Tuesday, October 06, 2015

Casey Luskin on Homo naledi

Casey Luskin has written a piece for Evolution, News and Views, in which he examines the hype surrounding the new Homo naledi find.  It would take more time than I have to tackle it point by point, but I will hit the high notes.  He writes:
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.
That's okay. It wasn't meant to. It fills in a bit of a puzzle, that is all. We now have more information than we had about this transition, which appears to have been complex.  He continues:
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 claimed to be 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.
One of the issues so far has been the dating of the remains.  Luskin, in quoting Carol Ward's concerns about lack of dating of the fossils, makes a legitimate objection in that we do not know how old the bones are.  That is a problem.  As I pointed out in my BioLogos post, we can date the cave floor, we can date the cave walls and we can date the cave ceiling but we have little to no idea when the bones were dropped in.  It is clear that the floor of this cave was not a living floor.  He writes:
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.
He is also correct about that.  It doesn't.  H. naledi has characteristics that link it with many different hominins and the combination of traits is unique.   This is not a problem it is, as Carol Ward commented, shows us that there was considerable hominin diversity at this time and, apparently at other times in the past.  The problem is that he uses selective passages and slanted wording to imply that the case for it having "human" traits is overblown.  For example:

  For example, Luskin writes:
The hands are claimed to be humanlike but they have key unique features and, 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."
All of this is done to shift the emphasis away from the fact that, while the hominin did possess primitive traits, it also possessed derived ones. Let's see what Berger actually writes about the find:
The hand shares many derived features of modern humans and Neandertals in the thumb, wrist, and palm, but has relatively long and markedly curved fingers (Kivell et al., 2015). The thumb is long relative to the length of the other digits, and includes a robust metacarpal with well-developed intrinsic (M. opponens pollicis and M. first dorsal interosseous) muscle attachments.
Note the characteristics that Luskin leaves out.  With regard to our understanding of its taxonomic designation, Luskin writes:
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.
He then puts in the following quote from an interview of Carol Ward in The Scientist:
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."
The problem is that he leaves out a rather significant statement about the find that precedes that. Here is the entire passage from the article (emphasis added):
“H. naledi possesses a combination of primitive and derived features not seen in the hand of any other hominin,” the authors wrote, but 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.”
Luskin has done two things here.  By removing the ellipses in from of the initial part of the quote, he suggests that this is a self-contained thought, which it clearly is not.  Further, by not quoting the initial statement of Ward's, he omits that she sees not just primitive but derived traits as well.  This is a pattern throughout his piece.

He phrases the rest of the piece in the form of four controversies.

The first is “How Old is Homo naledi?”  The irony here is that Luskin could very easily have focused on this topic and left it at that.  He has Berger over a barrel, here.  We don't know how old the bones are.  We have zero idea when they were dropped in. Gunter Brauer had a problem in the 1990s with an important skull in the transition to early modern humans, Eliye Springs, which washed out a bank where the spring entered Lake Turkana.  Great find, no idea what its age is.  As I mentioned in my blog post on Homo naledi, the South African cave sites present a serious problem for dating the hominins in which they are found.  Some success has been had but many finds are simply given wide chronological ranges.

Consequently, if Homo naledi is between 2 and 3 mya, it represents a find that is near where the transition is thought to occur, based on the presence of early Homo in East Africa, although the Ledi jaw may suggest an earlier transition.  On the other hand, if it is late, say 1 mya, then it simply represents a dead end that retained many primitive traits.

Here is what Luskin writes about this:
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.
This is not entirely true and Luskin knows it, or ought to. There are perfectly valid reasons to suggest that this find is this old, even if we do not, in fact, know its age. For one thing, we know that we have hominins in East Africa that have derived traits toward modern humans between 2.3 and 1.8 million years ago.  Consequently, we know that the transition to this form(s) took place somewhere between 2.5 and 3.0 million years ago.  Further, this tracks with the discovery of the Ledi jaw, which has a mix of australopithecine and early Homo traits and is dated to 2.8 mya.  Therefore, do we know how old the Homo naledi fossils are?  No, we don't.  Are the estimated dates just being driven by evolutionary assumptions?  Clearly not.

Controversy Number 2: “Is Homo naledi a single species?”  Luskin writes:
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.
Where Luskin is going with this is that, if there are, in fact, two different species in the cave, then some of them might just be modern human.  He continues:
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.
Here, he glosses over something very important:  even the human-like skeletal material has characteristics that are not modern for example:
  • The small heads have both angular and occipital tori, characteristics only found on Homo erectus.  No australopithecine has these and no modern human does, either.
  • The faces, while being small, lack australopithecine traits such as canine jugae and anterior pillars.  Further, there is limited post-orbital constriction, a more modern characteristic.
  • Even the “human-like” hands and feet have characteristics that are primitive. For example, Berger writes: 
    The talar head and neck exhibit strong, humanlike torsion; the horizontal angle is higher than in most humans, similar to that found in australopiths. The calcaneus is only moderately robust, but possesses the plantar declination of the retrotrochlear eminence and plantarly positioned lateral plantar process found in both modern humans and Au. afarensis...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...Overall, carpal shapes and articular configurations are very similar to those of modern humans and Neandertals, and unlike those of great apes and other extinct hominins. However, the H. naledi wrist lacks a third metacarpal styloid process, has a more radioulnarly oriented capitate-Mc2 joint, and has a relatively small trapezium-Mc1 joint compared to humans and Neandertals. Moreover, the phalanges are long (relative to the palm) and more curved than most australopiths.
Therefore, appeals to there being modern humans in the cave along with australopithecines are not warranted. Maybe there was more than one species in the cave and one of them was an australopithecine. What remained, however, was not modern human.  There are no modern humans walking around with angular and occipital tori.

But say there is more than one species down there, and these different species represent different times in the history of hominin evolution.  Would this be a bad thing? What it would mean is that there is a good deal of variability in the human fossil record, something we already suspected anyway.  We know that expanded diversity existed as far back as Ardipithecus.  Why would it not be present at other times?

Controversy No. 3: Did Homo naledi Bury Its Dead?  Here is Luskin:
A major claim being promoted in the media holds that Homo naledi ritualistically buried its dead, a testimony to its supposedly human-like intellect.
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...
Luskin is correct that the media attention to this is overblown and sensationalistic. Luskin goes on, then to quote many different researchers who are skeptical about this claim (skepticism that I think is warranted, by the way) But let's see what Berger et al. say about it. First, the word “burial” never appears once in the original paper by Berger et al. That information comes from the supplementary paper by Dirks et al., who write this:
The Dinaledi collection displays taphonomic characteristics indicative of a depositional history that involved several stages of burial with surface modifications and breakage patterns consistent with repeated reworking of at least part of the assemblage within the confines of the Dinaledi Chamber, involving both biotic and abiotic agents (Supplementary file 2). The distribution of bone material and skeletal part representation indicative of limited winnowing (Table 1) indicate that the fossils of H. naledi must have found their way into the chamber via a difficult route that precluded any other large vertebrates from finding a way in. The distribution of the fossils within reworked material derived from Unit 2, as well-articulated remains in Unit 3 suggests that H. naledi fossils entered the chamber over an extended period of time; that is, not all remains were deposited at once.
So that is what we know, and that is all we know.  First, I was wrong in my post on BioLogos, this is not a standard karst cave with a top opening of any kind.  Consequently, there is no way for the bones to have gotten there unless they were placed there.  There is one possible explanation that is not mentioned by Dirks that is not in the account by Luskin:
Flowstone formation continues today (Flowstone 3), changing the morphology of cave passages. This makes it possible that a more direct access-way or easier passage may have existed when hominins entered. A different entrance into the chamber may also explain the presence of rodent bone concentrations in Facies 1b. However, sedimentation patterns indicate that the accumulation of Unit 2 with fossils occurred below the current entry point into the chamber, and alternate routes did not involve vertical access shafts that connected directly to surface in either the Dinaledi Chamber or nearby Dragon's Back Chamber.
This doesn't address the reliability of the burial hypothesis but it does suggest that it might have been much easier for the bones to get where they were.  I suspect that Jungers is correct in his hypothesis about whether or not they were intentionally buried.

Controversy No. 4: Does "Homo" naledi Belong in Homo? Luskin writes:
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."
Much is left out of this series of statements. Let's go back to the ABC article from which it is quoted:
Researchers said the newly discovered species most resembles other hominids such as Homo erectus, Homo habilis and Homo rudolfensis.

Ian Tattersall, curator emeritus at the American Museum of Natural History in New York, said the find was incredibly important and could shed important light on Homo sapiens, modern humans, as a species and the many other early hominoids.

“It’s really very exciting,” Tattersall told ABC News. “What this is doing is definitely increasing the perception that we have -- that evolution of hominids was one of vigorous experimentation of evolution.”
Why would the find shed any light on Homo if it did not have any characteristics of Homo? As numerous researchers have commented, it absolutely does.  Many of the characteristics that align it with Homo erectus have been pointed out, as well as other deviations from australopithecines.

Much of what Luskin is trying to accomplish in this section is focused on the fact that, while the Dinaledi finds have characteristics that align them with Homo, they also have some that align them with australopithecines and maybe we are over-interpreting the early Homo ones.  He then vaguely supports his case by using a series of quotes from researchers who are skeptical that we know exactly what H. naledi actually is.  For example, he writes this:
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:
Interestingly, he then quotes Schwartz, who points out only the australopithecine portions of the anatomy, to the exclusion of any of the modern traits that it has.  While it is quite true that there are differing opinions about what these finds represent, it is equally true that there are Homo traits present.  Consequently, to simply lump them in with australopithecines is inaccurate.  It may be years before we have enough information to make a sound judgment about exactly where this fits in the pantheon of human evolution, but for now, we can safely say that, whether or not there are one species present or two, a hominin with some of the traits of early Homo was present at this cave.

But even if

it turns out that H. naledi is, in fact, Au. naledi, after much reflection.  Then it just means that australopithecine diversity is greater than we thought it was and that there were many different morphs that exhibited a wide range of traits, some of which were derived in the direction of Homo.  That we don't know exactly which form gave rise to an early Homo form is not a deal-breaker.  Remember, systematics does not reveal ancestor-descendant relationships, but, instead, sister taxa.  Au. sediba and H. naledi are two different forms that express a mosaic of traits, some of which are advanced and some which are not.  They are both considerably more advanced than the australopithecines that preceded them.

Luskin is focused on the fact that H. naledi is probably not the missing link between the australopithecines and early Homo that everybody was hoping for.  That is beside the point.  The point is we now know quite a bit more about this stage of human evolution.  Even if the bones turn out to be younger than we thought, it still gives us information about human evolutionary development that we did not already have.  That's okay.

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