Classification: Lumping & Splitting

In creating categories, there is a major intellectual divide between two, conflicting, orientations:

Orientation 1 (Lumping): Two things are in the same category unless there is some convincing reason to divide them.
Orientation 2 (Splitting): Two things are in different categories unless there is some convincing reason to unite them.

For most people most of the time, these two different orientations do not produce any significant conflicts. Apples and oranges and bananas are all fruit, and go into the fruit category. Or apples and oranges are round, while bananas are long, and so apples and oranges go into the round category while bananas go into the long one. Oranges and lemons and melons are all classed as fruit, but they readily break down into citrus fruit as against melons in other contexts. And so forth and so on.

In the study of only partially understood fossil animals, however, it is often difficult to decide how things should be classified, and the terms "lumpers" and "splitters" are frequently used to refer to people with different inclinations.

For example, the following picture shows an exhibit from the Museum of Man in San Diego's Balboa Park. The exhibit consists of casts of five fossil skulls, all classified into the genus Homo, all between one and a half and two million years old, and all from East Africa. The museum visitor is asked to decide how many species they represent. (Color makes no difference; that is a product of soil conditions. And the places that are restored make no difference: they are plaster filler where pieces of bone are missing.)

The answer is that experts disagree. Nearly all specialists would classify the two skulls on the left as "Homo habilis" and the two on the right as Homo ergaster (erectus). But what about the middle one?

For some specialists the middle one is different enough from the others that it gets a separate species name. (In this case, Homo rudolfensis.) For other specialists, it is not enough different from the two on the left to be called a separate species, and they lump it together with the left ones as Homo habilis. In the exhibit, and to the casual visitor, it actually looks more like the one to the right of it, making it Homo erectus. (The museum leaves visitors to decide for themselves.)

What is a species?

As a general rule of thumb, animals are considered to be of the same species if a mating between a fertile male and a fertile female can produce fertile offspring. And animals are considered to be of different species if that cannot occur. (This is why dogs are all of the same species despite their enormous differences in form, but horses and cows are of different species despite apparent similarities.)

So could the animal represented by skull number 3 have produced fertile offspring by mating with the animal represented by skull number 1 or number 2? If the answer is yes, then number 3 is Homo habilis. If not, then it is something else (tentatively named Homo rudolfensis). The fact is, of course, that we cannot actually know. We can study the skull in very close detail and make a best guess. But having done that, we often still don't really know.

So which is the best guess? Homo habilis, or Homo rudolfensis?

There is absolutely no way to be sure. And specialists are about equally divided. Those who default to Orientation 1 (the same unless obviously different) say Homo habilis. Those who default to Orientation 2 (different unless obviously the same) say Homo rudolfensis. And the rest of us just get confused.

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There is more implied by the distinction than just an intellectual prejudice about what is convenient. A very important issue in this is the conception of the length of time (and the conditions) necessary for two different groups of the same species to accumulate enough mutations that they can no longer mate. In other words, there is a difference in the understanding of speciation, the process by which accumulated genetic mutations become so numerous as to interfere with successful mating. Since we are talking about poorly known prehistoric populations of extinct animals, speciation is almost impossible to understand properly.

Lumpers tend to assume that separation of animal populations is rarely complete, that mutations do not occur often, and that the process of speciation is therefore not fast. The best guess is that members of two closely similar populations could mate just fine, given the opportunity to do so. That is why the best assumption is that similar forms are probably best classed in one category until there is some reason to do otherwise.
Splitters tend to assume that speciation occurs relatively quickly when breeding populations are separated, that separation is usually quite complete, and that therefore speciation is quite a rapid process.. Since speciation obviously results in different species, we should assume that there are and have always been lots of species out there, the vast majority of which are, of course, long extinct.

The important point here is that no two specialists balance lumping and splitting quite the same way, but it makes a huge difference to the terminology that they use, and to the historical possibilities they think about.

Lumping tends to reduce the number of genus and species terms. Non-specialists like lumping for this reason. Most popular authors tend to be lumpers. It makes for easier reading and avoids celebrating potentially trivial differences or needlessly multiplying names. Most popular authors therefore speak of Homo erectus, not about Asian forms being called Homo erectus but African forms being called Homo ergaster.
Splitting tends to increase the number of genus and species names, at least in the short term until they are clearly shown to be redundant. The Museum of Man curators, unlike many popular authors, label their African "Homo erectus" specimens as Homo ergaster, reflecting their sense that, on the whole, splitting is intellectually safer than lumping.

Often a new specimen is assigned to a new species pending better classification later. Thus the fossils (of several people) collectively referred to as Peking Man when originally found were classified initially as Sinanthropus pekinensis (Latin for "Chinese Person From Beijing"), although they were eventually classified with many other finds as examples of Homo erectus. Nobody imagines that there is a distinctive, non-human genus called Sinanthropus any more, and the term has vanished from all respectable sources (although not from disreputable ones).

(Caution: Nobody lumps or splits all the time. That would be stupid. The terms "lumper" and "splitter" refer to only to the default assumptions applied in cases that are ambiguous. I have heard of exam questions being designed with this in mind. Ahem.)

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