Let us begin by establishing the following facts as true:
– I am not a monkey’s uncle, yet, in a certain way, I am.
– Birds are indeed dinosaurs, yet, are indubitably not.
Knowledge is knowing that a bird is a dinosaur. Wisdom is not charging people extra to see your reconstituted Jurassic Park style dinosaur zoo when all you’ve got is a barn full of chickens.
Humans have always classified organisms in a variety of ways, depending on need. Centuries ago, the Linnaean system emerged, for reasons of Linneaus’s own, and that system eventually became the scientifically correct way to name things. Not so much to classify things, but to name them, but along with this naming system came a natural system of classification that scientists found very useful.
Then one day, a man named Willi Hennig hit apon a very important idea. (Use a German accent when you say these words to yourself, for effect.) No system of klassification kan be korrect unless it perfectly matches the underlying evolutionary reality. And with that, Willi invented Kladistics, which we now call Cladistics.
Cladistics had two main features. One, it requires that organisms be placed in relation to each other by means of characters (features) that were evolutionarily meaningful and that properly sorted out relationships. There are important details within that dictum that we will ignore here, but this has to do with knowing when a characteristic doesn’t really tell you about the relative position of species on a “tree of life” vs when the characteristic works for that purpose.
The second feature, which emerged from the first, is that no three (or more) organisms can share an ancestral node. In other words, you can’t say something like, “humans, gorillas, chimps, and bonobos are all in the same family and equally related to each other.” Rather, you’d have to say, “chimps and bonobos share a common ancestor to the exclusion of the others, that clade and humans share a common ancestor to the exclusion of the others, and that clade and gorillas share a common ancestor bla bla bla.”
In the old Linnaean system, there were entities such as “genus” or “family” or “order” that included organisms that seemed to share a set of common features, and that could be treated as meaningful. For example, you could say, “among mammals, only three families include species with a meaningful degree of root eating” or “within the family that includes lemmings and voles, there is a mixture of monogamous and polygynous species” and so on.
That was useful, and it is still useful, even if there are often major exceptions. Sometimes the exceptions are spectacular. For example, “the Ungulata include hooved animals with multi-chambered stomachs, except the whales.”
So, here’s the thing. Some sort of ancient fish-with-feet is the ancestors of humans. There were probably hundreds, maybe thousands, of species of fish-with-feet, and one population gave rise to the so-called “tetrapods,” animals with skeletons, arms, and legs, including modern snakes and living gorillas. Therefore, humans are fish with feet, right?
Yes. According to cladists, every organism is a member of any group of organisms that includes it. And that makes total sense and is a great thing to know. For example, it would make a huge amount of sense for suspensory feeding monkeys to have six, not four, limbs. But they can’t because the first tetrapod that emerged from the fish-with-feet group happen to have four limbs, and all we tetrapods are stuck with that feature. Humans who reproduce later in life risk genetic problems in the offspring. Why? Because the population of early mammals that gave rise to all the living mammals did not live long, and for some reason, the female gametes were all formed at once, even before the female was born, most likely. That was probably a great strategy for rapid reproduction … live very fast and die very young. That strategy determined the fact that all mammals would have the risk of somatic mutations that accumulate over time affecting their eggs. This is a key contributing reason for the existence of, and nature of, modern human grandmothers. Think about it.
But here’s the thing. When cladistics started to become popular, in the late 1970s and through the 1980s, the method proved to be a very powerful way to sort out classifications and evolutionary history among sexy organisms such as birds and hominins. In fact, much of the key early cladistic work was done on these two groups, and this was happening at the same time, roughly, as the early use of DNA analysis for the same purpose.
Since cladistics arrived on the scene as a sort of super-hero methodology, saving the day, solving previously intractable problems, it soon took on a somewhat cultish tone. People were interrogated. “Are you a kladist?” (Again, use the German accent whenever I throw in the letter ‘k.’) “Are you a kladist, or are you a pheneticist?” Not only did you need to “accept” cladistics (as though anyone wouldn’t) but you had to swear fealty to it, to the point of fully rejecting all other ways of putting animals in evolutionary, ecologically, or adaptively relevant groupings.
Somewhere in there someone noticed that since birds emerged from a therapod dinosaur, birds were therefore, according to cladistics, dinosaurs, and since kladistics is truth and all other ways of seeing the world are heresy, birds are dinosaurs and dinosaurs therefore never went extinct.
But since all living organims descend from a single ancient forms, which I hereby dub The Primanism (the first organism), and we are all therefore primaniams, no organisms have every gone extinct.
But it doesn’t really work that way, does it?
Here is how I like to think about it. Cladistics is not a way of looking at groups of organisms. Indeed, cladistics really only allows you to observe two types of groups of organisms. One: the dyads, the pairs, the sets of two that share a common ancestor. Two: all the organisms together. There is not really an in-between. Not one that is very useful, anyway.
Rather, cladistics is a way of looking at the process of evolution of species. Groupings are static, and as such are often useuful. The rules of groupings are dynamic. E. coli is a single species with a vast number of varieties, and the differences among them are enormously important. Leeches form a subclass, which is a very large scale taxonomic group, within which there is precious little actual variation. A term like “hooved animals” is monophyletic and a perfectly sound adaptive group, except for the whales. A cladist would cut off their right arm before using the term “hooved animals” (ungulata) and include in it only the animals with hooves (and a few other features) to refer to an evolutionarily and adaptively valid grouping. But it is in fact a valid grouping, as long as one remembers to not include the tomatoes in the fruit salad. Er, I mean, to not include the whales when you collect data on food type, body size, and ranging patterns.
Monkey is a common, not scientific, term. People use the terms “primate” and “monkey” interchangeably, and refer to such things as lemurs as monkeys. Therefore, the best name for the primordial primate is monkey, and therefore all humans, including my niece and nephew, are monkeys. That makes me a monkey’s uncle, but it really isn’t very useful to say that. Better to call me an ape’s uncle.
Dinosaurs went extinct, and birds are amazing anyway. That’s my story and I’m sticking to it.
I sort of get where you’re coming from, we come across these sorts of issues all the time on Wikipedia. The problem lies in how one applies common/vulgar/folk taxonomy to clades. For example, what is a duck? It isn’t a clade, it’s all the Anatidae that aren’t called geese or swans or sumsuch. A duck isn’t a meaningful word, taxonomically, but its still a thing. The problem only gets worse once you start travelling back in time.
Maybe we should rename dinosaurs proto-birds. That would put the Felidae amongst the Columbidae.
So why can’t three organisms share a node? Say you have an ancient species of thingamapod. Over time geological forces divide them into three disconnected populations. Different selective forces then cause them diverge into three different species.
Who gets to claim ownership of the ancestral node?
More generally it seems to me that nodes can be so close together that mapping them may be impossible. And since they are so closely related it does not seem very important to do so anyway.
Most of these “Come@me” opinions are sketchy but I’m with you on this one. Why does everyone make a big deal of birds “being” dinosaurs and not about mammals being reptiles or all of the above being fish?
Cladistics may be more “real” but phenetics seems a whole lot more useful for real life.
You’re not wrong, but if we don’t call birds dinosaurs I have no idea how to make toddlers appropriately cautious around chickens.
Why can’t we have three lineages on a node? Excellent question, I was hoping someone would ask that.
As per the logic you lay out, you probably sort of can, but there are two reasons you can’t, one pragmatic, one pedantic.
Pragmatically, the analytical methods that have been developed don’t work with trichotomies, or at least, work better with dichotomies. When there is a trichotomy, which can happen if there is not enough resolution, standard procedures include, essentially, coin flips to decide which way to construct the node.
So, it is a lot easier to just pretend there are only two lines coming out of every node.
Pedantically, even if there were three populations that came out of one as you suggest, there still would be one that was slightly sooner or earlier, just like with twins or triplets, one was still born first.
Duncan: I think one way to look at it is this. We can use our sensible groupings when they don’t directly conflict cladistics. Our analyses that use groupings ALWAYS must account for phylogenetic effects. So, we could say that quacking is wonderful because so many birds quack, while screaming is bogus because there are only a few screamers. But then, we notice that the reason there are so many quackers is because ducks diversified and the original duck quacked, while perhaps, therefore, screaming and quacking were each single evolutionary event.
Thee non-cladistic study of groups would privilege quacks and repress screaming, totally unfair.
So if we study groups, we can do that as long as we also apply cladistic reasoning to construct the groups, understand variability in them, and make proper excuses for when our groups are off kilter (like leaving whales out of a study of hooved animals)
We could divide the therapods into two groups: Protobirdia and Wannabebirdia. Those in the prodobirdia group that went extinct before birds arose could be Couldabeenbirdia.
Wait, what, were we supposed to be joking? Nobody told me! What the heck!
We’re gonna need a bigger chicken.