It is assumed that the origins of our genus, Homo, is derived from a species of Australopithecus, the “southern ape man from Africa,” discovered by Raymond Dart in the early part of the last century. Since the discovery of that find, the number of australopithecine species has reached ten, all dating to between 3.8 and 1.1 million years before the present. Bridging the gap between any of these species and our own line has been difficult, however. This owes in large part to the spotty paleontological record from the period of time between 2.0 and 3.0 million years ago, the period in which this transition is thought to have taken place.
Consequently, the search for our own beginnings has been a study in frustration, where vast stretches of time go unremarked with fossil evidence. Beginning around 2.0 million years ago, early Homo simply appears on the landscape, with no easily discernible antecedents. Further, there is rapid diversification of this form into what have come to be known as Homo habilis, Homo rudolfensis and Homo ergaster. These display a mix of large and small faces and brains.1 This has led to a confused understanding of where these hominins fit in the picture of human evolution. Now, with one fossil find, much light has been shed on this transition.
In January of 2013, a jaw fragment with teeth, catalog number LD 350-1, was unearthed in the arid region of Ledi-Geraru, in the Afar Triangle, which is part of the Horn of Africa and is known for its rich early hominin deposits. Radiometrically dated to between 2.75 and 2.8 million years ago, this find fills an important gap in the knowledge of our hominin ancestors and pushes the first known appearance of our own line back 400 thousand years.
Two papers in Science1,5 and one in Nature4 outline why this find is so important to the study of earlyHomo. These studies also point to the recent revolution in paleontology, in which the focus has shifted from the identification of transitional forms to transitional features in the fossil record. With this conceptual shift, transitional features are found in many different taxa and can be easily recognized.3 Then the question does not become “Is this a transitional form?” but rather “Does this form display transitional features?” This has allowed paleontologists to more clearly understand evolutionary relationships between related taxa.
Using this framework, an examination of the new find reveals that it shares traits with some current examples of early Homo to the exclusion of all but one australopithecine, Au. Afarensis, and shares some traits with that hominin. Those it shares with Au. Afarensis (the species of the famous Lucy fossil), include a shortened, smaller region where the chin would be and an overall smaller jaw, relative to later species. Traits that are derived relative to Lucy in the direction of early Homo are present in the form of the lack of a hollow spot behind the premolars, which characterizes all Au. Afarensis finds, the overall tooth and crown shape and the wear pattern of the teeth. As the describers note: “In the majority of traits that distinguish it from this species, LD 350-1 presents morphology that we interpret as transitional between Australopithecus and Homo”.5
These traits also place LD-350 further along the line leading to modern humans relative to other australopithecines. In all of the analyses involving tooth and mandibular size and shape, the LD-350 specimen plots away from all other australopithecines, who tend to have large cheek teeth and strong jaws. This is, perhaps one of the most important tentative conclusions that this find presents: with its constellation of traits and its early date, it relegates all of the australopithecines, save Au. afarensis, to a side branch of hominin prehistory. As such, they represent their own “human-like” evolutionary trajectories and, like so many other species in the fossil record, they all suffer the ignominious fate of extinction. Even Au. sediba, with its modern-looking hand, human-like spine and possible association with stone tools, because of its late date of 1.9 million years BP, cannot be ancestral to early Homo. This raises tantalizing questions regarding the possible cognitive advancements in late surviving australopithecines, which would have run parallel to those of our own genus. As the rich pageantry of human prehistory grows, hopefully these and other questions will have answers.