Primitive in the sense most relevant to phylogenetics means resembling evolutionary ancestors of living things and in particular resembling them in the nature of their anatomy and behaviour. For example, one might regard a flatworm, which has no legs, wings, or image-forming eyes, as more primitive than a beetle, that in its more advanced morphology has all these things. The term "primitive" might suggest simplicity, but it does not imply it; many "advanced" organisms have lost complex structures that were present in some of their ancestral forms. For example, the jaws of mammals are simpler than those of their ancestral fishes, in that they include fewer bones and usually fewer teeth.
Problems in formulating definitive meanings for the term
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Depending on context and the discipline under discussion, the term primitive has several meanings, and they are not clearly distinct. Even in the field of phylogeny it is a difficult concept to deal with unambiguously.
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.
Again, there is the vexed question of how to assess stabilising selection as contributing to advanced status; empirically we can see very little reason to regard say, a simple-looking living Onychophoran as anything but primitive, when it is hardly distinguishable from a fossil half a billion years old, but that resemblance is no coincidence; its stable anatomy endured for hundreds of millions of years during which any specimen deviating too far from the norm reproduced poorly in comparison to its more conservative contemporaries, or simply died out. In some species, for that matter, just upsetting a selective balance too badly could wipe out a population. Fig wasps provide some such examples.
Beyond apparent adaptive stasis, the concept of primitivity makes little sense in dealing with examples of obvious adaptive loss of functions that had been adaptively gained in the first place. For instance, our Rhipidistian ancestors spent perhaps tens of millions of years evolving legs, and then at various times over the next two hundred millions of years or so, some prominent, but unrelated groups among their descendants irrevocably discarded legs altogether. These included the Ophidians, Caecilians and legless lizards. The process was fully adaptive at all points; the ways of life of their terrestrial, limbed ancestors had begun to favour swimming, burrowing, or just wriggling through grass and the like, as is easy to believe when contemplating the anatomy of say, extant skinks with reduced limbs.
Again, abyssal fauna and troglobites, living in deep water or caves as they do, are notoriously prone to adaptive loss of eyes and pigments. And animals and plants pursuing parasitic or other commensal life strategies lose all sorts of functions, sometimes ending up effectively as shapeless reproductive masses, as has happened with Sacculina. However, even the scale on which such creatures have discarded adaptive functions pales in comparison to endosymbionts such as mitochondria and hydrogenosomes.
What is one to make of such gain and loss of function? When a population acquires functional or sophisticated features, it is easy to think of that as a notional move away from the primitive state; when a population remains apparently unchanged over a long period, while adapting ever more closely to an apparently unchanged ecological niche, that is less obvious, but not hard to understand as achieving a derived state. We see such an example in say, a tick or an Onychophoran.
However, it is harder to see actual, often irrevocable, loss of function as an advance from primitivity, and yet each apparently destructive change of that type occurs through adaptation by exactly the same selective mechanisms as the development of new functions. The difficulty of reconciling such assorted concepts in a single word is a good argument for preferring phylogenetic terms such as ancestral, basal, and derived states, to the likes of primitive and advanced, which irrelevantly, even misleadingly, might suggest a scale of inferiority or superiority.
One way or another, the concept of primitivity as applied to extant organisms is meaningful only while one remembers an important point: every liver fluke and Sacculina, every prokaryote and jellyfish, is of a lineage as ancient and adaptively evolved as that of any eagle or ape. Many popular works on evolution have described adaptive losses of function as degeneration (not to be confused with a completely different modern use of the word degeneration). However, even from the founding years of the field of study, early evolutionists authored remarkably sophisticated lines of thought; already in the mid-19th century some were arguing that the loss of unused functions should be regarded as specialisation rather than degeneracy; a move towards a derived, rather than a primitive state.
Only the more naively moralistic authors made much use of terms such as degeneracy or reversion to more primitive states. For instance the writings of John Langdon Down, for whom Down syndrome is named, actually asserted that degeneracy in human intellects directly amounted to reversion to the primitive level of primitive human races.
Modern usage and views
In the light of the foregoing examples, it should be clear why there has long been a tendency to avoid description of particular species or traits in terms such as primitive. The word may seem convenient, but it confuses concepts that need separate consideration. The word "primitive", in particular, makes no sense applied to modern species, clades, or taxa: all modern species are a mix of plesiomorphic ("primitive") and apomorphic ("advanced") characters. Professionals in fields such as phylogenetics prefer to use terms specific to the discipline. For example, the prominent discipline of cladistics uses terms such as symplesiomorphy and synapomorphy for particular aspects of basal and derived attributes. That might sound cumbersome to the layman, but their precision lends a degree of clarity and avoids the confusion and associated implications of concepts such as primitivity.
Evolutionary biologists hardly use the term primitive; many current textbooks do not mention it at all. The term primitive has been so frequent among popular writings however, that although it does occur in older technical books, it is surprisingly rare; the influential figures of the late 19th century had quickly recognised the associated pitfalls. Even Darwin and T. H. Huxley hardly used it at all.
Possibly popular writings have been the main reason of the prominence of the word; Doyle used it freely in "The Lost World" for example. Perhaps the implication of an evolutionary scale like a "ladder" in which each new addition is superior than organisms in the lower rungs, appeals to the popular imagination.
To the qualified biologist, such implications, far from being attractive, are an unacceptable nuisance; even if the idea of evolutionary superiority were of general applicability, it simply is not transitive. In other words, if it were possible to claim that A is more primitive than B, and B than C, it would not follow that one could claim that A is in all senses more primitive than C. In particular more recent or complex organisms are not automatically meaningfully superior to older, simpler organisms. For example, some archaea, forms of prokaryotic organisms, are able to survive efficiently in a much broader range of extreme environments than can "advanced" humans, or than eukaryotes in general.
In modern phylogeny the predominant view of evolutionary relationships takes the form of extending branches (with occasional exceptions arising from hybridisation or other forms of union between genetically separate populations). Instead of having the evolutionary system as a division between higher (superior) and lower (inferior) organisms, each branch extends outwards to represent temporal and developmental distance. Some terms that cladists prefer are respectively basal or plesiomorphic, and their antonyms, derived, or apomorphic.