BioLogos: How did paleontologists like Lee Berger determine that these fossils belong to a new member of the genus Homo (instead of a variant of an existing species)? Why not a member of Australopithecus (the genus in which most early hominins belong, including the famous “Lucy” fossil)?
James Kidder: According to Berger’s analysis of the fossil form (Berger et al., 2015), it is similar in size to early Homo (although cranial size, at 513 cc, is on the low end of this range) but the morphology is different from existing forms. There are characteristics of the head that are found in later hominins, such as an angular torus (a ridge of bone extending from behind the head to the back of the head), more extensive brow ridge development, an occipital torus (ring of bone on the back of the head), and sagittal keeling (a ridge of bone on the top of the skull). These are all traits found in Homo erectus (although to greater extent) and not found on any australopithecine. Interestingly, As Chris Stringer has also noted (Stringer, 2015), some of these traits also are found in the 1.8 mya Dmanisi sample, reported a few years ago, suggesting that they are widespread and may appear earlier than originally thought.
Although the face is lightly built, it lacks many of the facial characteristics of the most common gracile in the area, Au. Africanus—no maxillary anterior pillars and a flat clivus, the space between the roof of the mouth and the maxillary sinuses. Additionally, the palate is parabolic, resembling early Homo. The teeth, however, are very small and resemble those of australopithecines.
Below the head, the skeleton is also a mix of traits found on australopithecines and those found on early Homo. The hands are more modern in that there are stronger muscle markings for opposable thumb use and manipulation of the fingers. Also, the thumb is, on the whole, longer. These characteristics are not at modern levels, however and the fingers are long and curved suggesting a link with late australopithecines. The tibia and femur are like those of other early Homo, but the foot and ankle are particularly modern in morphology, with adaptations suggestive of facilitating long distance walking. Oddly, the pelvis seems to resemble the early Au. afarensis in that it is flared. Importantly, while the ribs appear primitive, the vertebrae and, especially their neural canals, are more modern. Expanded neural canals are directly linked with greater central nervous response—better hand/eye coordination, running ability and other similar advancements.
The fossils have not yet been dated. Are there ways to determine where this species fits in our evolutionary timeline even without accurate dating of the fossils themselves?
Not easily. The depositional aspects of this find are similar to those that plague other South African finds. As was indicated in the accompanying article by Dirks etal. (Dirks et al., 2015), this was not a place where these hominins lived. Rising Star Cave system is one of many different cave systems that form when water percolating through the rock dissolves the underlying limestone, creating large cavities under the surface of the ground. Eventually, as more water erodes the surface, these cavities are exposed. Dirks et al. indicate that the bones of H. naledi were carefully dropped in from up above and any decomposition and disarticulation occurred after this. As was pointed out in the Nova special, The Dawn of Humanity, the bottom of the cave is not even visible from the surface. The problem is that we have no idea when they were dropped in. We may be able to date the surface of the cave floor but there is no way to relate those dates to the bones, themselves.
Consequently, the morphological analysis will have to proceed in a systematic fashion, with an examination of which traits are derived or primitive relative to other species of Homo and Australopithecus. Such an analysis can never completely proceed in the absence of a chronological framework, however. We have to have some kind of anchor to tie it to.
Currently, several hypotheses are being examined for how the hominin remains got where they are, including mass catastrophe, deliberate “dropping” and predation. Dirks et al. suggest that the best model involves the deliberate placing of the hominins into the cave mouth simply because they show little to no signs of scavenging or other predatory marks. Whether this means that they were displaying advanced understanding of death and the complex notion of “burial,” as was implied during the Nova special, is not clear. The other hominins in the area may simply have wanted to get rid of the bodies because they did not want them around, a view also espoused by Bill Jungers.
Is there any significance to the location of the finding (in South Africa)? There have been hominin fossil finds before in South Africa, but not many of the Homo genus. Were you surprised about the location?
Yes, there is quite a bit of significance to this find being in South Africa. Since the late 1970s, there have been South African fossil finds that have been touted as being early Homo but the finds have been extremely fragmentary, so there has been no “smoking gun” that firmly establishes early Homo there. Further, the remains that have been found are restricted to one location: Swartkrans Cave, which is less than half a mile from the Rising Star system. Grine, in 2005, suggested that the “incomplete fragments and isolated teeth” found at Swartkrans cannot be unequivocally linked to early Homo. He noted at the time that such a hypothesis, while not unreasonable, simply had no direct support (Grine, 2005).
If the morphology of the H. naledi remains is not in dispute, then the presence of early Homo in South Africa is now clearly established. This is a game-changer because, arguments about Au. sediba’s potential role in the evolution of early Homo aside, it has always been assumed that the evolution to early Homo occurred in an East African context. That may have yet happened, but these finds make that argument harder to unequivocally support.
The other aspect of this find that is so striking is the sheer size of the cache of remains. The material from the Rising Star Cave comprises some 1550 fragments that can be attributed to at least fifteen individuals. Aside from the Middle Pleistocene remains at Atapuerca, in Spain, and the Zhoukoudian Homo erectus remains in China, we have never had a find this large. That these remains are from such a critical point in human prehistory, the transition from australopithecines to early Homo, is simply astounding. These two papers only scratch the surface of what we can learn about human evolution from these fossils.
In what way does this finding confirm the existing scientific understanding of human evolution?
This confirms the existing scientific understanding of human evolution in several ways. First, it provides us with yet another example of the mosaic nature of evolution, in which some traits have clearly evolved toward the modern condition, while others have yet to do so. Second, it reaffirms the trend towards modernity that we see in this range of species. As the traits are studied, patterns of evolutionary change can be delineated over time and space. Third, it confirms one of the principle predictions that Charles Darwin made in Descent of Man, that the ancestors of humans would be found near where our closest living relatives, the higher apes, live. Since the early 1920s, with the finding of the first australopithecine skull, that prediction has been shown to be true over and over. This new find, with its suite of modern and primitive traits, also reaffirms what we have thought for the last eighty years: that humans have evolved and share a common ancestor with the modern apes sometime in the late Miocene epoch.
In what way does the finding challenge our current understanding of human evolution?
It challenges it because, as noted above, it clearly places early Homo in South Africa, a place where, up until this point, we haven’t been sure it was there. If we can ever determine the date for these fossils or, better yet, discover other members of this species that can be dated, then we can better understand how early Homo got to South Africa.
One of the things I find somewhat mystifying about the article written by Berger et al. is its complete omission of any mention of the Ledi-Geraru find in the Afar Triangle of Ethiopia that, ostensibly, establishes early Homo at 2.8 million years. This becomes especially important since both the Rising Star and the Ledi-Geraru finds suggest the presence of primitive, australopithecine traits and more modern, early Homo ones, although this is disputed by John Hawks, who suggests that the LD 350-1 jaw cannot be unequivocally assigned to Homo4. Any overarching model of early Pleistocene human origins will have to address these geographically disparate finds.