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Homo naledi as Spin Detector


You have probably seen the headlines. Numerous fossils of a new species of hominin have been found in a nearly inaccessible cave in South Africa. Intriguingly, the fossils appear to be part of a very large deposit of bones, apparently left there deliberately by their own kind. Also intriguingly, the skeletons of these individuals may have some traits that are Homo (human-like) and some that are australopithicine (more ape-like). To add further to the mystery, these fossils may be anywhere from 1 to 3 million years old, potentially making them among the oldest fossils identified as being of our genus Homo.

In reading the coverage of Homo naledi, as the species is called now, it seems clear to me that the spin put on the actual bones depends on the assumptions of the writers. What do I mean? Bones can only tell us so much. The rest is a matter of interpretation, and one's point of view inevitably tends to color that interpretation.

Let me give two examples:

The first example is how writers interpret skull size. H. naledi had a small brain compared to ours, about the size of a chimpanzee's. To some writers that seems to indicate the probable lack of high levels of cognition. Only species with brain sizes near our own are considered intelligent. The data used to support that claim are (a) our current knowledge of chimp and gorilla brain sizes, and their lack of rational, abstract thought; and (b) the claim that a gradual progression in brain size exists from australopiths to Homo erectus to Neanderthals to us, indicating a gradual progression in intelligence, which fits the evolutionary story.

There are two problems with these interpretations. First, modern humans exhibit a range in brain sizes, and those differences do not correlate with intelligence. The fossil skulls of Homo erectus, the earliest fossil judged to be "human," exhibit a range in skull size also (but see Casey Luskin's post on Homo habilis). Second, if we were to judge intelligence based on brain size, Neanderthals would be smarter than us since their average brain sizes were more than average human brain sizes. Maybe they were smarter?

H. naledi has a mixture of ape-like and human-like traits. It depends on the desired outcome which traits will be emphasized, and where H. naledi will be placed on the putative fossil tree. The discoverers have placed her squarely on the human side based on her apparent behavior and her inferred ability for long distance walking.

By her apparent behavior I mean the fact that the deposit of bones was found deep in a lightless cave with difficult access. It would take much effort and perhaps some light to make the journey. Yet many, many individuals were found in that dark cave. The discoverers, chief among them Lee Berger, have claimed this as evidence of human intelligence. Such deliberate behavior required significant effort and some danger and indicates a special care for the dead, something that until now only humans were known to exhibit. (I am counting Neanderthals as human.) Yet there are those who seek to claim this is not evidence of ritual behavior or a special care for the dead, that it may be a form of animal behavior. I suspect these people may find the small skull size more significant than the behavior, or they have some other driving motivation for discounting it. Not everyone wants to find evidence of human intelligence and care for the dead in fossils so old or in brain sizes so small, and not everyone likes Lee Berger.

To quote a piece on PBS:

[William] Jungers, [chair of the department of anatomical sciences at Stony Brook University], doesn't dispute that the H. naledi bones belong in the genus Homo and were likely deposited deliberately, but he cautions against "trying to argue for complex social organization and symbolic behaviors." There may be a simple answer. "Dumping conspecifics down a hole may be better than letting them decay around you." He suggests it's possible that there was once another, easier, way to access the chamber where the bones were found. Until scientists can know the approximate age of the Homo naledi fossils, Jungers says they are "more curiosities than game changers. Intentional corpse disposal is a nice sound bite, but more spin than substance."

Jungers is more dismissive of Berger's suggestion that we may have inherited the practice of burying our dead from H. naledi, a creature with a much smaller brain than modern humans. "That's crazy speculation -- the suggestion that modern humans learned anything from these pin heads is funny."

So back to the small skulls. The two opposing views must argue that either (a) behavior does not correlate with intelligence because skull size trumps everything, or (b) behavior correlates with intelligence and skull size doesn't matter. In addition, behavior and intelligence trump other differences in morphology. True human status is assigned regardless of shoulder and pelvis shape, or the as yet undetermined age of the fossils.

Even the way H .naledi is described by science writers reflects a certain bias. The disagreement is not due to scanty evidence but rather to interpretation. Some emphasize that the hands are more chimp-like in their fingers, and that the shoulders appear to be suited for climbing. Thus H. naledi spent time in trees, making her more australopith-like. Others emphasize that limbs and feet appear to be mostly like ours, indicating long distance walking ability, and thus assign her status as Homo.

In fact there is a dispute about whether the find represents one species or two. Berger and his coauthors claim the find represents one species. But others disagree. Jeffrey Schwarz, professor of biological anthropology at the University of Pittsburgh, says this about Homo naledi and the subject of classification and preconceived notions:

...How do paleoanthropologists decide if a specimen belongs to a species - whether newly or already named - and if that species is a member of Homo? As the case of Homo habilis illustrates, it is primarily by chronology, not detailed morphology...

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.... 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.

What to do? As I recently advocated in the journal Science, it's about time paleoanthropologists acknowledged what a taxonomic and undefinable mess the genus Homo has become, and restudy the human fossil record without preconceived notions and the historical weight of overly used names.

In the interests of fairness, as far as I know neither Junger or Schwarz has examined the actual fossils first hand. So to give Berger and his extensive list of coauthors their chance to speak (Berger has welcomed the collaboration of many experts in their fields) I quote from their paper defining the species:

In addition to general morphological homogeneity including cranial shape, distinctive morphological configurations of all the recovered first metacarpals, femora, molars, lower premolars and lower canines, are identical in both surface-collected and excavated specimens ....These include traits not found in any other hominin species yet described. These considerations strongly indicate that this material represents a single species, and not a commingled assemblage....

The collection is morphologically homogeneous in all duplicated elements, except for those anatomical features that normally reflect body size or sex differences in other primate taxa.

This is a very interesting fossil find. I am sure what it means will be argued over extensively in years to come, as more data is collected and analyzed. However, the undisputed bias will undoubtedly be that the evolutionary story of our common ancestry with chimps is true, regardless of where H. naledi is assigned in the story.

Image credit: Lee Roger Berger research team (http://elifesciences.org/content/4/e09560) [CC BY 4.0], via Wikimedia Commons.