One of the more difficult conceptual problems the layperson has with biology lies in the simple word "primitive". It has many antonyms - "modern", "evolved" and "derived", and like many biological uses of ordinary words, everybody thinks they understand it, and doesn't.
It is a word from the Latin, of course, for "first fruits" or "first things of their kind", but in modern use it means "simple" or "undeveloped". And this is not - quite - what it means in biology.
It got its modern sense via the early 19th century social philosophy of a fellow named August Comte, who thought that societies evolved through predetermined stages - the theological, the metaphysical, and the scientific (thus starting the movement known as positivism). His ideas impressed a legal professor at Cambridge, Henry Maine, who concluded the society had society had passed through definite stages, from "primitive" to "progressive"; and these ideas spread from there.
Progress was then, as now, in the air. Things could only get better, and change of society was forced to improve. No sudden reversals... only improvement. It is obvious, therefore, how such ideas (which collectively go by the name The Great Chain of Being) would be transferred over to the living world when evolution, or transmutation as it was called, got applied to life. What was true of the political world had to be true of the living world also.
This is exactly how the two founders of evolution over time applied the Great Chain - Jean Baptiste de Lamarck, and Erasmus Darwin. That's right, Erasmus, not Charles; his grandfather was one of the first people to propose evolution. For Lamarck and Erasmus Darwin, life began simple and unorganized, and got more complex and sophisticated in what they could do. Eventually, of course, we came along, and behold! We were the pinnacle of evolution, weren't we (if we happened to be white European males of property, at any rate)?
But then along came Charles Darwin, and Alfred Russel Wallace, and they had a different notion of progress. In fact, they had only a local notion of it. Things, if they got better, did so locally and relatively. And they could do badly too... they could go extinct, a possibility not acceptable under the Great Chain (the good God would never permit part of his creation to disappear). Groups of organisms, or taxa, as they came to be known in the 20th century, could become hard to find. There could be, in other words, a dearth of taxa.
It is famously understood that Darwin used a tree diagram to represent evolutionary relationships. But tree diagrams can mislead. Darwin knew this - he had himself documented barnacles that became simpler as they evolved, not more complex, and he wrote to himself in a margin of a book "never say higher or lower" (advice he disregarded, by the way). We tend to think more like Lamarck or the first Darwin and think that something earlier in the evolutionary tree was somehow "not as good" as the modern taxa in that tree. But all that is required in Darwin's scheme is that each new species is better adapted to local conditions than the older ones. There is no insistence upon transitivity - if each species is better adapted at local conditions than its predecessors, it doesn't follow that the last in a series of species is better overall than the first in the series. Each species is well-adapted to its own conditions.
In the mid 20th century, a German entomologist named Willi Hennig (1950, 1966) proposed a way to classify taxa that was deliberately founded on Darwin's scheme of evolution. He used the characters of organisms and arranged them so that the more inclusive forms could be seen as those of the ancestors. Hennig coined many words from the Greek to replace the older and confusingly ambiguous terms. Two in particular were important.
Hennig defined the ancestral character with an adjective: plesiomorphous. This Greek term means "neighboring form", because it is shared by all the neighbors on the tree. Any character than is derived from the plesiomorphy is a version of it, but is changed. These he called apomorphous, from the Greek prefix that means "away from". Apomorphies defined branches on the evolutionary tree that had, by Ockham's Razor, likely changed later than the plesiomorphies that all the rest of that tree shared.
Hennig's terms remove the implications of Comte's view of history or the Great Chain. Evolution is now seen as a change from one form shared by all the other branches except those that have forms derived from it. There is no implication that this gets the species that share that trait a little closer to God, or perfection, or white European males.
By focusing on the traits of organisms, some have felt that Hennig's approach is a reversion to the older philosophy known as typology that preceded Darwin, and in a way, that is true. But this is not necessarily a flaw. The Canadian historian of biology Mary P Winsor (2003) recently pointed out that typology was in fact very similar to the modern approach of finding examples that approach the mode of a population. She calls this the "method of exemplars" and to my mind it really does capture what taxonomists were doing before, and after, Darwin.
So, "primitive" has been effectively abandoned by most biologists (Ayala 1988, Gould 1996, Mittelstrass et al. 1997), and those who do use it, are careful to avoid implications of progress in evolution (but see Ruse 1996).
Ayala, Francisco José. 1988. Can 'progress' be defined as a biological concept? In Evolutionary progress, edited by M. H. Nitecki. Chicago: University of Chicago Press.
Gould, Stephen Jay. 1996. Full house: the spread of excellence from Plato to Darwin. New York: Harmony Books.
Hennig, Willi. 1950. Grundzeuge einer Theorie der Phylogenetischen Systematik. Berlin: Aufbau Verlag.
---. 1966. Phylogenetic systematics. Translated by D. D. Davis and R. Zangerl. Urbana: University of Illinois Press.
Mittelstrass, Jürgen, Peter McLaughlin, and A. S. V. Burgen. 1997. The idea of progress, Philosophie und Wissenschaft, transdisziplinäre Studien; Bd. 13. New York: W. de Gruyter.
Ruse, Michael. 1996. Monad to man: the concept of progress in evolutionary biology. Cambridge, Mass.: Harvard University Press.
Winsor, Mary Pickard. 2003. Non-essentialist methods in pre-Darwinian taxonomy. Biology & Philosophy 18:387-400.