Aspects Related To Ancestry and Adaptation
In the field of evolution, primitive, when used as a descriptive term, is at its least disputable when applied to ancient species that had not yet undergone selective adaptation that later would cause their descendants to develop functional capabilities of interest in context. For example, prokaryotes such as bacteria are often described as primitive because they resemble life forms that occurred early in the evolutionary history of the planet, and are less complex than organisms that emerged later, such as eukaryotes.
Similarly, among larger organisms, there is no substantial doubt that for example, the most recent common ancestors of the Thysanura (silverfish etc.) and the Ephemeroptera (mayflies) were wingless, and that those wingless ancestors had no winged ancestors in turn. It would be reasonable to regard those ancestors as more primitive than the mayflies at least, and the Thysanura similarly more primitive than the mayflies in that they resemble those ancestors more closely.
Though this might seem obvious, it is appropriate to remember that the most recent common ancestors of both orders (Thysanura and Ephemeroptera) themselves would definitely be insects; as such they would already be very advanced organisms with many derived traits, the products of millions of years of evolution since the first undebatable insect appeared, not to mention even earlier, still more "primitive" invertebrate ancestors. They would have had perhaps 200 million years of evolution behind them since the emergence of the Arthropoda; this is some two to four times as long as the period that has elapsed since the disappearance of the dinosaurs. That in itself might seem like a great deal of evolution, but in turn the first arthropods had something like two or three billion years of evolution preceding them.
Clearly, even in the simplest linear terms, the concept primitive is a relative one.
Furthermore, that wingless Thysanuran silverfish may not look much more "advanced" than the hypothetical most recent insect ancestor that it shared with the mayflies, but fossils do not tell how much internal physiological adaptation might have occurred in the last four hundred million years or so, during which Thysanura showed little external change. It is a large assumption that the silverfish must be more primitive than the mayfly, just because it does not produce wings and other visible changes as it matures. Not all important changes are necessarily visible, as one can verify from any textbook of comparative physiology.
Read more about this topic: Primitive (phylogenetics), Problems in Formulating Definitive Meanings For The Term
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