Australopithecus garhi

''Australopithecus garhi'' is a species of australopithecine from the Bouri Formation in the Afar Region of Ethiopia 2.6–2.5 million years ago (mya) during the Early Pleistocene. The first remains were described in 1999 based on several skeletal elements uncovered in the three years preceding. ''A. garhi'' was originally considered to have been a direct ancestor to ''Homo'' and the human line, but is now thought to have been an offshoot. Like other australopithecines, ''A. garhi'' had a brain volume of ; a jaw which jutted out (prognathism); relatively large molars and premolars; adaptations for both walking on two legs (bipedalism) and grasping while climbing (arboreality); and it is possible that, though unclear if, males were larger than females (exhibited sexual dimorphism). One individual, presumed female based on size, may have been tall.

''A. garhi'' is the first pre-''Homo'' hominin postulated to have manufactured tools—using them in butchering—and may be counted among a growing body of evidence for pre-''Homo'' stone tool industries (the ability to manufacture tools was previously believed to have separated ''Homo'' from predecessors.) ''A. garhi'' possibly produced the Oldowan industry which was previously considered to have been invented by the later '' H. habilis'', though this may have instead been produced by contemporary ''Homo''.

Taxonomy

The Ethiopian ''Australopithecus garhi'' was first described in 1999 by palaeoanthropologists Berhane Asfaw, Tim D. White, Owen Lovejoy, Bruce Latimer, Scott Simpson, and Gen Suwa based on fossils discovered in the Hatayae Beds of the Bouri Formation in Middle Awash, Afar Region, Ethiopia. The first hominin remains were discovered here in 1990—a partial parietal bone (GAM-VP-1/2), left jawbone (GAM-VP-1/1), and left humerus (MAT-VP-1/1)—which are unassignable to a specific genus. The first identifiable ''Australopithecus'' fossils–an adult ulna (BOU-VP-11/1)–were found on 17 November 1996 by T. Assebework. A partial skeleton (BOU-VP-12/1) was discovered 13 days later by White, comprising a mostly complete left femur, right humerus, radius, and ulna, and a partial fibula, foot, and jawbone. The holotype specimen, a partial skull (BOU-VP-12/130), was discovered on 20 November 1997 by Ethiopian palaeoanthropologist Yohannes Haile-Selassie. More skull fragments (BOU-VP-12/87) were recovered south of BOU-VP-12/1. On 17 November 1997, French palaeoanthropologist Alban Defleur discovered a complete mandible (BOU-VP-17/1) about north in the Esa Dibo locality of the formation, and American palaeoanthropologist David DeGusta discovered a humerus (BOU-VP-35/1) north of BOU-VP-17/1. However, BOU-VP-11, -12, and -35 cannot conclusively be attributed to ''A. garhi''.

The remains are dated to about 2.5 million years ago (mya) based on argon–argon dating. When they were discovered, human evolution was obscured due to a paucity of remains from 3 to 2 mya, with the only hominins from this timespan being identified from South Africa ('' A. africanus'') and Lake Turkana, Kenya ('' Paranthropus aethiopicus''). Likewise, the classification of australopithecines and pre-''Homo erectus'' hominins has been the subject of much debate. The original describers considered ''A. garhi'' to be a descendant of the earlier '' A. afarensis'' which inhabited the same region, based mainly on dental similarities. Though they assigned the species to ''Australopithecus'', the original describers believed it could represent an ancestor to ''Homo'', which, if the case, would possibly lead to reclassification as ''H. garhi''. Because the characteristics of ''A. garhi'' are unexpected for a human ancestor at this stage, the specific name, ''garhi'', means "surprise" in the local Afar language. In 1999, American palaeoanthropologists David Strait and Frederick E. Grine concluded that ''A. garhi'' was instead an offshoot of the human line instead of an ancestor because ''A. garhi'' and ''Homo'' share no synapomorphies (traits unique to only them). In 2015, ''Homo'' was recorded from 2.8 mya, much earlier than ''A. garhi''.

Anatomy

Like other australopithecines, ''A. garhi'' had a brain volume of about , a sagittal crest running along the midline of the skull, and a prognathic jaw (the jaw jutted out). Relatively, the postcanine teeth, the molars and premolars, are massive ( post-canine megadontia), similar to or greater than those of other australopithecines and of the large-toothed ''Paranthropus robustus''.

Like the earlier ''A. afarensis'' from the same region, ''A. garhi'' had a humanlike humerus to femur ratio, and an apelike brachial index (lower to upper arm ratio) as well as curved phalanges of the hand. This is generally interpreted as adaptations for both walking on two legs (habitual bipedalism) as well as for grasping while climbing in trees (arboreality).

The BOU-VP-35/1 humerus specimen is notably larger than the humerus of the BOU-VP-12/1 specimen, which could potentially indicate size-specific sexual dimorphism with males larger than females to a similar degree to what is postulated in ''A. afarensis'', but it is unclear if this does not represent normal size variation of the same sex as this is based on only two specimens. Nonetheless, on the basis of size, BOU-VP-12/130 is considered male and BOU-VP-17/1 female. Contemporary hominins from Kenya are about the same size as ''A. garhi''. BOU-VP-17/1 may have been about tall.

''Australopithecus'' are thought to have had fast, apelike growth rates, lacking an extended childhood typical of modern humans. However, the legs of ''A. garhi'' are elongated, unlike those of other ''Australopithecus'', and, in humans, elongated limbs develop during the delayed adolescent growth spurt. This could mean that ''A. garhi'', compared to other ''Australopithecus'', either had a slower overall growth rate, or a more rapid leg growth rate.

Palaeoecology

The large teeth of ''Australopithecus'' species have historically been interpreted as having been adaptations for a diet of hard foods, but the durable teeth may instead have only served an important function during leaner times for harder fallback foods. That is, dental anatomy may not accurately portray normal ''Australopithecus'' diet, rather abnormal diet during times of famine.

Though it was not found with any tools, mammalian bones associated with the ''A. garhi'' remains exhibit cut and percussion marks made from stone tools: the left mandible