Sunday, December 24, 2023

Critical Hominin Theory

[Note: Both reviewers recommended publication of the following essay for Paleoanthropology, but it has not yet been accepted. If you like it and want to cite it, go ahead and cite this page. I'll let you know if it ever gets officially published.]

Introduction: Physical anthropology as pseudo-taxonomy

At one time, the field of physical anthropology was occupied primarily with establishing the number and natures of the elementary natural units of the human species. Radical biologists, like Ernst Haeckel (1868) , W. C. Osman Hill (1940) and Reginald R. Ruggles Gates (1944) even held the different varieties of living peoples to be themselves species, rejecting the familiar interbreeding criterion. Nevertheless, once that question was resolved to everyone’s satisfaction, in the early 1960s, (e.g., Barnicot 1963), the question still remained about how to understand the elementary units of the human species taxonomically below the species level. 

In Nature, Campbell (1962) formally identified H. sapiens sapiens, H. sapiens afer, H. sapiens asiaticus, H. sapiens americanus, H. sapiens australasicus, and H. sapiens neptunianus. Concurrently, based on serological genetic considerations, Boyd (1963) proposed thirteen human “races,” clustered into seven “groups” in Science.  Garn (1961) identified nine “geographical races” and 32 “local races”. Coon (1962) identified five subspecies of living Homo sapiens (ignoring the additional nonsense about their having evolved in parallel from five subspecies of Homo erectus, and its possible bearing on school integration), namely: Caucasoids, Mongoloids, Negroids, Capoids, and Australoids.

In the intervening decades, we stopped thinking about and teaching about human variation that way in physical anthropology.  We have learned to talk about human variation without formal biological taxonomy. The actual empirical biological structure of our species is regarded as constituting a widespread network of more or less interrelated, ecologically adapted and functional entities” (Weiner 1957: 80), or in contemporary language, a “structured metapopulation” that defies taxonomic precision.  The groups that compose our species are created for particular purposes, and while they may correlate with some biological patterns, they do not represent natural divisions, but political identities. Classifying our species as Linnaeus did turned out to be a square peg – round hole problem (Livingstone 1962). Race can thus be modeled on the classic anthropological example of African witchcraft (Evans-Pritchard 1937; Fields and Fields 2012), which structures people’s lives in spite of not having any material, naturalistic existence.

Indeed, partly as a result of the historical baggage associated with treating the human species in such a pseudo-taxonomic fashion, we terminated physical anthropology altogether and rebranded ourselves formally as biological anthropology. Yet even as biological anthropology, we retain the urge to try and make sense of the ostensible lineages in our ancestry taxonomically (e.g., Wood and Boyle, 2016; Reed et al., 2023), a daunting task that even Linnaeus himself never faced back in the 18th century.

A popular science bestseller, Harari’s (2014) Sapiens, told readers on its back cover that “[o]ne hundred thousand years ago, at least six different species of humans inhabited Earth”. That, of course, might indeed be true. What are the six species he identified 100,000 years ago? One, Homo sapiens, modern people. Fair enough, if a bit on the robust side. Two, Homo neanderthalensis. Indeed, according to the best genetic data thirty years ago (Krings et al. 1997); but now the geneticists say I may have 2% Neanderthal DNA, which presumably changes the status of Neanderthals, or the status of species, or both (Harvati and Ackermann 2022; Weasel 2022). Three, Homo denisova. Yet even the geneticists, who are the only people to whom it is visible, say this is a genomic subgroup of Neanderthals (Meyer et al., 2012) – who, as just noted, may not even be their own species in the first place. Four, Homo erectus. That is largely uncontroversial as a taxon; but not at 100,000 years ago, unless perhaps we regard species #6 as H. erectus. Five, Homo floresiensis. Clearly, the mysterious and isolated H. floresiensis was something (Madison 2023). And six, Homo soloensis, named for a different set of Indonesian fossils than H. floresiensis, which are generally regarded as archaic H. sapiens (Swisher et al., 1996) on account of their anatomical continuity with both H. erectus and H. sapiens.

All of a sudden, we are down from “at least six different species” to humans, the hobbit, and possibly Neanderthals. This is not to single out Harari for criticism, but rather to note that biological anthropologists know that all of these species are vexatious at best (e.g., Athreya and Hopkins 2021; Bae et al., 2023), while outside of biological anthropology, the nuanced winks that generally accompany these taxa tend to get lost.

The problem is not a new one. Reviewing mammalian systematics in 1945, G. G. Simpson was frustrated by the difference he encountered between paleontological and paleoanthropological taxonomy. Since human are mammals, it stands to reason that an expert on mammalian species should be able to make sense of extinct hominin[1] species. Like any other biological taxonomic enterprise, there is a proper taxonomic scheme, reflecting a proper understanding of the fossil species in that particular evolutionary lineage. It’s simply a matter of finding it. Some people see too few species (lumpers), and some people see too many species (splitters), while a competent paleontologist should produce results that are as Goldilocks found the baby bear’s porridge: just right.

The problem, however, is not simply that everybody fancies themselves to be the baby bear; but rather, lies more fundamentally in the assumption that the elementary units in paleoanthropology and the elementary units of paleontology are equivalent. I think it is time to call that assumption into question. The units of paleontology, and of biology more generally, are different from the units of paleoanthropology, in that the latter are units in a story of our ancestors, and the ancestors are invariably sacred.

What are primate species?

A species, like a culture or a gene, is infamously difficult to define precisely and satisfactorily. Species appear to be units of nature of some sort, but different kinds of units than species among plants or bacteria (Godfrey and Marks 1991; Barraclough 2019). Nevertheless, for multicellular animals at least, it appears to be an elemental unit of ecology, as a cell is an elemental unit of physiology, a genotype is an elemental unit of a population genetics, and an organism is an elemental unit of society. If we restrict ourselves just to the more familiar mammalian and primate species, we can ideally identify several properties possessed by a species: (1) it is composed of organisms related to one another as either potential mates or competitors for mates, thus bounded by the limits of a gene pool; (2) it constructs and is adapted to a niche, filling a role in a dynamic ecosystem; (3) it has a locus in space and a duration in time as an evolutionary lineage; and (4) it can replicate, making more species, each slightly different from itself. Different definitions of species, from Buffon (1753; Farber 1973) through Mayr (1942) and beyond, highlight one or another of these features.[2]

A more recently-appreciated feature of species is that they are also units of conservation legislation, and thus partly cultural as well. That is why the number of extant primate species has risen so dramatically in recent decades, from about 170 in the mid-1980s (Richard 1985) to over 500 today (Strier 2022). It is not simply that the number of species is being taxonomically inflated, and is thus less accurate than it used to be (Rosenberger 2012); or that new species are finally being recognized, and their number is thus more accurate than it used to be (Groves 2014). In a sense, comparing the number of living primate species 35 years ago and today is simply unfair, because they are actually tabulating different things than they used to.

 However entropic the universe of extant primate species seems to have become, it is disconnected from extinct primate species, which are of course unaffected by conservation legislation. They nevertheless have their own issues, and their status is constantly being negotiated by the community of primate paleontologists. Perhaps unsurprisingly, the closer we get to ourselves, the more complex things seem to become (Kimbel and Martin, 1993).

Hominin species seem just as vexed today as they did to Simpson back in 1945. His explanation was that the people doing the paleoanthropology were generally coming from medical anatomy, and simply didn’t understand taxonomy well enough: “much of the work on primates has been done by students who had no experience in taxonomy and who were completely incompetent to enter this field, however competent they may have been in other respects” (Simpson 1945:181). In particular, he noted, “Dart's placing of Australopithecus in a family ‘Homo-simiadae’ (1925) only served to exemplify the total ignorance of zoology so common among the special students of these higher primates (although, of course, Dart's work is excellent in his own field)” (Simpson 1945:188).

The ontology of hominin species

The underlying presupposition is that hominin biological history is composed of zoological units that are equivalent to species, however species may be defined biologically. Thus, Homo naledi, Homo longi, Homo heidelbergensis, Homo luzonensis, etc., either are or are not distinct phylogenetic lineages, and only a proper taxonomy will resolve and describe it.
Suppose, however, that there are no recoverable biological species in the history of our lineage, perhaps because much of that history is occupied biologically by a structured meta-population rather than by distinct species (Pääbo 2015; Scerri et al. 2019). Perhaps because there is more at stake in identifying our ancestors than there is in identifying the ancestors of other species. Perhaps because the names themselves represent not so much biological ancestors, as anthropological ancestors.
What do I mean by an anthropological ancestor? I mean the subject of a sacred narrative about our past, a story intended to explain who we are and how we came to be here. The diverse manners by which peoples construct relations of (horizontal) kinship and (vertical) ancestry was among the earliest discoveries of anthropology (Morgan 1871; Franklin & McKinnon 2001). And while we have talked for decades about looking at science as a culture (Snow 1959), we are only recently actually getting around to doing it (Franklin 1995). I want to suggest that the reason that hominin taxonomy is, and has always been, so vexed, is because its elements are not biological species. The species that compose our own lineage are mythic ancestors, which are intended to resemble biological species. But they are actually elements in a story, the bricolage (Lévi-Strauss 1962) out of which scientific narratives of our origins (Landau 1985) are constructed.
In one story, the actor in a creation story might be Mother Corn Spirit. In another, it might be Homo luzonensis. In still another, it might be Homo soloensis. And they might have precisely the same ontological status: as nonexistent naturalistic entities in human ancestry. As some other kind of entities, namely as characters in an origin narrative, they may have a very different kind of existence.
The persistent confusion over hominin taxonomy is a result of misunderstanding the nature and existence of hominin species. It is not that there is a correct taxonomic interpretation of the fossils, which will reveal itself under the proper analytic technique. It’s that (1) this taxonomic practice is different from other ostensibly zoological taxonomic practices, by virtue of being reflexive; (2) the origin of this difference lies in the fact that it is naming and describing our own ancestors, and (3) ancestors are invariably sacred (in the broad anthropological sense of “special”, rather than in the narrower sense of “holy,” although the ancestors may of course sometimes be that too; Zerubavel 2012).
Figure 1: A notoriously sacred ancestor,
Eoanthropus dawsoni, or Piltdown Man.
Anthropologically speaking, hominin species are sacred ancestors. In some cases, this is more obvious than others. The role of Eoanthropus dawsoni as a sacred British ancestor (Figure 1), which effectively precluded its exposure as a hoax for decades, is well-known (Weiner 1955). Another notable example is Sinanthropus pekinensis. We often retell the story of the loss of the Peking Man fossils on Pearl Harbor Day. They were being transported for safe keeping to the naval base in Hawaii to protect them from the Japanese, who were very interested in taking control of the fossils (Roberts et al. 2021). We never tell a parallel story in Europe: After all, there were hominin fossils scattered around Europe in 1941, but they didn’t require special protection from the Germans. Why did Peking Man require protection from the Japanese? Presumably, because the fossils were regarded as ancestors of the Chinese (Sautman 2001; Schmalzer 2008), and thus possessed symbolic power and value to East Asians, which was quite different from the cultural meanings of European fossils.
And it wasn’t even Homo erectus at the time, but Sinanthropus pekinensis. That is what all of these names represent: not elements of biological history, but elements of stories about biological history. They are attempts to create formal biological characters and distinctive elements in order to convey a story of human origins, where there are, in fact, no such formal biological distinctions in nature.  This situation is familiar from the neontological end of physical anthropology, in the form of two centuries of misguided racial classification. I am suggesting that the problem with paleoanthropological taxonomy today is the same problem with racial taxonomy decades ago: namely, a fundamental confusion of categories, mistaking units of symbolic culture for units of biological nature.
Figure 2: An ancestor (STS-5 or Mrs. Ples)
on a postage stamp 
Some famous fossil ancestors have appeared on national postage stamps (Figure 2), an unusual location for scientific data. My point is that studying and narrating one’s own ancestry is a culturally powerful activity. The elements of a scientific story of our ancestors are species, and those species are not like other species, by virtue of being our ancestors (Walker et al., 2021). We are humans studying human ancestors; the reflexivity is built into the system. In order to break out of the loop (Huxley 1863:69), we can pretend to be space aliens (“scientific Saturnians, if you will”), but that is hardly a scientific solution to any problem.
We no longer teach the pattern of extant human variation as being taxonomically structured, because we have come to realize that the taxonomies of race did not in fact represent what they purported to: the existence of a fairly small number of fairly discrete and fairly natural clusters of people. By shifting the ontological status of race from the biological to the cultural (obviously, sometimes correlated with a bit of biology), race has become no longer a subject of formal biological taxonomy. Consequently, whether the International Code of Zoological Nomenclature recognizes Homo sapiens afer as a valid subspecies is irrelevant, for it isn’t actually a biological thing. It is simply one of the many cultural ways that humans form politically salient groups, and consequently one of the identities that are available for people to adopt in a particular time and place. 
Similar reasoning can be applied to human ancestry. We may begin to regard Homo heidelbergensis, for example, as ontologically equivalent to Homo sapiens asiaticus. That is to say, as an attempt to apply scientific taxonomic rigor where it is inapplicable, by naming something and thus willing it into existence. This is the fallacy of reification, or misplaced concreteness. There is no Homo sapiens asiaticus, although we can certainly discuss the peoples of East Asia, their gene pools, and their adaptations. We do not recognize Linnaean subspecies taxa as describing our species at present. Are the Linnaean species taxa that ostensibly describe our ancestry any realer? Or are they primarily serving to conceal the actual structure of human prehistoric biology? It is, after all, at least conceivable that the human lineage was more like a structured metapopulation than like a clade, all the way down. If that were true, then gene flow would presumably be occurring, which becomes a mechanism for the horizontal transmission of derived characters. If derived characters are transmitted horizontally by gene flow, rather than vertically by common ancestry, that would render any ostensible phylogenetic analyses untenable; in which case the goal of establishing a proper taxonomy and phylogeny for recent hominin fossils (e.g., Bae et al., 2023) would be impossible.


Figure 3: Five official kinds of living people, from 1911 

            We talk today about a species Homo sapiens, and even about a subspecies, Homo sapiens sapiens, without further formal taxonomic structure, although with real and studiable patterns of biological diversity below that level. But instead of characterizing Homo sapiens europaeus as if it were a naturalistic taxon, today we ask instead why scientists thought it existed as such for two or three centuries when it actually doesn’t (Figure 3). Instead of asking how many and what the human races are, we ask how and why they get made, and what work they do. We may look similarly at the question in paleoanthropology of how hominin species get made and manipulated – without necessarily assuming that hominin species are biological things, because that is what we think our prehistory is supposed be composed of. It may be that the quest for a proper and correct taxonomy of hominin species is itself a vain one.

The 18th century Linnaean tree of classification does not map perfectly on to the 19th century Darwinian tree of phylogeny. Why not? Because adaptive Darwinian divergence creates paraphyletic Linnaean categories – such as invertebrates (minus vertebrates), fish (minus tetrapods), reptiles (minus birds), monkeys (minus hominoids), and apes (minus humans). Consequently, 21st century cladistic classifications often have weird components, as they attempt to make all recognized taxa monophyletic (Withgott, 2000). Nevertheless, even Linnaeus was not faced with trying to incorporate extinct taxa into his work; he didn’t even believe in extinction. Moreover, Linnaeus did not know about macro-and micro-evolution when he formulated his system, and the distinction has vexed systematists ever since, since speciation is a process and not an event, and thus reproductive isolation (if that is your criterion of species) is often incomplete (Dobzhansky, 1937).
The recent paper by Meneganzin and Bernardi (2023) about Neanderthal specieshood underscores the problem. With imprecise definitions of what “human,” “Neanderthal,” and “species” mean, the Linnaean system simply breaks down here. They may well be right: Neanderthals may indeed be a “good” species, assuming that species actually mean something real and biological in this context. But I would suggest that Neanderthals are neither a “good” species nor a “bad” species; the imposition of biological species labels where the species are at best biologically ambiguous is the problem. That would be a problem anywhere on the “tree” of life; but Neanderthals have the added burden of sitting on the boundary between the human and the nonhuman, which is about as singularly symbolically charged as any place you can imagine. 
Perhaps hominin species do more harm than good at present, when it comes to understanding the bio-historical processes and patterns that got us here. Perhaps hominin species are examples of scientific pareidolia: projecting patterns like faces onto objects like clouds and tortillas. If you really want them to be there, you can always find them. The problem lies in convincing everyone else that they are there.


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[1] Although I reject the two premises that led to the adoption of “hominin” (only naming clades, and privileging our genetic intimacy with the apes over our ecological difference from them), I use the term for its hipness.

[2] I regard a species to be a group of organisms composed of potential mates and competitors for mates, and representing a temporarily stable state in a biopolitical field of evolution, ecological relations, capitalism, conservation science, and governmental action.

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