Hominid Hype and Homo naledi: A Unique "Species" of Unclear Evolutionary Importance
Casey Luskin September 20, 2015 6:40 PM |
The media is once again abuzz over the discovery of a new hominin fossil. The fossil is named Homo naledi, represented by hundreds of bones found in a cave near Johannesburg, South Africa. Biologic Institute biologist Ann Gauger has already commented on the find here and here.
It has long been recognized that we are missing fossils documenting the supposed transition from the apelike genus Australopithecus to the humanlike Homo. Despite what you may be hearing in the media, Homo naledi does not solve this problem.
Some have envisioned the hallowed intermediate link being a creature with an apelike body and a human-like head. For some time, Homo habilis was such a candidate -- until cooler heads prevailed, as I noted earlier . Others have hoped we'd uncover something with a more Homo-like postcranial (below the head) skeleton but a more australopith-ape-like body. Indeed, almost exactly four years ago, in a post titled "Hominid Hype and the Election Cycle, I noted these precise arguments with regard to Australopithecus sediba.
Coincidently, we're right now in almost exactly the same place in the election cycle, and seeing almost identical claims about this new fossil discovery. Indeed, Homo naledi was discovered (and is being promoted) by the same researcher, Lee Berger, that unveiled (and promoted) sediba, although, as we'll see, naledi has a very different and unique set of traits from sediba. Numerous recent articles have hailed Homo naledi as the newest human ancestor: CNN: "Homo naledi: New species of human ancestor discovered in South Africa," PBS: "Trove of fossils from a long lost human ancestor," Vocativ: "Meet Homo Naledi: Your Newest Human Ancestor," Daily Mail: "Scientists discover skull of new human ancestor Homo Naledi," NBC New York: "Scientists Discover Homo Naledi, Early Human Ancestor," and so on.
Ironically, all of these claims are pure hype because, as we'll see, no one knows how old these bones are, and so it's entirely possible they're younger than our own species, making it, at present, a very real possibility that it's chronologically impossible for them to be anything remotely close to our "ancestor."
The main claim about Homo naledi is that it is a small-brained hominin (when compared to humans) that has other features that are very humanlike -- especially its hands and feet. As the news headlines suggest, there has been an immense amount of hype about this species, consistent with the hype surrounding Australopithecus sediba, which again was discovered and promoted by the same researcher, Lee Berger. However, while there are some humanlike aspects of its body plan, my overall impression is that this is a highly unique species that doesn't fit well into previously established categories.
The technical paper, "Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa," appeared in a lesser-known journal, eLife. It's a great find due to the sheer number of bones that were find, but to my mind its publication in eLife is immediate hint that this fossil isn't an earthshattering "transitional form," because if it were, we almost unquestionably would have seen the fossil published in Science or Nature. Many comments there reveal this fossil to be unique -- precisely with regard to features that are said to be humanlike:
Regarding the head, the technical paper states, "Cranial morphology of H. naledi is unique..." The paper continues:
H1 is further distinguished from H. neanderthalensis and H. sapiens by its relatively small facets for the Mc1 and scaphoid on the trapezium, its low angle between the Mc2 and Mc3 facets on the capitate, and by its long and curved proximal and intermediate phalanges on rays 2-5.
H1 is differentiated from all known hominins in having a Mc1 that combines a mediolaterally narrow proximal end and articular facet with a mediolaterally wide distal shaft and head, a dorsopalmarly flat and strongly asymmetric (with an enlarged palmar-lateral protuberance) Mc1 head, and the combination of an overall later Homo-like carpal morphology combined with proximal and intermediate phalanges that are more curved than most australopiths. H1 also differs from all other known hominins except H. neanderthalensis in having non-pollical distal phalanges with mediolaterally broad apical tufts (relative to length).
The hands are claimed to be humanlike but they have key unique features and are, unlike human hands, are tailored for climbing. ABC News reported: "Homo naledi had human-like hands and feet, but Tattersall said it was impressive that it also had climbing features, more similar to an ape." CNN reports: "Its hands are superficially humanlike, but the finger bones are locked into a curve -- a trait that suggests climbing and tool-using capabilities." And even Berger states: "It's pretty clear from those fingers that they're [for] climbing." Moreover, the technical paper notes the hand's "unique first metacarpal morphology" and states: "One of the most unique aspects of H. naledi is the morphology of the first metacarpal; the derived aspects of this anatomy are present in every one of seven first metacarpal specimens in the collection."
The femur is unique: "The femur of H. naledi differs from those of all other known hominins in its possession of two well-defined, mediolaterally-running pillars in the femoral neck. The pillars run along the superoanterior and inferoposterior margins of the neck and define a distinct sulcus along its superior aspect."
The tibia is unique: "The tibia of H. naledi differs from those of all other known hominins in its possession of a distinct tubercle for the pes anserinus tendon."
The foot has unique traits: "The foot of H. naledi can be distinguished from the foot of H. sapiens only by its flatter lateral and medial malleolar facets on the talus, its low angle of plantar declination of the talar head, its lower orientation of the calcaneal sustentaculum tali, and its gracile calcaneal tuber." Moreover, the technical paper states:
The phalanges are moderately curved, slightly more so than in H. sapiens. The only primitive anatomies found in the foot of H. naledi are the talar head and neck declination and sustentaculum tali angles, suggestive of a lower arched foot with a more plantarly positioned and horizontally inclined medial column than typically found in modern humans.
The lower limb has unique features: "The lower limb of H. naledi is defined not only by a unique combination of primitive and derived traits, but also by the presence of unique features in the femur and tibia. ... Unique features in the lower limb of H. naledi include a depression in the superior aspect of the femoral neck that results in two mediolaterally oriented pillars inferoposteriorly and superoanteriorly, and a strong distal attachment of the pes anserinus on the tibia."
Even Berger admits, "It doesn't look a lot like us." He also states: "There may be debate over the Homo designation" since "the species is quite different from anything else we have seen."
It wouldn't be surprising if later analyses change our understanding of the fossil. One scientist suggests new interpretations may be forthcoming:
Carol Ward, a professor of pathology and anatomical sciences at the University of Missouri who was not involved with the study said she was disappointed by the lack of empirical data presented in the paper. "There are only tiny composite pictures of the fossils, so you can't see them and there are no comparative data comparing it to anything else," said Ward. "There's nothing we can use to make our own judgments about the validity of what they are saying."
An "Anatomical Mosaic"
For now, the promoters of Homo naledi are are calling it an "anatomical mosaic." That terminology raises a red flag. In the parlance of evolutionary biology, it means the fossil is a strange organism that doesn't fit well into the standard phylogeny. Indeed, just four years ago Australopithecus sediba was the transitional form du jure between Australopithecus and Homo. If people now want to advocate Homo naledi as the new transition, then they have to radically change the phylogeny. As Berger confirms, the two species have very different mixtures of traits:
"Naledi is almost the mirror of sediba," says Berger. "Almost everywhere in the sediba skeleton where you see primitive features, in naledi you see derived features. And almost everywhere that sediba is derived, naledi is primitive."
If Australopithecus sediba somehow turned into Homo naledi, then that would be a very odd, unparisimonious evolutionary story. Yet both species have now been called a human ancestor in the past few years. The claims, it would seem, cannot both be true.
Another major challenge to claims for naledi as a transitional form is the fact that the age of these newly reported fossils is currently totally unknown. Thus it's not at all clear how it might fit into any evolutionary scheme. For all we know at present, the fossils are very young (say, less than 250,000 years old), and are far removed from any hypothetical evolutionary transition between Australopithecus and Homo.
There are at least four controversies surrounding Homo naledi we're probably going to be hearing about for a long time. These strike at the heart of whether the species -- if it is a single species -- is a some kind of a "transitional form" or just another weird hominin that might not fit anywhere else:
Controversy 1: How Old Is Homo naledi?
No one knows the age -- either generally or precisely -- of the bones currently assigned to Homo naledi. As The Scientist reports:
The team has not yet been able to determine an approximate age for the fossils. Ward said this information will be key to interpreting their significance to human evolution. "It's an amazing collection of fossils, unprecedented and stunning," she said, "but without a date, they don't tell us much other than there was another kind of hominin out there."
Likewise Scientific American notes,
The age of the fossils has yet to be determined, leaving other scientists unsure of what to make of them. ... exactly where H. naledi belongs in the human family tree, apart from somewhere on the Homo branch, is unclear. The confusion arises in large part from the fact that thus far the researchers have been unable to determine the age of the bones.
But some of naledi's advocates think they know what to make of the fossils, despite the compete current lack of an age for these fossils. How do they know? Evolutionary assumptions, which drive a desire among some that the bones should turn out to be somewhere between 2.5 and 3 million years old. As the Associated Press reports:
Lee Berger, a professor at the University of the Witwatersrand in Johannesburg who led the work, said naledi's anatomy suggest that it arose at or near the root of the Homo group, which would make the species some 2.5 million to 2.8 million years old. The discovered bones themselves may be younger, he said.
It is evolutionary interpretations, not the data, that are guiding the proposed dates of the bones. Smithsonian paleoanthropologist Rick Potts brings the conversation back to earth:
Rick Potts, director of the human origins program at the Smithsonian Institution's Natural History Museum, who was not involved in the discovery, said that without an age, "there's no way we can judge the evolutionary significance of this find."
If the bones are about as old as the Homo group, that would argue that naledi is "a snapshot of ... the evolutionary experimentation that was going on right around the origin" of Homo, he said. If they are significantly younger, it either shows the naledi retained the primitive body characteristics much longer than any other known creature, or that it re-evolved them, he said.
Until the ages are determined, it's better to remain cautious about interpretations of these fossils -- something the media has not been doing lately.
Controversy 2: Is Homo naledi a single species?
The question of whether the bones currently assigned to Homo naledi represent a single species may seem like an academic one but it actually could bear directly upon whether it's something like a transitional form, or nothing of the kind. Jeffrey Schwartz, an anthropologist at the University of Pittsburgh, thinks the bones represent multiple species because of the two different types of skulls found in the cave:
Inevitably, though, there are dissenting views. "To me, having studied virtually the entire human fossil record, the specimens lumped together as Homo naledi represent two cranial morphs," says Jeffrey Schwartz at the University of Pittsburgh in Philadelphia.
[...]
As for the Dinaledi finds, Schwartz and Tattersall point out that although the foreheads of some of the new skulls are gently sloped, one skull has a taller forehead with a distinct brow ridge -- suggesting two species are present. "Putting these fossils in the genus Homo adds to the lack of clarity in trying to sort out human evolution," says Schwartz.
That's a very significant observation. There are two different types of skulls found in the cave. Normally that would suggest multiple species, but these authors want there to be a single species with a small brain and human-like hands and feet -- the "transitional mosaic."
Thus, according to New Scientist, Lee Berger explains away the criticisms of Schwartz and Tattersall. He says the different skulls "can be explained by differences between males and females of the same species." Perhaps so, but it's also a very real possibility that there are multiple species within the cave.
The fact that Berger appeals to sexual dimorphism (different morphologies between males and females of a single species) to explain the different skulls is revealing. It shows that there is indeed a challenge to his "single species" claim. However, if there are multiple species, then you don't necessarily know that humanlike hands and feet didn't come from something more like us, whereas the small heads came from another species more like an australopith. We just don't know. As Nature News reports:
Jeffrey Schwartz, an evolutionary biologist at the University of Pittsburgh in Pennsylvania, thinks that the material is too varied to represent a single species. "I could show those images to my students and they would say that they're not the same," he says. One of the skulls looks more like it comes from an australopithecine, he says, as do certain features of the femurs.
Complicating all of this is the fact that many of the bones were jumbled up in the cave. CNN reports:
"The first thing that you would see, especially in the early stages of the investigation, was just bones. Bone debris everywhere," says K. Lindsay Hunter, an American scientist and one of the "astronauts" on the Rising Star expeditions, which were conducted in November 2013 and March 2014.
Likewise ABC News reports:
"Everywhere that my headlamp shone, I could see that there was bone on the floor. Not full bone but fragments of material," Marina Elliott, one of the authors of the study detailing the find, told BBC News. "It was an incredible thing to see."
Observations like these show how it can be very difficult to determine which hands or feet were connected to what craniums and other bones in a living individual. The discoverers/promoters of the fossils claim they all belong to a single species, but do we know that's true?
Controversy 3: Did Homo naledi Bury Its Dead?
A major claim being promoted in the media holds that Homo naledi ritualistically buried its dead, a testimony to its supposedly human-like intellect. The New York Times described it this way:
Besides introducing a new member of the prehuman family, the discovery suggests that some early hominins intentionally deposited bodies of their dead in a remote and largely inaccessible cave chamber, a behavior previously considered limited to modern humans. Some of the scientists referred to the practice as a ritualized treatment of their dead, but by "ritual" they said they meant a deliberate and repeated practice, not necessarily a kind of religious rite.
Even if this story is true, it's not the case that this species buried its dead in any manner like humans bury their dead. The bones weren't buried in the ground. Rather, it seems like the bodies were just tossed into the back crevice of a cave and left there to rot, as New Scientist explains:
The cave layout seems not to have changed in thousands of years, so either bodies were dragged into the caves and "posted" down the shaft, or else people crawled down there alive and subsequently died.
The bones were found at the very back of a large cave in a small chamber that was separated from the cave through a narrow bottleneck, kind of like a stomach that's separated by something like an esophagus. As New Scientist explains, "To reach the chamber, you have to descend through a narrow, 12-metre vertical shaft."
The cave layout means that if this species did "bury" its dead, then it would have had to crawl very far back into a deep dark cave, dragging a body with it. Some have suggested that would require using torches. Is that likely? This species had a small brain not much larger than a chimp, and we definitely don't see any extant evidence that species with that kind of intelligence could use fire, or bury its dead. Burial by torchlight seems highly unlikely for a species of this level of intelligence. As Scientific American explains:
The suggestion that small-brained H. naledi was systematically disposing of its dead has likewise raised eyebrows. "It would be quite radical," says Alison Brooks of George Washington University. "There are people who think Neandertals didn't bury their dead," she observes. (Neandertals are our closest relatives; they had brains as large as our own and engaged in a host of sophisticated behaviors. Whether or not they buried their dead is a matter of some debate. ) "I don't want to rule it out entirely that they're right, but I just think it is so far out there that they really need a higher standard of proof." Brooks adds that the team would have a better case if it could show that the remains all date to the same time period.
Another problem is that "burying" the dead in this manner would require shimmying through a very narrow crevice in the cave while dragging a body -- something that could be physically challenging for any hominin of any level of intelligence to do. The Scientist explains what modern-day researchers had to go through to get to the chamber where the bones were found:
To reach the ancient specimen-rich chamber, researchers had to squeeze through a tiny gap just 7.5 inches wide. The remains appear to have arrived in the cave fully intact and decomposed after deposition. Researchers found no indication of predation. ... Ward is also skeptical of the intentional burial explanation. "If it's really that hard to get to the cave, how do you get to that long dark cave carrying your dead grandmother?" she asked.
Indeed, even if there is deliberate disposal of the dead, that doesn't necessarily mean that members of Homo naledi were the ones doing the burying, as the Associated Press reminds us: "Potts said a deliberate disposal of dead bodies is a feasible explanation, but he added it's not clear who did the disposing. Maybe it was some human relative other than naledi, he said."
An article at IFL Science frames the problem well:
So how did this collection of individuals arrive in this dark, isolated and extremely difficult to access cave? And difficult is not an understatement: one of the narrowest cracks was a mere 17.5 centimeters wide, and as far as the group can tell, there were no other entrances to the tiny chamber. So unwelcoming that no other species were found here, aside from a few rodent and bird bones.
Unless the cave has changed significantly, it would be very difficult to drag a dead body through such a small gap.
So what happened? Much more likely, the individuals who were found in the back of the cave got there alive, wriggling their way in on their own.
There is a good explanation of how these bones got there, an explanation that's consistent with all the evidence: the individuals were running away from a predator and fled into the deep darkness of a cave where perhaps they got stuck or lost. Perhaps they were afraid to leave the cave. Perhaps it was too dark to find their way out. In any event, in this scenario, they died. Whatever the reason, it's easy to imagine these apelike creatures fleeing into a deep, dark cave as the most expedient way to escape when being chased by some unfriendly African predator (pick your species). At least one person, Travis Pickering of the University of Wisconsin, seems to feel the same way, as Scientific American reports:
Pickering adds that it is impossible to say whether the H. naledi individuals were lured or pushed into the cave to be murdered, or whether they were placed there, once dead, as part of a ritual. In fact, another, contemporaneous human species might have disposed of H. naledi's bones in that spot."
And even if the individuals were placed at the back of the cave by their friends after they died, does that mean they were "buried"? William Jungers says no. He explains in The Scientist:
William Jungers, chair of anatomical sciences at Stony Brook University in New York who was not involved with the study, cautioned against attributing too much meaning to the notion of intentional burial. "Dumping conspecifics down a hole may just be better than letting them decay around you," he said. Jungers added that there may once have been another, easier to access, entrance to the cave.
Given the lack of evidence and the availability of plausible alternatives, why do naledi's discoverers prefer the theory that the species ritualistically buried its dead ? IFL Science suggests an explanation:
After ruling all of the probable scenarios, such as mass death, transport by water and predation, the team was left with the improbable: this species was deliberately, repeatedly disposing of its dead in a protected area, away from the external environment. Before now, we thought that was a characteristic specific to modern humans.
Actually I don't think this is entirely correct. The team that discovered Homo naledi seems to have ignored the obvious interpretation -- that the individuals were trying to escape predators -- in favor of the view that they were burying their dead. The species' promoters preferred this explanation because they want a small-brained species that has human-like behavior. Evolutionary considerations, again, are what's driving the conclusions here.
Controversy 4: Does "Homo" naledi Belong in Homo?
As I discussed recently, the definition of the genus Homo is controversial, and definitions are often influenced by evolutionary biases. In that article I recounted how Jeffrey Schwartz and Ian Tattersall argued that species have often been sloppily shoehorned into Homo. Given that Schwartz and Tattersall have been critical of placing naledi within Homo, their recent piece in Science may have been a preemptive strike against the promoters of naledi placing it within "Homo."
Ian Tattersall told ABC News: "We're [probably] looking at a cousin rather than an ancestor, but who knows."
"Who knows..." That is exactly right. Even Berger stated: "We need to be very cautious about proclaiming everything we find as the direct ancestors of humans, it's clear there are a lot of experiments going on out there."
Isn't that the central message here? It should be, which therefore makes it odd that Berger told the New York Times that naledi is a "new species of human ancestor."
In any case, other scientists are skeptical that it belongs in Homo, as Scientific American reports:
The team's claims have met with skepticism. "I find [the discovery] fabulous but confusing," says Susan Antón of New York University, who studies the evolution of Homo. She notes that the remains highlight an ongoing debate among paleoanthropologists about what constitutes the genus in the first place. Early Homo fossils tend to be scrappy at best, which makes it hard to figure out which traits first distinguished our genus from Australopithecus. H. naledi has multiple body parts preserved, but "we don't have any idea how old this stuff is or whether it's relevant to the origin of Homo," Antón comments.
Bernard Wood of George Washington University agrees with the authors that the remains represent a new species, but he does not think that they will force experts to revise the overarching story of human evolution. Instead he suspects that bones represent a relic population that might have evolved its odd traits in relative isolation in South Africa, which he describes as a cul de sac at the bottom of the African continent. Wood points to another small-brained species of Homo, H. floresiensis from the island of Flores in Indonesia, as another example of such a relic population.
As I noted above in listing "Homo" naledi's unique feature, the species is primarily said to belong in Homo on the basis of its hands and feet -- but even those parts of the body have unique features. Another major feature said to place the species within Homo is its relatively tall height of about 5 feet. But such a height is known within australopithecines as well, as the technical paper explains:
H. naledi has a range of body mass similar to small-bodied modern human populations, and is similar in estimated stature to both small-bodied humans and the largest known australopiths. ... Some large australopiths also had long tibiae and presumably comparably tall statures, as evidenced by the KSD-VP 1/1 skeleton from Woranso-Mille.
So its stature was not necessarily humanlike and could be australopithlike. For me, here's the most significant reason why it doesn't belong in Homo: "The endocranial volume of all H. naledi specimens is clearly small compared to most known examples of Homo." That's an understatement. The paper continues: "The resulting estimates of approximately 560cc and 465cc, respectively, overlap entirely with the range of endocranial volumes known for australopiths. Within the genus Homo, only the smallest specimens of H. habilis, one single H. erectus specimen, and H. floresiensis overlap with these values." (And as we've noted recently, habilis probably doesn't belong in Homo.)
Schwartz himself wrote a scathing op-ed in Newsweek, "Why the Homo Naledi Discovery May Not Be Quite What it Seems." He argued that "Homo naledi" may in fact represent multiple species, and probably doesn't belong in Homo:
Enter the newly announced species, Homo naledi, which is claimed to be our direct ancestor because it has features of australopiths and Homo. Why is it a species of Homo? Because some specimens seem to be like us. Why australopith? Because other specimens have some of their features. Why do all belong to the same species? Because they were found in the same cave. but, the published images tell a different story.
Viewed from the side, two partial skulls are long and low, with a long gently sloping forehead that flows smoothly into the brow -- nothing like us, or most specimens regarded as Homo. A third partial skull is very short and rounded, with a high-rising forehead that is distinguished from a distinct, well-defined brow by a shallow gutter -- not like the other skulls, and not like us or most specimens regarded as Homo. The femur has a small head (the ball end that fits in the hip socket) that is connected to the shaft of the bone by a long neck, and, below the neck, is a "bump" of bone that points backward. These features are seen in every australopith femur. In us, and all other living primates, the head of the femur is large and the neck short, and the "bump" points inward. Further, the teeth are very similar to those from a nearby fossil site that has yielded various kinds of australopith. Even at this stage of their being publicized, the "Homo naledi" specimens reflect even greater diversity in the human fossil record than their discoverers will admit.
The specimens found in this cave are very diverse, suggesting they might belong to multiple species. But if that's the case, then we don't know that we've found one single species with a mixture of human-like and australopithecine traits. The supposedly transitional "mosaic" falls apart.
This brings us back, in a full circle, to Australopithecus sediba. As I mentioned at the beginning, exactly four years ago the media were eagerly promoting that species as the latest "human ancestor" or "transitional form." And what happened? What always happens happened: cooler heads prevailed and it was found that sediba was from the wrong time period and had the wrong set of traits to be a link between humanlike members of the genus Homo and the apelike members of Australopithecus. What will become of "Homo naledi" remains to be seen. So far, though, its pathway reminds me of other hominin fossils whose "transitional" or "ancestral" status ultimately went belly up. A strong dose of healthy skepticism is warranted.
Casey Luskin September 20, 2015 6:40 PM |
The media is once again abuzz over the discovery of a new hominin fossil. The fossil is named Homo naledi, represented by hundreds of bones found in a cave near Johannesburg, South Africa. Biologic Institute biologist Ann Gauger has already commented on the find here and here.
It has long been recognized that we are missing fossils documenting the supposed transition from the apelike genus Australopithecus to the humanlike Homo. Despite what you may be hearing in the media, Homo naledi does not solve this problem.
Some have envisioned the hallowed intermediate link being a creature with an apelike body and a human-like head. For some time, Homo habilis was such a candidate -- until cooler heads prevailed, as I noted earlier . Others have hoped we'd uncover something with a more Homo-like postcranial (below the head) skeleton but a more australopith-ape-like body. Indeed, almost exactly four years ago, in a post titled "Hominid Hype and the Election Cycle, I noted these precise arguments with regard to Australopithecus sediba.
Coincidently, we're right now in almost exactly the same place in the election cycle, and seeing almost identical claims about this new fossil discovery. Indeed, Homo naledi was discovered (and is being promoted) by the same researcher, Lee Berger, that unveiled (and promoted) sediba, although, as we'll see, naledi has a very different and unique set of traits from sediba. Numerous recent articles have hailed Homo naledi as the newest human ancestor: CNN: "Homo naledi: New species of human ancestor discovered in South Africa," PBS: "Trove of fossils from a long lost human ancestor," Vocativ: "Meet Homo Naledi: Your Newest Human Ancestor," Daily Mail: "Scientists discover skull of new human ancestor Homo Naledi," NBC New York: "Scientists Discover Homo Naledi, Early Human Ancestor," and so on.
Ironically, all of these claims are pure hype because, as we'll see, no one knows how old these bones are, and so it's entirely possible they're younger than our own species, making it, at present, a very real possibility that it's chronologically impossible for them to be anything remotely close to our "ancestor."
The main claim about Homo naledi is that it is a small-brained hominin (when compared to humans) that has other features that are very humanlike -- especially its hands and feet. As the news headlines suggest, there has been an immense amount of hype about this species, consistent with the hype surrounding Australopithecus sediba, which again was discovered and promoted by the same researcher, Lee Berger. However, while there are some humanlike aspects of its body plan, my overall impression is that this is a highly unique species that doesn't fit well into previously established categories.
The technical paper, "Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa," appeared in a lesser-known journal, eLife. It's a great find due to the sheer number of bones that were find, but to my mind its publication in eLife is immediate hint that this fossil isn't an earthshattering "transitional form," because if it were, we almost unquestionably would have seen the fossil published in Science or Nature. Many comments there reveal this fossil to be unique -- precisely with regard to features that are said to be humanlike:
Regarding the head, the technical paper states, "Cranial morphology of H. naledi is unique..." The paper continues:
H1 is further distinguished from H. neanderthalensis and H. sapiens by its relatively small facets for the Mc1 and scaphoid on the trapezium, its low angle between the Mc2 and Mc3 facets on the capitate, and by its long and curved proximal and intermediate phalanges on rays 2-5.
H1 is differentiated from all known hominins in having a Mc1 that combines a mediolaterally narrow proximal end and articular facet with a mediolaterally wide distal shaft and head, a dorsopalmarly flat and strongly asymmetric (with an enlarged palmar-lateral protuberance) Mc1 head, and the combination of an overall later Homo-like carpal morphology combined with proximal and intermediate phalanges that are more curved than most australopiths. H1 also differs from all other known hominins except H. neanderthalensis in having non-pollical distal phalanges with mediolaterally broad apical tufts (relative to length).
The hands are claimed to be humanlike but they have key unique features and are, unlike human hands, are tailored for climbing. ABC News reported: "Homo naledi had human-like hands and feet, but Tattersall said it was impressive that it also had climbing features, more similar to an ape." CNN reports: "Its hands are superficially humanlike, but the finger bones are locked into a curve -- a trait that suggests climbing and tool-using capabilities." And even Berger states: "It's pretty clear from those fingers that they're [for] climbing." Moreover, the technical paper notes the hand's "unique first metacarpal morphology" and states: "One of the most unique aspects of H. naledi is the morphology of the first metacarpal; the derived aspects of this anatomy are present in every one of seven first metacarpal specimens in the collection."
The femur is unique: "The femur of H. naledi differs from those of all other known hominins in its possession of two well-defined, mediolaterally-running pillars in the femoral neck. The pillars run along the superoanterior and inferoposterior margins of the neck and define a distinct sulcus along its superior aspect."
The tibia is unique: "The tibia of H. naledi differs from those of all other known hominins in its possession of a distinct tubercle for the pes anserinus tendon."
The foot has unique traits: "The foot of H. naledi can be distinguished from the foot of H. sapiens only by its flatter lateral and medial malleolar facets on the talus, its low angle of plantar declination of the talar head, its lower orientation of the calcaneal sustentaculum tali, and its gracile calcaneal tuber." Moreover, the technical paper states:
The phalanges are moderately curved, slightly more so than in H. sapiens. The only primitive anatomies found in the foot of H. naledi are the talar head and neck declination and sustentaculum tali angles, suggestive of a lower arched foot with a more plantarly positioned and horizontally inclined medial column than typically found in modern humans.
The lower limb has unique features: "The lower limb of H. naledi is defined not only by a unique combination of primitive and derived traits, but also by the presence of unique features in the femur and tibia. ... Unique features in the lower limb of H. naledi include a depression in the superior aspect of the femoral neck that results in two mediolaterally oriented pillars inferoposteriorly and superoanteriorly, and a strong distal attachment of the pes anserinus on the tibia."
Even Berger admits, "It doesn't look a lot like us." He also states: "There may be debate over the Homo designation" since "the species is quite different from anything else we have seen."
It wouldn't be surprising if later analyses change our understanding of the fossil. One scientist suggests new interpretations may be forthcoming:
Carol Ward, a professor of pathology and anatomical sciences at the University of Missouri who was not involved with the study said she was disappointed by the lack of empirical data presented in the paper. "There are only tiny composite pictures of the fossils, so you can't see them and there are no comparative data comparing it to anything else," said Ward. "There's nothing we can use to make our own judgments about the validity of what they are saying."
An "Anatomical Mosaic"
For now, the promoters of Homo naledi are are calling it an "anatomical mosaic." That terminology raises a red flag. In the parlance of evolutionary biology, it means the fossil is a strange organism that doesn't fit well into the standard phylogeny. Indeed, just four years ago Australopithecus sediba was the transitional form du jure between Australopithecus and Homo. If people now want to advocate Homo naledi as the new transition, then they have to radically change the phylogeny. As Berger confirms, the two species have very different mixtures of traits:
"Naledi is almost the mirror of sediba," says Berger. "Almost everywhere in the sediba skeleton where you see primitive features, in naledi you see derived features. And almost everywhere that sediba is derived, naledi is primitive."
If Australopithecus sediba somehow turned into Homo naledi, then that would be a very odd, unparisimonious evolutionary story. Yet both species have now been called a human ancestor in the past few years. The claims, it would seem, cannot both be true.
Another major challenge to claims for naledi as a transitional form is the fact that the age of these newly reported fossils is currently totally unknown. Thus it's not at all clear how it might fit into any evolutionary scheme. For all we know at present, the fossils are very young (say, less than 250,000 years old), and are far removed from any hypothetical evolutionary transition between Australopithecus and Homo.
There are at least four controversies surrounding Homo naledi we're probably going to be hearing about for a long time. These strike at the heart of whether the species -- if it is a single species -- is a some kind of a "transitional form" or just another weird hominin that might not fit anywhere else:
Controversy 1: How Old Is Homo naledi?
No one knows the age -- either generally or precisely -- of the bones currently assigned to Homo naledi. As The Scientist reports:
The team has not yet been able to determine an approximate age for the fossils. Ward said this information will be key to interpreting their significance to human evolution. "It's an amazing collection of fossils, unprecedented and stunning," she said, "but without a date, they don't tell us much other than there was another kind of hominin out there."
Likewise Scientific American notes,
The age of the fossils has yet to be determined, leaving other scientists unsure of what to make of them. ... exactly where H. naledi belongs in the human family tree, apart from somewhere on the Homo branch, is unclear. The confusion arises in large part from the fact that thus far the researchers have been unable to determine the age of the bones.
But some of naledi's advocates think they know what to make of the fossils, despite the compete current lack of an age for these fossils. How do they know? Evolutionary assumptions, which drive a desire among some that the bones should turn out to be somewhere between 2.5 and 3 million years old. As the Associated Press reports:
Lee Berger, a professor at the University of the Witwatersrand in Johannesburg who led the work, said naledi's anatomy suggest that it arose at or near the root of the Homo group, which would make the species some 2.5 million to 2.8 million years old. The discovered bones themselves may be younger, he said.
It is evolutionary interpretations, not the data, that are guiding the proposed dates of the bones. Smithsonian paleoanthropologist Rick Potts brings the conversation back to earth:
Rick Potts, director of the human origins program at the Smithsonian Institution's Natural History Museum, who was not involved in the discovery, said that without an age, "there's no way we can judge the evolutionary significance of this find."
If the bones are about as old as the Homo group, that would argue that naledi is "a snapshot of ... the evolutionary experimentation that was going on right around the origin" of Homo, he said. If they are significantly younger, it either shows the naledi retained the primitive body characteristics much longer than any other known creature, or that it re-evolved them, he said.
Until the ages are determined, it's better to remain cautious about interpretations of these fossils -- something the media has not been doing lately.
Controversy 2: Is Homo naledi a single species?
The question of whether the bones currently assigned to Homo naledi represent a single species may seem like an academic one but it actually could bear directly upon whether it's something like a transitional form, or nothing of the kind. Jeffrey Schwartz, an anthropologist at the University of Pittsburgh, thinks the bones represent multiple species because of the two different types of skulls found in the cave:
Inevitably, though, there are dissenting views. "To me, having studied virtually the entire human fossil record, the specimens lumped together as Homo naledi represent two cranial morphs," says Jeffrey Schwartz at the University of Pittsburgh in Philadelphia.
[...]
As for the Dinaledi finds, Schwartz and Tattersall point out that although the foreheads of some of the new skulls are gently sloped, one skull has a taller forehead with a distinct brow ridge -- suggesting two species are present. "Putting these fossils in the genus Homo adds to the lack of clarity in trying to sort out human evolution," says Schwartz.
That's a very significant observation. There are two different types of skulls found in the cave. Normally that would suggest multiple species, but these authors want there to be a single species with a small brain and human-like hands and feet -- the "transitional mosaic."
Thus, according to New Scientist, Lee Berger explains away the criticisms of Schwartz and Tattersall. He says the different skulls "can be explained by differences between males and females of the same species." Perhaps so, but it's also a very real possibility that there are multiple species within the cave.
The fact that Berger appeals to sexual dimorphism (different morphologies between males and females of a single species) to explain the different skulls is revealing. It shows that there is indeed a challenge to his "single species" claim. However, if there are multiple species, then you don't necessarily know that humanlike hands and feet didn't come from something more like us, whereas the small heads came from another species more like an australopith. We just don't know. As Nature News reports:
Jeffrey Schwartz, an evolutionary biologist at the University of Pittsburgh in Pennsylvania, thinks that the material is too varied to represent a single species. "I could show those images to my students and they would say that they're not the same," he says. One of the skulls looks more like it comes from an australopithecine, he says, as do certain features of the femurs.
Complicating all of this is the fact that many of the bones were jumbled up in the cave. CNN reports:
"The first thing that you would see, especially in the early stages of the investigation, was just bones. Bone debris everywhere," says K. Lindsay Hunter, an American scientist and one of the "astronauts" on the Rising Star expeditions, which were conducted in November 2013 and March 2014.
Likewise ABC News reports:
"Everywhere that my headlamp shone, I could see that there was bone on the floor. Not full bone but fragments of material," Marina Elliott, one of the authors of the study detailing the find, told BBC News. "It was an incredible thing to see."
Observations like these show how it can be very difficult to determine which hands or feet were connected to what craniums and other bones in a living individual. The discoverers/promoters of the fossils claim they all belong to a single species, but do we know that's true?
Controversy 3: Did Homo naledi Bury Its Dead?
A major claim being promoted in the media holds that Homo naledi ritualistically buried its dead, a testimony to its supposedly human-like intellect. The New York Times described it this way:
Besides introducing a new member of the prehuman family, the discovery suggests that some early hominins intentionally deposited bodies of their dead in a remote and largely inaccessible cave chamber, a behavior previously considered limited to modern humans. Some of the scientists referred to the practice as a ritualized treatment of their dead, but by "ritual" they said they meant a deliberate and repeated practice, not necessarily a kind of religious rite.
Even if this story is true, it's not the case that this species buried its dead in any manner like humans bury their dead. The bones weren't buried in the ground. Rather, it seems like the bodies were just tossed into the back crevice of a cave and left there to rot, as New Scientist explains:
The cave layout seems not to have changed in thousands of years, so either bodies were dragged into the caves and "posted" down the shaft, or else people crawled down there alive and subsequently died.
The bones were found at the very back of a large cave in a small chamber that was separated from the cave through a narrow bottleneck, kind of like a stomach that's separated by something like an esophagus. As New Scientist explains, "To reach the chamber, you have to descend through a narrow, 12-metre vertical shaft."
The cave layout means that if this species did "bury" its dead, then it would have had to crawl very far back into a deep dark cave, dragging a body with it. Some have suggested that would require using torches. Is that likely? This species had a small brain not much larger than a chimp, and we definitely don't see any extant evidence that species with that kind of intelligence could use fire, or bury its dead. Burial by torchlight seems highly unlikely for a species of this level of intelligence. As Scientific American explains:
The suggestion that small-brained H. naledi was systematically disposing of its dead has likewise raised eyebrows. "It would be quite radical," says Alison Brooks of George Washington University. "There are people who think Neandertals didn't bury their dead," she observes. (Neandertals are our closest relatives; they had brains as large as our own and engaged in a host of sophisticated behaviors. Whether or not they buried their dead is a matter of some debate. ) "I don't want to rule it out entirely that they're right, but I just think it is so far out there that they really need a higher standard of proof." Brooks adds that the team would have a better case if it could show that the remains all date to the same time period.
Another problem is that "burying" the dead in this manner would require shimmying through a very narrow crevice in the cave while dragging a body -- something that could be physically challenging for any hominin of any level of intelligence to do. The Scientist explains what modern-day researchers had to go through to get to the chamber where the bones were found:
To reach the ancient specimen-rich chamber, researchers had to squeeze through a tiny gap just 7.5 inches wide. The remains appear to have arrived in the cave fully intact and decomposed after deposition. Researchers found no indication of predation. ... Ward is also skeptical of the intentional burial explanation. "If it's really that hard to get to the cave, how do you get to that long dark cave carrying your dead grandmother?" she asked.
Indeed, even if there is deliberate disposal of the dead, that doesn't necessarily mean that members of Homo naledi were the ones doing the burying, as the Associated Press reminds us: "Potts said a deliberate disposal of dead bodies is a feasible explanation, but he added it's not clear who did the disposing. Maybe it was some human relative other than naledi, he said."
An article at IFL Science frames the problem well:
So how did this collection of individuals arrive in this dark, isolated and extremely difficult to access cave? And difficult is not an understatement: one of the narrowest cracks was a mere 17.5 centimeters wide, and as far as the group can tell, there were no other entrances to the tiny chamber. So unwelcoming that no other species were found here, aside from a few rodent and bird bones.
Unless the cave has changed significantly, it would be very difficult to drag a dead body through such a small gap.
So what happened? Much more likely, the individuals who were found in the back of the cave got there alive, wriggling their way in on their own.
There is a good explanation of how these bones got there, an explanation that's consistent with all the evidence: the individuals were running away from a predator and fled into the deep darkness of a cave where perhaps they got stuck or lost. Perhaps they were afraid to leave the cave. Perhaps it was too dark to find their way out. In any event, in this scenario, they died. Whatever the reason, it's easy to imagine these apelike creatures fleeing into a deep, dark cave as the most expedient way to escape when being chased by some unfriendly African predator (pick your species). At least one person, Travis Pickering of the University of Wisconsin, seems to feel the same way, as Scientific American reports:
Pickering adds that it is impossible to say whether the H. naledi individuals were lured or pushed into the cave to be murdered, or whether they were placed there, once dead, as part of a ritual. In fact, another, contemporaneous human species might have disposed of H. naledi's bones in that spot."
And even if the individuals were placed at the back of the cave by their friends after they died, does that mean they were "buried"? William Jungers says no. He explains in The Scientist:
William Jungers, chair of anatomical sciences at Stony Brook University in New York who was not involved with the study, cautioned against attributing too much meaning to the notion of intentional burial. "Dumping conspecifics down a hole may just be better than letting them decay around you," he said. Jungers added that there may once have been another, easier to access, entrance to the cave.
Given the lack of evidence and the availability of plausible alternatives, why do naledi's discoverers prefer the theory that the species ritualistically buried its dead ? IFL Science suggests an explanation:
After ruling all of the probable scenarios, such as mass death, transport by water and predation, the team was left with the improbable: this species was deliberately, repeatedly disposing of its dead in a protected area, away from the external environment. Before now, we thought that was a characteristic specific to modern humans.
Actually I don't think this is entirely correct. The team that discovered Homo naledi seems to have ignored the obvious interpretation -- that the individuals were trying to escape predators -- in favor of the view that they were burying their dead. The species' promoters preferred this explanation because they want a small-brained species that has human-like behavior. Evolutionary considerations, again, are what's driving the conclusions here.
Controversy 4: Does "Homo" naledi Belong in Homo?
As I discussed recently, the definition of the genus Homo is controversial, and definitions are often influenced by evolutionary biases. In that article I recounted how Jeffrey Schwartz and Ian Tattersall argued that species have often been sloppily shoehorned into Homo. Given that Schwartz and Tattersall have been critical of placing naledi within Homo, their recent piece in Science may have been a preemptive strike against the promoters of naledi placing it within "Homo."
Ian Tattersall told ABC News: "We're [probably] looking at a cousin rather than an ancestor, but who knows."
"Who knows..." That is exactly right. Even Berger stated: "We need to be very cautious about proclaiming everything we find as the direct ancestors of humans, it's clear there are a lot of experiments going on out there."
Isn't that the central message here? It should be, which therefore makes it odd that Berger told the New York Times that naledi is a "new species of human ancestor."
In any case, other scientists are skeptical that it belongs in Homo, as Scientific American reports:
The team's claims have met with skepticism. "I find [the discovery] fabulous but confusing," says Susan Antón of New York University, who studies the evolution of Homo. She notes that the remains highlight an ongoing debate among paleoanthropologists about what constitutes the genus in the first place. Early Homo fossils tend to be scrappy at best, which makes it hard to figure out which traits first distinguished our genus from Australopithecus. H. naledi has multiple body parts preserved, but "we don't have any idea how old this stuff is or whether it's relevant to the origin of Homo," Antón comments.
Bernard Wood of George Washington University agrees with the authors that the remains represent a new species, but he does not think that they will force experts to revise the overarching story of human evolution. Instead he suspects that bones represent a relic population that might have evolved its odd traits in relative isolation in South Africa, which he describes as a cul de sac at the bottom of the African continent. Wood points to another small-brained species of Homo, H. floresiensis from the island of Flores in Indonesia, as another example of such a relic population.
As I noted above in listing "Homo" naledi's unique feature, the species is primarily said to belong in Homo on the basis of its hands and feet -- but even those parts of the body have unique features. Another major feature said to place the species within Homo is its relatively tall height of about 5 feet. But such a height is known within australopithecines as well, as the technical paper explains:
H. naledi has a range of body mass similar to small-bodied modern human populations, and is similar in estimated stature to both small-bodied humans and the largest known australopiths. ... Some large australopiths also had long tibiae and presumably comparably tall statures, as evidenced by the KSD-VP 1/1 skeleton from Woranso-Mille.
So its stature was not necessarily humanlike and could be australopithlike. For me, here's the most significant reason why it doesn't belong in Homo: "The endocranial volume of all H. naledi specimens is clearly small compared to most known examples of Homo." That's an understatement. The paper continues: "The resulting estimates of approximately 560cc and 465cc, respectively, overlap entirely with the range of endocranial volumes known for australopiths. Within the genus Homo, only the smallest specimens of H. habilis, one single H. erectus specimen, and H. floresiensis overlap with these values." (And as we've noted recently, habilis probably doesn't belong in Homo.)
Schwartz himself wrote a scathing op-ed in Newsweek, "Why the Homo Naledi Discovery May Not Be Quite What it Seems." He argued that "Homo naledi" may in fact represent multiple species, and probably doesn't belong in Homo:
Enter the newly announced species, Homo naledi, which is claimed to be our direct ancestor because it has features of australopiths and Homo. Why is it a species of Homo? Because some specimens seem to be like us. Why australopith? Because other specimens have some of their features. Why do all belong to the same species? Because they were found in the same cave. but, the published images tell a different story.
Viewed from the side, two partial skulls are long and low, with a long gently sloping forehead that flows smoothly into the brow -- nothing like us, or most specimens regarded as Homo. A third partial skull is very short and rounded, with a high-rising forehead that is distinguished from a distinct, well-defined brow by a shallow gutter -- not like the other skulls, and not like us or most specimens regarded as Homo. The femur has a small head (the ball end that fits in the hip socket) that is connected to the shaft of the bone by a long neck, and, below the neck, is a "bump" of bone that points backward. These features are seen in every australopith femur. In us, and all other living primates, the head of the femur is large and the neck short, and the "bump" points inward. Further, the teeth are very similar to those from a nearby fossil site that has yielded various kinds of australopith. Even at this stage of their being publicized, the "Homo naledi" specimens reflect even greater diversity in the human fossil record than their discoverers will admit.
The specimens found in this cave are very diverse, suggesting they might belong to multiple species. But if that's the case, then we don't know that we've found one single species with a mixture of human-like and australopithecine traits. The supposedly transitional "mosaic" falls apart.
This brings us back, in a full circle, to Australopithecus sediba. As I mentioned at the beginning, exactly four years ago the media were eagerly promoting that species as the latest "human ancestor" or "transitional form." And what happened? What always happens happened: cooler heads prevailed and it was found that sediba was from the wrong time period and had the wrong set of traits to be a link between humanlike members of the genus Homo and the apelike members of Australopithecus. What will become of "Homo naledi" remains to be seen. So far, though, its pathway reminds me of other hominin fossils whose "transitional" or "ancestral" status ultimately went belly up. A strong dose of healthy skepticism is warranted.
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