon. The recovery of 39 additional postcranial and three mandibular and dental hominid specimens (1990–2012) has increased the estimate for the minimum number of individuals at A.L. 333 from 13 to at least 17 (nine adults, three adolescents and five juveniles).

Previous speculation proffered the notion that the A.L. 333 hominid assemblage was the result of a flash flood. However, detailed microstratigraphic study of the geological strata at the site indicated the hominins were interred in a shallow depression (swale) in the upper reaches of an abandoned channel, which originally flowed northwards (Behrensmeyer, 2008 and Behrensmeyer et al., 2003).

Preservation of some fragile elements and still-articulated hand bones indicates little postmortem fluvial transport. Lack of abrasion and extensive weathering suggest rapid burial perhaps within weeks of death. The under representation of axial skeletal elements, as well as some postmortem carnivore damage, may reflect some level of feline predation.

Beginning in 1990, strategically focused field survey confirmed the relative paucity of fossils in the Kada Hadar Member. Yet recognition of a significant number of new hominid localities, many above the KHT, has doubled the temporal range for A. afarensis at Hadar from 200,000 to 400,000 years (Kimbel and Delezene, 2009 and Kimbel et al., 1994). Noteworthy was the recovery of male (A.L. 444-2) and female (A.L. 822-1) skulls and a partial skeleton (A.L. 438-1) in the upper reaches of the Hadar Formation (Fig. 13). Between 1990 and 2012, a total of 187 additional fossil hominin specimens were collected bringing the total to 427.

In November 1994, a Homo maxilla (Fig. 14) was discovered at A.L. 666 (Kimbel et al., 1996) in the Makaamitalu Basin in “upper” Kada Hadar Member. The maxilla and associated stone tools scattered on the hillside eroded from a siltstone horizon 80 cm below the BKT-3, thus providing a minimum age of 2.33 ± 0.07 Mya for this locality. A.L. 666 preserves the oldest known co-occurrence of Homo and Oldowan stone tools.

The presumably male (large canine) maxilla lacks Australopithecus morphology, and 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 angled relative to the floor of the nasal cavity, and square anterior maxillary profile.

Many of the morphological features resemble later Homo and are probably apomorphic within the Homo clade, making it difficult to assign A.L. 666-1 to a later species of Homo. Bearing this in mind, the narrowing of the M¹ and the “rhomboidal” shape of the M³ suggest similarities to H. habilis (Kimbel et al., 1997).

Excavations at A.L. 894, penecontemporaneous with A.L. 666 in the Makaamitalu Basin, uncovered in situ bone and lithic artifacts. The excavated lithics consist of small flakes and flake fragments as well as some larger flakes and a core. That these artifacts were produced “on the spot” as is confirmed by the refitting of an angular flakes onto cores (Hovers, 2009).

A. afarensis fossil remains are now known from other sites in Ethiopia: Maka (White et al., 2000 and White, 1993), Dikika (Alemseged et al., 2006), Galili (Macchiarelli et al., 2004), Belohdelie (Asfaw, 1987), Lake Turkana (Kimbel, 1988), Woranso-Mille (Haile-Selassie et al., 2010), and Omo (Suwa, 1990). The Bahr-el-Ghazal mandible from Chad (Brunet et al., 1996) is assigned here to A. afarensis.

A. afarensis is, therefore, the most geographically widely ranging and the longest temporally enduring species – 800,000 years – of Australopithecus. The Hadar sample, now consisting of more 400 specimens, represents more than 90% of the A. afarensis hypodigm.

One crucial missing element in understanding A. afarensis anatomy was a complete skull. An adult male skull reconstruction of an A. afarensis skull was based on 13 specimens from the Hadar Formation (Kimbel and White, 1988 and Kimbel et al., 1984). This composite served as a guide to skull architecture, but an associated cranium and mandible was needed to better understand details of the cranio-facial architecture.

The recovery in 1990 of a 3.0 Mya male skull from A.L. 444 fulfilled a vital gap in our knowledge (Kimbel et al., 1994) of A. afarensis cranio-facial anatomy (Fig. 13). The A.L. 444 skull from an elderly individual based on heavy dental wear constitutes the largest Australopithecus skull thus far discovered.

Overall, the male skull confirms earlier observations made on the composite skull, but provides further anatomical detail of regions missing in that reconstruction. A small sagittal crest is present posteriorly, rather than commencing on the frontal squama, as in A. boisei. Like other A. afarensis occipitals, this skull has an ape-like compound temporal/nuchal crest. In facial view, the orbits are squared off laterally and the face is very tall. While little can be said of endocranial gyri and sulci patterns, the endocranial volume is estimated to be 550 ± 10 cm³; larger in comparison to estimates for A.L. 162 (female) of 400 cm³, and for A.L. 333-45 (male) of 485 cc. (see Kimbel et al., 2004, for a monographic treatment of the anatomy of A.L. 444-1).

A slightly older, 3.1 Mya, but nearly complete second skull (Fig. 13) was found in 2002 at A.L. 822 (Johanson and Edgar, 1996). The anatomy of this small, presumably female, skull demonstrates classic A. afarensis cranial anatomy with expected minor anatomical variations.

While small in size, the A.L. 822-1 is larger than A.L. 288-1, confirming that “Lucy” is one of the smallest examples of her species. The A.L. 822 mandible is intermediate in size between the smaller A.L. 288 and the much larger A.L. 444 mandibles.

While the majority of Hadar hominid specimens recovered since 1990 are dental and cranial remains, a partial, very large, skeleton from A.L. 438 (Kimbel et al., 1994) offers some insight into both locomotion and size variation of A. afarensis. This specimen dated to 3.0 Mya preserves the right half an edentulous mandible, a fragment of frontal bone, a complete left ulna, two second metacarpals, one third metacarpal and portions of a humerus, radius, right ulna and fragment of clavicle.

Although the slightly curved A.L. 438 ulna is nearly twice the size of the “Lucy” ulna, the two ulnae are otherwise anatomically identical, further confirming a high level of sexual dimorphism in A. afarensis. Drapeau et al. (2005) concluded that the anatomy of the metacarpal and ulna are shared with humans. Rather than interpreting the marked muscularity in the ulna as indicating significant arboreal acitivity, she posits that it may reflect activities associated with food gathering.

Examples of anagenesis in early hominin evolution have been notoriously challenging to identify. This has been due to a number of factors, most notably the paucity of well-dated and complete fossils, but with enlarged data sets we can begin to extend a number of reasonable hypotheses by assessing ancestor-descendant anatomies. A case in point is the proposed A. afarensis, A. aethiopicus, A. boisei lineage outlined above.

A. anamensis, first identified in the Lake Turkana region at two sites, Kanapoi (Fig. 15) and Allia Bay (Leakey et al., 1995, Leakey et al., 1998 and Ward et al., 2001) is dated to between 4.2 and 3.9 Mya. This is chronologically just prior to the appearance of A. afarensis, hence it was logical to undertake a detailed comparison of the 80 A. anamensis specimens with the more than 400 A. afarensis fossils.

Following exhaustive comparative investigation of some 20 characters found in the mandible, maxilla and dentition, we conducted a phylogenetic analysis employing McClade 4.07 to generate trees based on the collected character-state data. It was concluded that the most parsimonious explanation, consistent with both chronology and character-state was anagensis, meaning that A. anamensis was the ancestor to A. afarensis (Kimbel et al., 2006).

Attempts to understand the ancient roots of the Homo lineage have been hampered by the dearth of Homo fossils dating in excess of 2.0 Mya. By roughly 2.0 Mya we see some diversity in Homo species: H. habilis, H. rudolfensis, H. ergaster and perhaps H. georgicus (Gabounia et al., 2002) hinting at a pre-2.0 Mya origin of the genus. From a paleontological point of view, it is worth observing we know much more about the genus Australopithecus than we do about our own genus.

Recovery of the A.L. 666-1 Homo mandible dated to 3.4 Mya bridged part of the temporal gap between 1.8-2.0 Mya. However, this still leaves a 600,000 gap between the last occurrence of A. afarensis and Homo at 2.4 Mya in the Makaamitalu Basin.

The recent fortuitous discovery of a 2.8 Mya partial mandible with teeth from the Ledi-Geraru research area in the Afar Triangle attributed to Homo allows further testing of the hypothesis that A. afarensis was ancestral to later Homo (Villmoare et al., 2015). Analysis of the anatomy of LD 350-1 (Fig. 16) revealed symmetrical premolars and narrow, mesiodistally enlongated molars as well as an evenly- proportioned mandible, from front to back; features all shared with later mandibles of Homo.

On the other hand, the mandible preserves A. afarensis-like anatomy in the anterior portion of the jaw. The primitive anterior corpus, inclined symphyseal cross section, bulbous anterior symphyseal face and the projecting inferior transverse torus are typical of A. afarensis.

The discovery of a 2.5 Mya cranium from Bouri, Middle Awash, Ethiopia (Asfaw et al., 2004) also appears to be a lineal descendant of A. afarensis (Fig. 17) The cranium (BOU-VP-112/130) with a small cranial capacity of 450 cc, exhibits a number of features that link it to A. afarensis particularly in the marked subnasal prognathism, posteriorly placed sagittal crest, large canine and U-shaped dental arcade.

What distinguishes this specimen, assigned to A. gahri, is the megadont postcanine dentition, shared with robust Australopithecus. In A. boisei, the large postcanine dentition is linked to a orthognathic, dish-shaped face, but in A. garhi it is linked to a very prognathic face, like in A. afarensis. It therefore appears that A. garhi may also be an evolutionary descendant of A. afarensis; one that apparently left no desendants.

It thus appears that A. afarensis gave rise to at least three descendant lineages: Homo, A. garhi, and A. aethiopicus to A. boisei. Furthermore, A. anamensis is the mostly likely ancestor to A. afarensis. There is no doubt that additional hominin discoveries may alter this hypothesis, but for the moment it is consistent with the anatomical and chronological data (Fig. 18).

From this brief overview it is clear that A. afarensis continues to play a pivotal role in our ever-expanding knowledge of early hominin evolution. The large data set for this species, and numerous specimens that have not yet been described, make A. afarensis, the most comprehensively represented ancient hominin species, over time and geography.

The discovery and naming of A. afarensis during the 1970s stimulated four decades of novel research approaches to early hominin paleoanthropology. This species serves as a critical resource for interpreting other Pliocene fossil hominin discoveries.

The disappearance of A. afarensis around the 3.0 Mya data and the subsequent diversification into at least three lineages is provocative. There is much debate about climate change and resulting speciation among hominins, but A. afarensis currently presents an ape-like species that sits at a pivotal place on the human family tree between more primitive species and more derived species.

While A. afarensis is clearly not the most ancient or most ape-like hominin species, it is by far the foremost fossil record for major transitions in the locomotor, cranial and masticatory systems. And, as a result of the intense focus by the IARE and the HRP at Hadar and the surrounding area, other teams are finding crucial fossils, further fleshing out our comprehension of the genus Australopithecus, but also the origins of the human genus, Homo.