Urbilaterian
Encyclopedia
The urbilaterian is the hypothetical last common ancestor of the bilateria
n clade
, i.e., all animal
s having a bilateral symmetry.
Its appearance is a matter of debate, for no representative has been (or is ever likely to be) identified in the fossil record
; the reconstructed morphology
it would display largely depends on whether the acoelomorpha
are included within the bilaterian clade. Since all protostome
s and deuterostome
s share features, such as blood circulation systems
and guts, that are only useful in relatively large (macroscopic
) organisms, their common ancestor
ought also to have been macroscopic. However, such large animals should have left traces in the sediment
in which they moved, and evidence of such traces first appear relatively late in the fossil record
— long after the urbilaterian would have lived. This leads to suggestions of a small urbilaterian, which is the supposed state of the ancestor of protostomes, deuterostomes and acoelomorphs.
s in sediments towards the end of the Ediacaran period
(about ), and the first fully accepted fossil of a bilaterian organism is Kimberella
, dating to . There are earlier, controversial fossils: Vernanimalcula
has been interpreted as a bilaterian, but may simply represent a fortuitously infilled bubble. Interestingly, Fossil embryos
are known from around the time of Vernanimalcula , but none of these have bilaterian affinities. This may reflect a genuine absence of bilateria, but caution is due — it could be that bilateria didn't lay eggs in sediment, where they would be likely to fossilise.
Molecular techniques
can generate expected dates of the divergence between the bilaterian clades, and thus an assessment of when the urbilaterian lived. These dates have huge margins of error, though they are becoming more accurate with time. More recent estimates are compatible with an Ediacaran bilaterian, although it is possible, especially if early bilaterians were small, that the bilateria had a long cryptic history before they left any evidence in the fossil record.
A traditional approach to reconstructing the urbilaterian considers it as a "roundish flatworm" consisting of as few features as possible. This concept of a very simple animal was abandoned when it was realised that the vast array of traits common to bilaterians would produce quite a complex animal, but has had a resurgence with the inclusion of a phylum of flatworms within the bilateria.
The urbilaterian is often considered to have possessed a gut and internal organs, a segmented body
and a centralised nervous system
, as well as a biphasic life cycle
(i.e. consisting of larvae and adults) and some features of embryonic development
. However, this need not necessarily be the case. If these features were of key importance to the evolution of large size, it is possible that bilaterian lineages each stumbled upon them independently, and perhaps even co-opted the same underlying genetic machinery from a different role.
The presence of genetic machinery (the Pax6 and Six genes) common to eye formation in all bilaterians suggests that this machinery - and hence eyes - was present in the urbilaterian. The most likely candidate eye type is the simple pigment-cup eye, which is the most widespread among the bilateria.
Since two types of opsin, the c-type and r-type, are found in all bilaterians, the urbilaterian must have possessed both types - although they may not have been found in a centralised eye, but used to synchronise the body clock to daily or lunar variations in lighting.
However, as biologists' understanding of the major bilaterian lineages increases, it is beginning to appear that some of these features may have evolved independently in each lineage. Further, the bilaterian clade has recently been expanded to include the acoelomorphs — a group of relatively simple flatworms. This lineage lacks key bilaterian features, and if it truly does reside within the bilaterian "family", many of the features listed above are no longer common to all bilateria. Instead, some features — such as segmentation and possession of a heart — are restricted to a sub-set of the bilateria, the deuterostomes and protostomes. Their last common ancestor would still have to be large and complex, but the bilaterian ancestor could be much simpler. However, some scientists stop short of including the acoelomorph clade in the bilateria. This shifts the position of the cladistic node
which is being discussed; consequently the urbilaterian in this context is farther out the evolutionary tree and is more derived
than the common ancestor of deuterostomes, protostomes and acoelomorphs.
Genetic reconstructions are unfortunately not much help. They work by considering the genes common to all bilateria, but problems arise because very similar genes can be co-opted for different roles. For instance, the gene Pax6
has a key role in eye development, but is absent in some animals with eyes; some cnidaria
have genes which in bilateria control the development of a layer of cells that the cnidaria don't even possess. This means that even if a gene can be identified as present in the urbilaterian, we can't tell what the gene was coding for. Before this was realised, genetic reconstructions implied a surprisingly complex urbilateria.
Furthermore, a reconstruction of the urbilateria must rest on identifying morphological similarities between all bilateria. While some bilateria live attached to a substrate
, this appears to be a secondary adaptation, and the urbilaterian was probably mobile. Its nervous system
was probably dispersed, but with a small central "brain". Since acoelomorphs lack a heart, coelom or organs, the urbilaterian probably did too — it would presumably have been small enough for diffusion
to do the job of transporting compounds through the body. A small, narrow gut was probably present, which would only have had one opening — a combined mouth and anus.
Functional considerations suggest that the surface of the bilaterian was probably covered with cilia, which it could have used for locomotion or feeding.
There is still no consensus on whether the characteristics of the deuterostomes and protostomes evolved once or many times. Features such as a heart and blood circulation system may therefore not have been present even in the deuterostome-protostome ancestor, which would mean that this too could have been small (hence explaining the lack of fossil record).
Bilateria
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...
n 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...
, i.e., all animal
Animal
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...
s having a bilateral symmetry.
Its appearance is a matter of debate, for no representative has been (or is ever likely to be) identified in the fossil record
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
; the reconstructed 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....
it would display largely depends on whether the acoelomorpha
Acoelomorpha
The Acoelomorpha are a disputed phylum of animals with planula-like features that were considered to belong to the phylum Platyhelminthes. In 2004 molecular studies demonstrated that they are a separate phylum, although their position in the tree of life is contentious; most researchers believe...
are included within the bilaterian clade. Since all protostome
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s and deuterostome
Deuterostome
Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s share features, such as blood circulation systems
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
and guts, that are only useful in relatively large (macroscopic
Macroscopic
The macroscopic scale is the length scale on which objects or processes are of a size which is measurable and observable by the naked eye.When applied to phenomena and abstract objects, the macroscopic scale describes existence in the world as we perceive it, often in contrast to experiences or...
) organisms, their common ancestor
Common descent
In evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....
ought also to have been macroscopic. However, such large animals should have left traces in the sediment
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
in which they moved, and evidence of such traces first appear relatively late in the fossil record
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
— long after the urbilaterian would have lived. This leads to suggestions of a small urbilaterian, which is the supposed state of the ancestor of protostomes, deuterostomes and acoelomorphs.
Dating the urbilaterian
The first evidence of bilateria in the fossil record comes from trace fossilTrace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
s in sediments towards the end of the Ediacaran period
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
(about ), and the first fully accepted fossil of a bilaterian organism is Kimberella
Kimberella
Kimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the molluscs, although its affinity with this group is contentious.Specimens were first found in...
, dating to . There are earlier, controversial fossils: Vernanimalcula
Vernanimalcula
Vernanimalcula guizhouena is a fossil believed by some to represent the earliest known member of the Bilateria . It is known from deposits dating to . The fossils are between 0.1 and 0.2 mm across...
has been interpreted as a bilaterian, but may simply represent a fortuitously infilled bubble. Interestingly, Fossil embryos
Fossil embryos
Fossil embryos are the preserved remains of organisms that have yet to hatch or be born. Many fossils of the Doushantuo formation have been interpreted as embryos; embryos are also common throughout the Cambrian fossil record.-Preservation:...
are known from around the time of Vernanimalcula , but none of these have bilaterian affinities. This may reflect a genuine absence of bilateria, but caution is due — it could be that bilateria didn't lay eggs in sediment, where they would be likely to fossilise.
Molecular techniques
Molecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
can generate expected dates of the divergence between the bilaterian clades, and thus an assessment of when the urbilaterian lived. These dates have huge margins of error, though they are becoming more accurate with time. More recent estimates are compatible with an Ediacaran bilaterian, although it is possible, especially if early bilaterians were small, that the bilateria had a long cryptic history before they left any evidence in the fossil record.
Characteristics of the urbilaterian
The urbilaterian must have possessed all those traits common to living bilateria that are thought only to have evolved once. The defining features are a three-layered (triploblastic) embryo and two main axes of symmetry — a top-to-bottom axis and a front-to-back axis.A traditional approach to reconstructing the urbilaterian considers it as a "roundish flatworm" consisting of as few features as possible. This concept of a very simple animal was abandoned when it was realised that the vast array of traits common to bilaterians would produce quite a complex animal, but has had a resurgence with the inclusion of a phylum of flatworms within the bilateria.
The urbilaterian is often considered to have possessed a gut and internal organs, a segmented body
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...
and a centralised nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
, as well as a biphasic life cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
(i.e. consisting of larvae and adults) and some features of embryonic development
Embryogenesis
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...
. However, this need not necessarily be the case. If these features were of key importance to the evolution of large size, it is possible that bilaterian lineages each stumbled upon them independently, and perhaps even co-opted the same underlying genetic machinery from a different role.
Eyes
Light detection (photosensitivity) is present in organisms as simple as seaweeds; the definition of a true eye varies, but in general eyes must have directional sensitivity, and thus have screening pigments so only light from the target direction is detected. Thus defined, they need not consist of more than one photoreceptor cell.The presence of genetic machinery (the Pax6 and Six genes) common to eye formation in all bilaterians suggests that this machinery - and hence eyes - was present in the urbilaterian. The most likely candidate eye type is the simple pigment-cup eye, which is the most widespread among the bilateria.
Since two types of opsin, the c-type and r-type, are found in all bilaterians, the urbilaterian must have possessed both types - although they may not have been found in a centralised eye, but used to synchronise the body clock to daily or lunar variations in lighting.
Complex or simple?
Proponents of a complex urbilaterian point to the shared features and genetic machinery common to all bilateria. They argue that (1) since these are similar in so many respects, they can only have evolved once; and (2) since they are common to all bilateria, they must have been present in the ancestral bilaterian animal.However, as biologists' understanding of the major bilaterian lineages increases, it is beginning to appear that some of these features may have evolved independently in each lineage. Further, the bilaterian clade has recently been expanded to include the acoelomorphs — a group of relatively simple flatworms. This lineage lacks key bilaterian features, and if it truly does reside within the bilaterian "family", many of the features listed above are no longer common to all bilateria. Instead, some features — such as segmentation and possession of a heart — are restricted to a sub-set of the bilateria, the deuterostomes and protostomes. Their last common ancestor would still have to be large and complex, but the bilaterian ancestor could be much simpler. However, some scientists stop short of including the acoelomorph clade in the bilateria. This shifts the position of the cladistic node
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
which is being discussed; consequently the urbilaterian in this context is farther out the evolutionary tree and is more 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...
than the common ancestor of deuterostomes, protostomes and acoelomorphs.
Genetic reconstructions are unfortunately not much help. They work by considering the genes common to all bilateria, but problems arise because very similar genes can be co-opted for different roles. For instance, the gene Pax6
PAX6
Paired box protein Pax-6 also known as aniridia type II protein or oculorhombin is a protein that in humans is encoded by the PAX6 gene.- Function :PAX6 is a member of the Pax gene family...
has a key role in eye development, but is absent in some animals with eyes; some cnidaria
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,...
have genes which in bilateria control the development of a layer of cells that the cnidaria don't even possess. This means that even if a gene can be identified as present in the urbilaterian, we can't tell what the gene was coding for. Before this was realised, genetic reconstructions implied a surprisingly complex urbilateria.
Reconstructing the urbilaterian
The absence of a fossil record gives a starting point for the reconstruction — the urbilaterian must have been small enough not to leave any traces as it moved over or lived in the sediment surface. This means it must have been well below a centimetre in length. As all Cambrian animals are marine, it is reasonable to assume that the urbilaterian was too.Furthermore, a reconstruction of the urbilateria must rest on identifying morphological similarities between all bilateria. While some bilateria live attached to a substrate
Substrate (biology)
In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
, this appears to be a secondary adaptation, and the urbilaterian was probably mobile. Its nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
was probably dispersed, but with a small central "brain". Since acoelomorphs lack a heart, coelom or organs, the urbilaterian probably did too — it would presumably have been small enough for diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
to do the job of transporting compounds through the body. A small, narrow gut was probably present, which would only have had one opening — a combined mouth and anus.
Functional considerations suggest that the surface of the bilaterian was probably covered with cilia, which it could have used for locomotion or feeding.
There is still no consensus on whether the characteristics of the deuterostomes and protostomes evolved once or many times. Features such as a heart and blood circulation system may therefore not have been present even in the deuterostome-protostome ancestor, which would mean that this too could have been small (hence explaining the lack of fossil record).