Articulata Hypothesis
Encyclopedia
The Articulata Hypothesis is the grouping in a higher taxon of animals with segmented bodies, consisting of Annelida and Panarthropoda
. This theory states that these groups are descended from a common segmented ancestor. The Articulata Hypothesis is an alternative to the hypothesis that ecdysis
(the shedding of outer cuticle) is a primitive character - this would place Panarthropoda in the group Ecdysozoa
.
kingdom according to structural similarities resulting from what he referred to as special “ground-plans”, which are analogous to blueprints. Each of these “grounds-plan”, he further argued, evolved separately from the others and structural similarities were due to common function and not to common ancestry. From these “ground plans”, Cuvier separated the known animal kingdom into four branches or “embranchements”: Vertebrata, Articulata
, Mullusca, and Radiata
. From this phylogenetic grouping, the articulata hypothesis was born.
The articulata hypothesis, simply stated, is the phylogenetic grouping of the phylum annelida, (which includes polychaetes, oligochaetes, and leeches), together with the phylum arthropoda, (arachnids, insects and crustaceans) into the common taxa, articulata. Cuvier grouped these diverse phyla together according the common structural feature: the segmented body plan. This hypothesis further implies that all segmented organisms have a common ancestral origin.
, DNA sequencing
, genomics
).
. While the general acceptance of the theory of evolution weakened Cuvier’s general theory of the unique “ground-plans” as the origin of modern taxa, it strengthened the articulata hypothesis by organizing annelids and arthropods into a clade
descended from a common segmented ancestor.
, advances in molecular biology
techniques lead to further data supporting the articulata hypothesis but also lead to the development of conflicting theories. Advances in confocal microscopy
technology lead to the discovery of embryonic
cleavage (embryo)
patterns, which differs between the annelids and arthropods. Annelids are all spiral cleavers, meaning that each embryonic cleavage occurs at progressive 90 degree angles with respect to the animal- vegetal axis. Arthropods, on the other hand, display a heterogeneous mix of embryonic cleavage patterns including spiral-like cleavage and radial cleavage patterns. This led researchers to two theories: The first was that the arthropods lineage must have lost the ability to spiral cleave since differentiating from the last common ancestor between annelids and arthropods. The second is that this showed similarities between annelids and mollusks who also spirally cleaves but lacks that the segmented body plan. This was not the only interpretation of this data but other hypotheses were seen to have less data or merit. Other studies such as those looking a neural patterns within the articulata clade
showed mixed patterns and thus mixed results.
The Articulata Hypothesis is the grouping in a higher taxon of animals with segmented bodies, consisting of Annelida and Panarthropoda
. This theory states that these groups are descended from a common segmented ancestor. The Articulata Hypothesis is an alternative to the hypothesis that ecdysis
(the shedding of outer cuticle) is a primitive character - this would place Panarthropoda in the group Ecdysozoa
.
kingdom according to structural similarities resulting from what he referred to as special “ground-plans”, which are analogous to blueprints. Each of these “grounds-plan”, he further argued, evolved separately from the others and structural similarities were due to common function and not to common ancestry. From these “ground plans”, Cuvier separated the known animal kingdom into four branches or “embranchements”: Vertebrata, Articulata
, Mullusca, and Radiata
. From this phylogenetic grouping, the articulata hypothesis was born.
The articulata hypothesis, simply stated, is the phylogenetic grouping of the phylum annelida, (which includes polychaetes, oligochaetes, and leeches), together with the phylum arthropoda, (arachnids, insects and crustaceans) into the common taxa, articulata. Cuvier grouped these diverse phyla together according the common structural feature: the segmented body plan. This hypothesis further implies that all segmented organisms have a common ancestral origin.
, DNA sequencing
, genomics
).
. While the general acceptance of the theory of evolution weakened Cuvier’s general theory of the unique “ground-plans” as the origin of modern taxa, it strengthened the articulata hypothesis by organizing annelids and arthropods into a clade
descended from a common segmented ancestor.
, advances in molecular biology
techniques lead to further data supporting the articulata hypothesis but also lead to the development of conflicting theories. Advances in confocal microscopy
technology lead to the discovery of embryonic
cleavage (embryo)
patterns, which differs between the annelids and arthropods. Annelids are all spiral cleavers, meaning that each embryonic cleavage occurs at progressive 90 degree angles with respect to the animal- vegetal axis. Arthropods, on the other hand, display a heterogeneous mix of embryonic cleavage patterns including spiral-like cleavage and radial cleavage patterns. This led researchers to two theories: The first was that the arthropods lineage must have lost the ability to spiral cleave since differentiating from the last common ancestor between annelids and arthropods. The second is that this showed similarities between annelids and mollusks who also spirally cleaves but lacks that the segmented body plan. This was not the only interpretation of this data but other hypotheses were seen to have less data or merit. Other studies such as those looking a neural patterns within the articulata clade
showed mixed patterns and thus mixed results.
The Articulata Hypothesis is the grouping in a higher taxon of animals with segmented bodies, consisting of Annelida and Panarthropoda
. This theory states that these groups are descended from a common segmented ancestor. The Articulata Hypothesis is an alternative to the hypothesis that ecdysis
(the shedding of outer cuticle) is a primitive character - this would place Panarthropoda in the group Ecdysozoa
.
kingdom according to structural similarities resulting from what he referred to as special “ground-plans”, which are analogous to blueprints. Each of these “grounds-plan”, he further argued, evolved separately from the others and structural similarities were due to common function and not to common ancestry. From these “ground plans”, Cuvier separated the known animal kingdom into four branches or “embranchements”: Vertebrata, Articulata
, Mullusca, and Radiata
. From this phylogenetic grouping, the articulata hypothesis was born.
The articulata hypothesis, simply stated, is the phylogenetic grouping of the phylum annelida, (which includes polychaetes, oligochaetes, and leeches), together with the phylum arthropoda, (arachnids, insects and crustaceans) into the common taxa, articulata. Cuvier grouped these diverse phyla together according the common structural feature: the segmented body plan. This hypothesis further implies that all segmented organisms have a common ancestral origin.
, DNA sequencing
, genomics
).
. While the general acceptance of the theory of evolution weakened Cuvier’s general theory of the unique “ground-plans” as the origin of modern taxa, it strengthened the articulata hypothesis by organizing annelids and arthropods into a clade
descended from a common segmented ancestor.
, advances in molecular biology
techniques lead to further data supporting the articulata hypothesis but also lead to the development of conflicting theories. Advances in confocal microscopy
technology lead to the discovery of embryonic
cleavage (embryo)
patterns, which differs between the annelids and arthropods. Annelids are all spiral cleavers, meaning that each embryonic cleavage occurs at progressive 90 degree angles with respect to the animal- vegetal axis. Arthropods, on the other hand, display a heterogeneous mix of embryonic cleavage patterns including spiral-like cleavage and radial cleavage patterns. This led researchers to two theories: The first was that the arthropods lineage must have lost the ability to spiral cleave since differentiating from the last common ancestor between annelids and arthropods. The second is that this showed similarities between annelids and mollusks who also spirally cleaves but lacks that the segmented body plan. This was not the only interpretation of this data but other hypotheses were seen to have less data or merit. Other studies such as those looking a neural patterns within the articulata clade
showed mixed patterns and thus mixed results.
hypothesis is generally accepted today as the best supported evolutionary hypothesis for annelids and arthropods.
Panarthropoda
Panarthropoda is a taxon combining the phyla Arthropoda, Tardigrada, and Onychophora. Dunn et al. in 2008 suggested that the tardigrades could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods...
. This theory states that these groups are descended from a common segmented ancestor. The Articulata Hypothesis is an alternative to the hypothesis that ecdysis
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
(the shedding of outer cuticle) is a primitive character - this would place Panarthropoda in the group Ecdysozoa
Ecdysozoa
Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...
.
Origins
The articulata hypothesis originates from the phylogenetic analyses of George Cuvier in his 1817 published work, Le Règne animal, distribué après son organization. In this work, Cuvier theorized that all organisms exist as a functional whole, meaning that all of the physiological structures of an organism are important for survival. By studying these physiological structures, Cuvier was able to group the known animalAnimal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
kingdom according to structural similarities resulting from what he referred to as special “ground-plans”, which are analogous to blueprints. Each of these “grounds-plan”, he further argued, evolved separately from the others and structural similarities were due to common function and not to common ancestry. From these “ground plans”, Cuvier separated the known animal kingdom into four branches or “embranchements”: Vertebrata, Articulata
Articulata
Articulata has four meanings in zoology:*Articulata , one of two main divisions of the brachiopods having two valves with an articulating hinge* Articulata , a subclass of crinoids, the only such to survive past the Paleozoic era...
, Mullusca, and Radiata
Radiata
The Radiata are the radially symmetric animals of the Eumetazoa subkingdom. The term Radiata has had various meanings in the history of classification...
. From this phylogenetic grouping, the articulata hypothesis was born.
The articulata hypothesis, simply stated, is the phylogenetic grouping of the phylum annelida, (which includes polychaetes, oligochaetes, and leeches), together with the phylum arthropoda, (arachnids, insects and crustaceans) into the common taxa, articulata. Cuvier grouped these diverse phyla together according the common structural feature: the segmented body plan. This hypothesis further implies that all segmented organisms have a common ancestral origin.
Evolution of the Articulata Hypothesis
Since its original formulation in 1817, there have been significant challenges and modifications to the articulata hypothesis as new theories have been accepted (Darwin’s theory of evolution) and new technologies have become available (confocal microscopyConfocal microscopy
Confocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a micrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that are thicker than the focal plane. It enables the reconstruction of...
, DNA sequencing
DNA sequencing
DNA sequencing includes several methods and technologies that are used for determining the order of the nucleotide bases—adenine, guanine, cytosine, and thymine—in a molecule of DNA....
, genomics
Genomics
Genomics is a discipline in genetics concerning the study of the genomes of organisms. The field includes intensive efforts to determine the entire DNA sequence of organisms and fine-scale genetic mapping efforts. The field also includes studies of intragenomic phenomena such as heterosis,...
).
Theory of Evolution
Darwin’s Theory of Evolution had a large, yet often understated impact of the articulata hypothesis. Cuvier’s original articulata hypothesis was based on his assumption that current species no longer evolved because to evolve would cause loss of integral structures necessary for the survival of the speciesSpecies
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...
. While the general acceptance of the theory of evolution weakened Cuvier’s general theory of the unique “ground-plans” as the origin of modern taxa, it strengthened the articulata hypothesis by organizing annelids and arthropods into a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
descended from a common segmented ancestor.
Confocal Microscopy and Modern Molecular Biology Techniques
While each advance in modern molecular biology has shaken the phylogenetic tree of bilateriaBilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
, advances in molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
techniques lead to further data supporting the articulata hypothesis but also lead to the development of conflicting theories. Advances in confocal microscopy
Confocal microscopy
Confocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a micrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that are thicker than the focal plane. It enables the reconstruction of...
technology lead to the discovery of embryonic
Embryonic
Embryonic received general acclaim from critics upon release, garnering a 81/100 critic score on Metacritic. The New Musical Express noted that "ten years after their last masterpiece, The Flaming Lips have finally produced another one," while Paste Magazine described the record as "a wonderfully...
cleavage (embryo)
Cleavage (embryo)
In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
patterns, which differs between the annelids and arthropods. Annelids are all spiral cleavers, meaning that each embryonic cleavage occurs at progressive 90 degree angles with respect to the animal- vegetal axis. Arthropods, on the other hand, display a heterogeneous mix of embryonic cleavage patterns including spiral-like cleavage and radial cleavage patterns. This led researchers to two theories: The first was that the arthropods lineage must have lost the ability to spiral cleave since differentiating from the last common ancestor between annelids and arthropods. The second is that this showed similarities between annelids and mollusks who also spirally cleaves but lacks that the segmented body plan. This was not the only interpretation of this data but other hypotheses were seen to have less data or merit. Other studies such as those looking a neural patterns within the articulata clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
showed mixed patterns and thus mixed results.
The Articulata Hypothesis is the grouping in a higher taxon of animals with segmented bodies, consisting of Annelida and Panarthropoda
Panarthropoda
Panarthropoda is a taxon combining the phyla Arthropoda, Tardigrada, and Onychophora. Dunn et al. in 2008 suggested that the tardigrades could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods...
. This theory states that these groups are descended from a common segmented ancestor. The Articulata Hypothesis is an alternative to the hypothesis that ecdysis
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
(the shedding of outer cuticle) is a primitive character - this would place Panarthropoda in the group Ecdysozoa
Ecdysozoa
Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...
.
Origins
The articulata hypothesis originates from the phylogenetic analyses of George Cuvier in his 1817 published work, Le Règne animal, distribué après son organization. In this work, Cuvier theorized that all organisms exist as a functional whole, meaning that all of the physiological structures of an organism are important for survival. By studying these physiological structures, Cuvier was able to group the known animalAnimal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
kingdom according to structural similarities resulting from what he referred to as special “ground-plans”, which are analogous to blueprints. Each of these “grounds-plan”, he further argued, evolved separately from the others and structural similarities were due to common function and not to common ancestry. From these “ground plans”, Cuvier separated the known animal kingdom into four branches or “embranchements”: Vertebrata, Articulata
Articulata
Articulata has four meanings in zoology:*Articulata , one of two main divisions of the brachiopods having two valves with an articulating hinge* Articulata , a subclass of crinoids, the only such to survive past the Paleozoic era...
, Mullusca, and Radiata
Radiata
The Radiata are the radially symmetric animals of the Eumetazoa subkingdom. The term Radiata has had various meanings in the history of classification...
. From this phylogenetic grouping, the articulata hypothesis was born.
The articulata hypothesis, simply stated, is the phylogenetic grouping of the phylum annelida, (which includes polychaetes, oligochaetes, and leeches), together with the phylum arthropoda, (arachnids, insects and crustaceans) into the common taxa, articulata. Cuvier grouped these diverse phyla together according the common structural feature: the segmented body plan. This hypothesis further implies that all segmented organisms have a common ancestral origin.
Evolution of the Articulata Hypothesis
Since its original formulation in 1817, there have been significant challenges and modifications to the articulata hypothesis as new theories have been accepted (Darwin’s theory of evolution) and new technologies have become available (confocal microscopyConfocal microscopy
Confocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a micrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that are thicker than the focal plane. It enables the reconstruction of...
, DNA sequencing
DNA sequencing
DNA sequencing includes several methods and technologies that are used for determining the order of the nucleotide bases—adenine, guanine, cytosine, and thymine—in a molecule of DNA....
, genomics
Genomics
Genomics is a discipline in genetics concerning the study of the genomes of organisms. The field includes intensive efforts to determine the entire DNA sequence of organisms and fine-scale genetic mapping efforts. The field also includes studies of intragenomic phenomena such as heterosis,...
).
Theory of Evolution
Darwin’s Theory of Evolution had a large, yet often understated impact of the articulata hypothesis. Cuvier’s original articulata hypothesis was based on his assumption that current species no longer evolved because to evolve would cause loss of integral structures necessary for the survival of the speciesSpecies
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...
. While the general acceptance of the theory of evolution weakened Cuvier’s general theory of the unique “ground-plans” as the origin of modern taxa, it strengthened the articulata hypothesis by organizing annelids and arthropods into a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
descended from a common segmented ancestor.
Confocal Microscopy and Modern Molecular Biology Techniques
While each advance in modern molecular biology has shaken the phylogenetic tree of bilateriaBilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
, advances in molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
techniques lead to further data supporting the articulata hypothesis but also lead to the development of conflicting theories. Advances in confocal microscopy
Confocal microscopy
Confocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a micrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that are thicker than the focal plane. It enables the reconstruction of...
technology lead to the discovery of embryonic
Embryonic
Embryonic received general acclaim from critics upon release, garnering a 81/100 critic score on Metacritic. The New Musical Express noted that "ten years after their last masterpiece, The Flaming Lips have finally produced another one," while Paste Magazine described the record as "a wonderfully...
cleavage (embryo)
Cleavage (embryo)
In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
patterns, which differs between the annelids and arthropods. Annelids are all spiral cleavers, meaning that each embryonic cleavage occurs at progressive 90 degree angles with respect to the animal- vegetal axis. Arthropods, on the other hand, display a heterogeneous mix of embryonic cleavage patterns including spiral-like cleavage and radial cleavage patterns. This led researchers to two theories: The first was that the arthropods lineage must have lost the ability to spiral cleave since differentiating from the last common ancestor between annelids and arthropods. The second is that this showed similarities between annelids and mollusks who also spirally cleaves but lacks that the segmented body plan. This was not the only interpretation of this data but other hypotheses were seen to have less data or merit. Other studies such as those looking a neural patterns within the articulata clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
showed mixed patterns and thus mixed results.
The Articulata Hypothesis is the grouping in a higher taxon of animals with segmented bodies, consisting of Annelida and Panarthropoda
Panarthropoda
Panarthropoda is a taxon combining the phyla Arthropoda, Tardigrada, and Onychophora. Dunn et al. in 2008 suggested that the tardigrades could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods...
. This theory states that these groups are descended from a common segmented ancestor. The Articulata Hypothesis is an alternative to the hypothesis that ecdysis
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
(the shedding of outer cuticle) is a primitive character - this would place Panarthropoda in the group Ecdysozoa
Ecdysozoa
Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...
.
Origins
The articulata hypothesis originates from the phylogenetic analyses of George Cuvier in his 1817 published work, Le Règne animal, distribué après son organization. In this work, Cuvier theorized that all organisms exist as a functional whole, meaning that all of the physiological structures of an organism are important for survival. By studying these physiological structures, Cuvier was able to group the known animalAnimal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
kingdom according to structural similarities resulting from what he referred to as special “ground-plans”, which are analogous to blueprints. Each of these “grounds-plan”, he further argued, evolved separately from the others and structural similarities were due to common function and not to common ancestry. From these “ground plans”, Cuvier separated the known animal kingdom into four branches or “embranchements”: Vertebrata, Articulata
Articulata
Articulata has four meanings in zoology:*Articulata , one of two main divisions of the brachiopods having two valves with an articulating hinge* Articulata , a subclass of crinoids, the only such to survive past the Paleozoic era...
, Mullusca, and Radiata
Radiata
The Radiata are the radially symmetric animals of the Eumetazoa subkingdom. The term Radiata has had various meanings in the history of classification...
. From this phylogenetic grouping, the articulata hypothesis was born.
The articulata hypothesis, simply stated, is the phylogenetic grouping of the phylum annelida, (which includes polychaetes, oligochaetes, and leeches), together with the phylum arthropoda, (arachnids, insects and crustaceans) into the common taxa, articulata. Cuvier grouped these diverse phyla together according the common structural feature: the segmented body plan. This hypothesis further implies that all segmented organisms have a common ancestral origin.
Evolution of the Articulata Hypothesis
Since its original formulation in 1817, there have been significant challenges and modifications to the articulata hypothesis as new theories have been accepted (Darwin’s theory of evolution) and new technologies have become available (confocal microscopyConfocal microscopy
Confocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a micrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that are thicker than the focal plane. It enables the reconstruction of...
, DNA sequencing
DNA sequencing
DNA sequencing includes several methods and technologies that are used for determining the order of the nucleotide bases—adenine, guanine, cytosine, and thymine—in a molecule of DNA....
, genomics
Genomics
Genomics is a discipline in genetics concerning the study of the genomes of organisms. The field includes intensive efforts to determine the entire DNA sequence of organisms and fine-scale genetic mapping efforts. The field also includes studies of intragenomic phenomena such as heterosis,...
).
Theory of Evolution
Darwin’s Theory of Evolution had a large, yet often understated impact of the articulata hypothesis. Cuvier’s original articulata hypothesis was based on his assumption that current species no longer evolved because to evolve would cause loss of integral structures necessary for the survival of the speciesSpecies
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...
. While the general acceptance of the theory of evolution weakened Cuvier’s general theory of the unique “ground-plans” as the origin of modern taxa, it strengthened the articulata hypothesis by organizing annelids and arthropods into a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
descended from a common segmented ancestor.
Confocal Microscopy and Modern Molecular Biology Techniques
While each advance in modern molecular biology has shaken the phylogenetic tree of bilateriaBilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
, advances in molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
techniques lead to further data supporting the articulata hypothesis but also lead to the development of conflicting theories. Advances in confocal microscopy
Confocal microscopy
Confocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a micrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that are thicker than the focal plane. It enables the reconstruction of...
technology lead to the discovery of embryonic
Embryonic
Embryonic received general acclaim from critics upon release, garnering a 81/100 critic score on Metacritic. The New Musical Express noted that "ten years after their last masterpiece, The Flaming Lips have finally produced another one," while Paste Magazine described the record as "a wonderfully...
cleavage (embryo)
Cleavage (embryo)
In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
patterns, which differs between the annelids and arthropods. Annelids are all spiral cleavers, meaning that each embryonic cleavage occurs at progressive 90 degree angles with respect to the animal- vegetal axis. Arthropods, on the other hand, display a heterogeneous mix of embryonic cleavage patterns including spiral-like cleavage and radial cleavage patterns. This led researchers to two theories: The first was that the arthropods lineage must have lost the ability to spiral cleave since differentiating from the last common ancestor between annelids and arthropods. The second is that this showed similarities between annelids and mollusks who also spirally cleaves but lacks that the segmented body plan. This was not the only interpretation of this data but other hypotheses were seen to have less data or merit. Other studies such as those looking a neural patterns within the articulata clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
showed mixed patterns and thus mixed results.
DNA sequencing and Genomics
The advancements in DNA sequencing techniques and the development of phylogenetic analysis algorithms lead to the splitting of the articulata clade. Original phylogenetic studies on the sequences of 18S and 28S ribosomal DNA sequence lead to the suggestions that the annelids and arthropods had evolutionarily diverged much earlier than was previously thought but such limited genetic studies lead to limited and often mixed results. As more genes were added to the studies, it became apparent that arthropods were genetically closer to nematodes and other molting organisms whereas the annelids were closer evolutionary to mollusk. This ecdysozoaEcdysozoa
Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...
hypothesis is generally accepted today as the best supported evolutionary hypothesis for annelids and arthropods.