Platypus and the problem with primitive.

The concept of “primitive” is one that is very often misunderstood. Properly defined, “primitive” means “more like a particular ancestor”, refers only to individual characteristics (not whole species or lineages), and is contrasted with “derived” (not “advanced” or “more evolved”). I have covered this and other misunderstandings of evolutionary concepts in various articles and I try to clarify these in my courses. But the intuitive interpretation in which one species is deemed more primitive than another is very hard to shake, including in the scientific literature.

An example:

This is the duck-billed platypus, Ornithorhynchus anatinus. It is a mammal, but as a monotreme it also lays eggs. It has fur and produces milk, but it does not have nipples. Because of these features, it is often considered a “primitive” species and people often compare features of the platypus to those of placental mammals in order to draw conclusions about the evolution of “modern” placental mammals.

It’s true that platypuses lay eggs and lack nipples. They also have a bill, can sense electric currents in the water, and (in males) have a spur that delivers venom. Does this mean that the ancestor of platypuses and placental mammals had bills and venom? No, it doesn’t. That’s because these are derived traits that evolved in the platypus lineage after the monotreme and placental mammal lineages diverged from a common ancestor.

There is no such thing as a primitive species, only primitive or derived traits. The platypus, like every species, is a mixture of the two. As such, comparing it with other living species of mammals does not automatically tell you anything about what the ancestral mammal was like.

What about the claim that the monotremes are an “older” group of mammals? It’s possible that, as a recognizable group, monotremes appeared before placentals. However, the lineage of which the modern platypus is a representative and the lineage of which humans are a representative have been evolving for exactly the same amount of time. This is simply a result of the fact that they share a common ancestor at some point in the past. The two lineages diverged from this ancestor and then began evolving separately. Some traits found in the ancestor were maintained in the platypus lineage, others in the human lineage, and in both cases there were new features that evolved that were not found in the common ancestor.

The take-home message is this. You can’t consider one species “primitive” and another “derived” and assume that any given feature in the former was found in the ancestor of the two. You need more information before you can interpret the evolutionary history of a particular characteristic.


5 comments to Platypus and the problem with primitive.

  • Why not say “ancestral” instead of “primitive”? “Primitive” sounds slightly judgmental, akin to “advanced,” “higher,” and other such terms.

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  • Friendly neighbourhood protistologist approves of this message ;-)
     
    Ancestral is better; esp. since the derived form does not have to be “better” or more “complicated”. Blindness is a derived feature in various cave-dwelling creatures; thus, the eyes of their sister lineages can be called ‘primitive’ with respect to eyelessness; perhaps ancestral does make more sense there.
     
    With regard to species, perhaps the best is to speak of sister clades, or if it’s several lineages, “early branching with respect to [blah]” to emphasise that they’re only early diverging along a certain lineage, and can be quite late diverging with respect to something else. A bit wordy to use in everyday conversation though, so we end up using ‘basal’ and ‘stem groups’, which is admittedly rather…meh.
     
    Human language didn’t really evolve with discussing phylogenies in mind, did it?

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  • Lesley F

    Hmmm, as a learning biology student studying the platypuses, I am very intrigued that they share so many traits with birds/reptiles and also so many other traits with mammals and placental mammals.  They do seem to hold a key place in the evolutionary tree and they do seem to be a ‘mix’ of features.
    How can I express that  it is a mammal with traits of both reptiles and mammals, and it seems to have many traits (not the derived ones) that do seem to tie in from its common ancestors which also gave rise to reptiles?
    …without making any incorrect inferences?  Or can I?
     
    Thanks….

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