Here we have (a cast of) one of the most famous fossils in the world, designated HMN 1880 (or MB. Av. 101, depending on who you ask). It is a single slab of limestone containing around 150 bones. If one were inclined to be particularly reductive, she might say that the fossil is a collection of billions of billions of billions of atoms. By what logic can we claim that this collection of various other things is itself a single thing?

Answer that question and you're doing metaphysics. The field is sometimes derided as "learning how to count," but let's not so easily dismiss the difficulties of counting. (Heck, western culture didn't get it right by current standards until the thirteenth century.) Just getting up to one begs a few philosophical questions.

In his excellent book on the subject, Gracia (1988) summarizes the definition of individuality implicitly shared by most contemporary philosophers:

[Individuals] (a) lose their fundamental character if they are divided into parts; (b) are distinct from all other entities, even from those that share some features from them; (c) are part of a group-type or class which has or can have several members; (d) can remain fundamentally the same through time and various changes; and (e) are the subject of predication but not predicated of other things. (28)

Applying these criteria to HMN 1880, the fossil would qualify as an individual object if it (a) would cease to be if some maniac smashed it to pieces; (b) is distinct in time and space from all other individual objects; (c) shares various similarities with other fossils; (d) remains the same fossil despite a number of preparations over time; and (e) is something different from its many copies. If all of these points are true, then the fossil would qualify as an individual--it would be part of what metaphysicists call "the furniture of the world."

Individuality (in biology)

Our concern here isn't for the furniture of the world, though; our concern is for the furniture of the biology department (proverbially, at least). The debate over those furnishings started long before the current imbroglio among philosophers of biology.

In the mid-nineteenth century, Thomas Henry Huxley, Louis Agassiz, and Ernst Haeckel debated whether siphonophores (such as the Portuguese man-of-war) should be considered individuals or colonies (see Gould 1984). The focus of their dispute was life cycles. Various parts of a siphonophore seem capable of survival and reproduction when separated from the whole. Agassiz took this as evidence that the parts of a siphonophore satisfy Gracia's criteria (a), (b), (c), and (d); Huxley disagreed. In this way, the disputants were essentially having the same debate that philosophers did. The only particularly biological aspect was the choice of example.

As is so often the case in biology, Darwin changed things with the Origin (1859). In chapter VII of that work, he argued that social insect colonies (in particular, bee hives) should be considered biological individuals. His reasoning was driven not by metaphysics, but by the needs of his theory. Species evolve by natural selection when individuals within the species differentially reproduce and pass on their variable traits. Sterile insect castes therefore present a problem: sterility ought to be bred out of a species because it's a trait that, by definition, can't be passed on to the next generation. Darwin resolved this problem by noting that sterility occurs only among social insects that live in well-integrated colonies; perhaps, then, colonies could be considered the relevant individuals (1859, 235). Unlike Agassiz or Huxley, Darwin added a uniquely biological criterion to his definition of individuality. Biological individuals in his account satisfied Gracia's (a) through (e), but also participated in the natural selection process.

When philosophers of biology now debate individuality, they do so in Darwin's context. Their goal is to identify the so-called "units of selection" (or "replicators" in some accounts; see Godfrey-Smith 2009). Richard Dawkins popularized one side of this debate in "The Selfish Gene": following Williams (1966), Dawkins and other genetic reductionists argue that all biological traits, including social insect sterility, are the result of the differential propagation of individual genes. On another side of the debate, multilevel emergentists argue that some traits--for example, cancer susceptibility or altruistic childcare--can only evolve through selection of cell lines, organisms, or well-integrated groups of organisms (see also Okasha 2006). The question at hand is whether there is only one kind of furnishing in the biological world or several.

This is why Oregon's humongous fungus qualifies as the world's largest organism: every part of the enormous network shares the same genome, and so the network would qualify as a single biological thing any which way the debate over biological individuality gets resolved. In fact, the biological individuality of something like a whale or a sauropod is even more controversial than the individuality of the fungal network: some philosophers of biology dispute the biological individuality of organisms, but relatively few dispute that genes are biological individuals. The fungus didn't cheat to win the "biggest organism" competition; it just played by a set of rules with which few of us are familiar.

Individuality (in paleontology)

To return to an earlier metaphor and another example: a cast of HMN 1880 is one of the furnishings of my office, hanging beside a bookcase full of dinosaur models and toys. I keep the cast as a material representation of all my professional commitments--to the study of evolutionary theory; to the appreciation of nature's truths; to the love of paleontology and its philosophical background. But it must be noted that my office is located in a building situated across Linfield's quad from our biology department. We might conceive paleontology as one among the various life sciences--a point raised several weeks ago in Joyce's great review of Martin Rudwick's "The Meaning of Fossils"--but paleontologists don't refine the concept of individuality in the same way that biologists do.

Darwin likened the fossil record to 'a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines' (1859, 310-311). One point raised here is paleontology's resolution problem. The vast majority of organisms--and their various interactions--have been erased from the historical record. Because of the fossil record's poor resolution, paleontologists have developed theories about large-scale trends visible in the fossil record. These trends might be explained by species-level differences, suggesting that species participate in a sorting process similar to natural selection. If so, then species would qualify as biological individuals--and paleontologists would share some of the same kinds of furniture with biologists.

As shown in the debate between Joyce and Derek on this very blog, however, species selection remains controversial among neontological biologists. Among the many reasons for the controversy is that species, unlike genes or organisms, don't meet biologists' criteria for individuality--in fact, they may not even meet Gracia's broader criteria. Criterion (d) is unfulfilled if species evolve and give rise to new species; criterion (e) is unfulfilled if each organism in a species is somehow representative of the species.

Perhaps fossils are more properly the furniture of paleontology (if nothing else, fossils adorn the offices of many paleontologists). Paleontological theories depend on individual fossils: for example, placement of the group Avialae within the group Archosauria turns on evidence from individual specimens such as HMN 1880. Insofar as fossil specimens represent fossil species, theories of species sorting also refer to individual fossils. Whatever the case, individual fossils seem to be the individual things that paleontologists talk about.

Fossils most certainly do not satisfy the biological criterion of individuality. As I've argued before, there is a significant metaphysical distinction between organisms and their fossils. No fossil per se participates in any kind of natural selection process: fossils neither replicate nor interact, and so fossils do not differentially transmit their traits to subsequent generations of fossils. Fossils are geologically manifested traces of dead organisms. This criterion refines the philosophical standard of individuality, but it is not a refinement of the biological criterion. Paleontological individuality is therefore an alternative to, rather than a subset of, biological individuality.

Towards a Biological Theory of Paleontology

Prothero (2009) argues that paleontology has not yet taken its proper seat at biology's "high table." This is a point that has recurred throughout my essays here. The considerations given above may go some way towards explaining this disconnect between the one and the other: paleontological furniture is different from neontological furniture, and so the paleontologist's chair doesn't fit neatly at the biologists' table.

One might argue that I've missed two important points: organisms that could qualify as biological individuals are causally responsible for the fossils paleontologists study, and sometimes paleontologists theorize about those organisms. For example, HMN 1880 is an important data point in attempts to resolve whether or not Archaeopteryx was capable of powered flight. Fossils may not qualify as biological individuals per se, but they give us plenty of information about things like avialans and whales and fungi.

And this is true. Paleontologists do draw conclusions about organisms from fossils--even though the former are biological individuals and the latter are not--and this is why we have paleobiological theories. But it's also important to recognize that paleobiology functions as a sort of inter-theoretical bridge between paleontology and biology. Neontologists develop biomechanical models of powered flight in birds; paleobiologists model how the biomechanical features leave geological traces; paleontologists verify the presence or absence of those traces. When paleontologists theorize about organisms, they do so by crossing the paleobiological bridge.

But in order for an inter-theoretical bridge to work, it must make connections between the concepts of the theory on the one side and the concepts of the theory on the other. Paleontologists can only cross the paleobiological bridge if that bridge successfully provides some means of connecting the paleontological individual with biological individuals. Part of that success comes from recognizing where lies the gap between the two theories. So maybe paleontology's chair can fit at biology's table--but only if conceptual analysis can first show why the two didn't initially fit together.

References