Primitive (biology)
Encyclopedia
Primitive in the sense most relevant to phylogenetics
means resembling the first living things and in particular resembling them in the simple 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.
it is a difficult concept to deal with unambiguously.
, 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, prokaryote
s such as bacteria are often described as primitive because they are older in the evolutionary time scale, and are less complex than later organism
s such as eukaryote
s.
On a macroscopic scale, 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.
Though this might seem obvious, it is appropriate to remember that the most recent common ancestors 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. They would have had perhaps 200 million years of evolution behind them since the emergence of the Arthropod
a; 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.
The point is that even in the simplest linear terms, the concept primitive is a very relative one.
But that is not all. That wingless Thysanuran silverfish may not have looked much more "advanced" than the hypothetical most recent insect ancestor that it shared with the mayflies, but the 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 to make if one is to call the silverfish 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.
s 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 Rhipidistia
n 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
, Caecilian
s and legless lizard
s. The process was fully adaptive; 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 skink
s with reduced limbs.
Again, abyssal
fauna and troglobite
s, 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 hydrogenosome
s.
, 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 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.
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. Professionals in fields such as phylogenetics prefer to use terms specific to the discipline, such as symplesiomorphy
, synapomorphy
, basal, or derived. These might sound cumbersome to the layman, but their precision lends a degree of clarity and avoids the confusion 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 could claim that A is more primitive than C. 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 eukaryote
s in general.
In modern phylogeny the view of evolutionary relationships takes the form of extending branches. 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. The terms that cladists prefer are basal
; its antonym
is derived
.
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
means resembling the first living things and in particular resembling them in the simple nature of their anatomy and behaviour. For example, one might regard a flatworm
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
, which has no legs, wings, or image-forming eyes, as more primitive than a beetle, that in its more advanced morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
has all these things.
Problems in formulating definitive meanings for the term
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 phylogenyPhylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
it is a difficult concept to deal with unambiguously.
Aspects related to ancestry and adaptation
In the field of evolutionEvolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
, 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, prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
s such as bacteria are often described as primitive because they are older in the evolutionary time scale, and are less complex than later organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s such as eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s.
On a macroscopic scale, there is no substantial doubt that for example, the most recent common ancestors of the Thysanura
Thysanura
Thysanura is an order of insects, encompassing silverfish and firebrats, known for their three long caudal filaments.The families Machilidae and Meinertellidae of the jumping bristletails were once included with Thysanura....
(silverfish
Silverfish
Lepisma saccharina, frequently called silverfish, fishmoths, carpet sharks or paramites, are small, wingless insects in the order Thysanura...
etc) and the Ephemeroptera
Mayfly
Mayflies are insects which belong to the Order Ephemeroptera . They have been placed into an ancient group of insects termed the Palaeoptera, which also contains dragonflies and damselflies...
(mayflies
Mayfly
Mayflies are insects which belong to the Order Ephemeroptera . They have been placed into an ancient group of insects termed the Palaeoptera, which also contains dragonflies and damselflies...
) 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.
Though this might seem obvious, it is appropriate to remember that the most recent common ancestors 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. They would have had perhaps 200 million years of evolution behind them since the emergence of the Arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
a; 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.
The point is that even in the simplest linear terms, the concept primitive is a very relative one.
But that is not all. That wingless Thysanuran silverfish may not have looked much more "advanced" than the hypothetical most recent insect ancestor that it shared with the mayflies, but the 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 to make if one is to call the silverfish 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.
Aspects related to simplicity and complexity
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 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 some species, for that matter, just upsetting a selective balance too badly could wipe out a population. Fig waspFig wasp
Fig wasps are wasps of the family Agaonidae which pollinate figs or are otherwise associated with figs, a coevolutional relationship that has been developing for at least 80 million years...
s 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 Rhipidistia
Rhipidistia
The Rhipidistia were lobe-finned fishes that are the ancestors of the tetrapods. Taxonomists traditionally considered the Rhipidistia a subgroup of Crossopterygii that described a group of fish that lived during the Devonian consisting of the Porolepiformes and Osteolepiformes...
n 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
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
, Caecilian
Caecilian
The caecilians are an order of amphibians that superficially resemble earthworms or snakes. They mostly live hidden in the ground, making them the least familiar order of amphibians. All extant caecilians and their closest fossil relatives are grouped as the clade Apoda. They are mostly...
s and legless lizard
Legless lizard
Legless lizard may refer to any of several groups of lizards which have independently lost limbs or reduced them to the point of being of no use in locomotion. It is the common name for the family Pygopodidae, but often refers to other groups, such as limbless anguids, depending upon the region of...
s. The process was fully adaptive; 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 skink
Skink
Skinks are lizards belonging to the family Scincidae. Together with several other lizard families, including Lacertidae , they comprise the superfamily or infraorder Scincomorpha...
s with reduced limbs.
Again, abyssal
Abyssal zone
The abyssal zone is the abyssopelagic layer or pelagic zone that contains the very deep benthic communities near the bottom of oceans. "Abyss" derives from the Greek word ἄβυσσος, meaning bottomless. At depths of 4,000 to 6,000 metres , this zone remains in perpetual darkness and never receives...
fauna and troglobite
Troglobite
Troglobites are small cave-dwelling animals that have adapted to their dark surroundings. Troglobite species include spiders, insects, fish and others. They live permanently underground and cannot survive outside the cave environment. Troglobite adaptations and characteristics include a heightened...
s, 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
Commensalism
In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...
life strategies lose all sorts of functions, sometimes ending up effectively as shapeless reproductive masses, as has happened with Sacculina
Sacculina
Sacculina is a genus of barnacles that is a parasitic castrator of crabs. The adults bear no resemblance to the barnacles that cover ships and piers; they are recognised as barnacles because their larval forms are like other members of the barnacle class Cirripedia...
. However, even the scale on which such creatures have discarded adaptive functions pales in comparison to endosymbionts such as mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
and hydrogenosome
Hydrogenosome
A hydrogenosome is a membrane-enclosed organelle of some anaerobic ciliates, trichomonads and fungi. The hydrogenosomes of trichomonads produce molecular hydrogen, acetate, carbon dioxide and ATP by the combined actions of pyruvate:ferredoxin oxido-reductase, hydrogenase, acetate:succinate CoA...
s.
Aspects related to loss and gain of adaptations
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 nicheEcological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
, that is less obvious, but not hard to understand as achieving a derived state. We see such an example in say, a tick
Tick
Ticks are small arachnids in the order Ixodida, along with mites, constitute the subclass Acarina. Ticks are ectoparasites , living by hematophagy on the blood of mammals, birds, and sometimes reptiles and amphibians...
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
Symplesiomorphy
In cladistics, a symplesiomorphy or symplesiomorphic character is a trait which is shared between two or more taxa, but which is also shared with other taxa which have an earlier last common ancestor with the taxa under consideration...
, basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
, 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
Trematoda
Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic flatworms, commonly referred to as "flukes".-Taxonomy and biodiversity:...
and Sacculina
Sacculina
Sacculina is a genus of barnacles that is a parasitic castrator of crabs. The adults bear no resemblance to the barnacles that cover ships and piers; they are recognised as barnacles because their larval forms are like other members of the barnacle class Cirripedia...
, every prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
and jellyfish
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
, is of a lineage as ancient and evolved as that of any eagle
Eagle
Eagles are members of the bird family Accipitridae, and belong to several genera which are not necessarily closely related to each other. Most of the more than 60 species occur in Eurasia and Africa. Outside this area, just two species can be found in the United States and Canada, nine more in...
or ape
Ape
Apes are Old World anthropoid mammals, more specifically a clade of tailless catarrhine primates, belonging to the biological superfamily Hominoidea. The apes are native to Africa and South-east Asia, although in relatively recent times humans have spread all over the world...
. 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
Degeneration
The idea of degeneration had significant influence on science, art and politics from the 1850s to the 1950s. The social theory developed consequently from Charles Darwin's Theory of Evolution...
). 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
Generalist and specialist species
A generalist species is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources . A specialist species can only thrive in a narrow range of environmental conditions or has a limited diet. Most organisms do not all fit neatly into either...
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
John Langdon Down
John Langdon Haydon Down was a British doctor best known for his description of a relatively common genetic disorder that is now called Down syndrome.-Education:...
, for whom Down syndrome
Down syndrome
Down syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
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
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
or traits
Trait (biology)
A trait is a distinct variant of a phenotypic character of an organism that may be inherited, environmentally determined or be a combination of the two...
in terms such as primitive. The word may seem convenient, but it confuses concepts that need separate consideration. Professionals in fields such as phylogenetics prefer to use terms specific to the discipline, such as symplesiomorphy
Symplesiomorphy
In cladistics, a symplesiomorphy or symplesiomorphic character is a trait which is shared between two or more taxa, but which is also shared with other taxa which have an earlier last common ancestor with the taxa under consideration...
, synapomorphy
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
, basal, or derived. These might sound cumbersome to the layman, but their precision lends a degree of clarity and avoids the confusion 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 could claim that A is more primitive than C. More recent or complex organisms are not automatically meaningfully superior to older, simpler organisms. For example, some archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, forms of prokaryotic organisms, are able to survive efficiently in a much broader range of extreme environments than can "advanced" humans, or than eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s in general.
In modern phylogeny the view of evolutionary relationships takes the form of extending branches. 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. The terms that cladists prefer are basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
; its antonym
Antonym
In lexical semantics, opposites are words that lie in an inherently incompatible binary relationship as in the opposite pairs male : female, long : short, up : down, and precede : follow. The notion of incompatibility here refers to the fact that one word in an opposite pair entails that it is not...
is derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
.