The Pliocene hominin diversity conundrum: Do more fossils mean less clarity?

Abstract

Recent discoveries of multiple middle Pliocene hominins have raised the possibility that early hominins were as speciose as later hominins. However, debates continue to arise around the validity of most of these new taxa, largely based on poor preservation of holotype specimens, small sample size, or the lack of evidence for ecological diversity. A closer look at the currently available fossil evidence from Ethiopia, Kenya, and Chad indicate that Australopithecus afarensis was not the only hominin species during the middle Pliocene, and that there were other species clearly distinguishable from it by their locomotor adaptation and diet. Although there is no doubt that the presence of multiple species during the middle Pliocene opens new windows into our evolutionary past, it also complicates our understanding of early hominin taxonomy and phylogenetic relationships.If one looks back over the controversies of human evolution, they have one element in common: new discoveries, theories, methods came along which no one in the controversy anticipated. The "facts" changed, and consequently people were not right or wrong in any simple way.S. L. Washburn and R. L. Ciochon, 1974 (1) New fossil discoveries and analytical methods that have proliferated during the last few decades have fundamentally changed how we study and interpret hominin fossils and understand human evolution. The discovery and subsequent naming of Australopithecus afarensis in the late 1970s was one of the major milestones in paleoanthropology (2). Its discovery not only pushed the record of hominins to earlier than 3 million years ago (Ma) (2), but also demonstrated the antiquity of human-like bipedality (3). However, the taxonomic homogeneity of the Au. afarensis hypodigm has been questioned since its naming (4-7), even though the Hadar fossil sample appears to be no more variable than other living ape species (8-11). A consensus emerged during the 1980s in which Au. afarensis, dated to between 3.7 and 2.9 Ma, was considered to be the sole early hominin species older than 3 Ma, largely supported by the lack of fossil evidence to indicate otherwise.When Australopithecus bahrelghazali was named in 1995 based on an approximately 3.5-Ma partial mandible from Chad (12), it was quickly dismissed as a geographic variant of Au. afarensis (13-15). The initial descriptions of Ardipithecus ramidus (16) and Australopithecus anamensis (17), followed by the naming of even earlier hominin species, such as Orrorin tugenensis (18), Ardipithecus kadabba (19,20), and Sahelanthropus tchadensis (21), extended the antiquity of our lineage as far back as >6 Ma. These early hominins initially appeared to show no temporal or spatial overlap, and hence reinforced the idea that the early phases of hominin evolution were characterized by phenetic continuity and phyletic gradualism, with only one hominin species existing in a region at any given time >3 Ma (e.g., ref. 22; see discussions below).The discovery of the Burtele partial foot from Ethiopia (23), the naming of Kenyan...

Evidence of a chimpanzee-sized ancestor of humans but a gibbon-sized ancestor of apes

“…One important reason for this omission is the paucity of African fossil hominids during the period when the chimpanzee and human lineages are believed to have diverged, perhaps 4–6 Ma (million years ago) 17 or earlier at 6–8 Ma 18–20 , with the notable exceptions of putative basal hominins Orrorin tugenensis (~6 Ma) 21 , Sahelanthropus tchadensis (6–7 Ma) 22 , Ardipithecus kadabba (5.5–6.4 Ma) 23 , and the later Ardipithecus ramidus (4.4 Ma) 7 . In addition, body sizes in the more well-sampled Miocene hominoid (all living and extant apes and humans) taxa (e.g., Proconsul ) appear to be extremely variable (e.g., refs.…”

Mammal extinctions and the increasing isolation of humans on the tree of life

“…By the end of the Miocene, the Earth had become littered with extinct ape lineages (Lovejoy, ). The same trend is seen in hominins: H. sapiens is the sole member of a much larger evolutionary bush of hominin species (Haile‐Selassie, Melillo, & Su, ). Why other hominins went extinct is still controversial.…”

“…). The same trend is seen in hominins: H. sapiens is the sole member of a much larger evolutionary bush of hominin species(Haile-Selassie, Melillo, & Su, 2016).Why other hominins went extinct is still controversial. In the case of Homo, one of the existing hypotheses is that H. sapiens was the cause of the extinction of other close relatives, such as Neanderthals…”

Mammal extinctions and the increasing isolation of humans on the tree of life

“…By the end of the Miocene, the Earth had become littered with extinct ape lineages [49]. The same trend is seen in hominins: H. sapiens is the sole member of a much larger evolutionary bush of hominin species [50]. Why other hominins went extinct is still controversial.…”