Why are animals symmetrical

Thanks to this, the bilateral body can move forward very efficiently, and it can also produce a greater pushing force in sideways directions compared to other streamlined symmetry types, thus ensuring the maximisation of turning forces [ 88 ] Fig. This is also helped by the bilaterally positioned appendages with which the bilateral body can further augment its sideways resistance without losing too much on skin friction, hence effectuating a kind of trade-off between the slowing effect due to the increased surface and the gained pushing force stemming from resistance picture the body of a fish, for example.

This clearly cannot be optimised to such an extent in a radially symmetrical body in which the theoretical, radially arranged appendages, besides offering the possibility to turn in many directions without twisting the body, would augment the surface and so skin friction superfluously, because the appendages which did not actually work in the given body movement would represent an unnecessary burden or would have to be instantaneously retracted and stuck out, continuously.

The process is best carried out with the use of bilaterally ordered appendages combined with body twists and turns. The bilaterally symmetrical body plan of most animals is generated by two, perpendicularly acting diffusible morphogen gradients: Wnt and BMP. The figure has been inspired by Fig.

Note that the BMP gradient is oriented in the opposite direction in chordates. Radially a , biradially b and bilaterally c symmetrical bodies with the projection of pushing surfaces created in a watery environment. Grids indicate the approximate magnitude of resistance necessary to produce turning forces.

To complete the picture, it is important to mention the role of gravity in the determination of dorsoventral polarity [ 1 , 90 ].

To produce sideways turning forces it is enough to have a laterally flattened body Fig. However, in dimensions characterised by even greater Reynolds numbers, the viscosity of the fluid will be not enough to hold the body, and hydrostatic pressure will not be able to fully counteract gravity.

In this realm, the dorsoventral polarisation, which produces a different profiling of the dorsal and ventral sides of the body, and, most importantly, of the appendages, will help to produce a lifting force. This dorsoventral polarisation leads to the advent of the second polarity axis, thus reducing the number of the two symmetry planes of a biradial body to one, generating a bilaterally symmetrical body.

Later on in evolution, bilaterally symmetrical locomotor apparati proved to be useful both on land, where locomotion essentially occurs in a 2D environment, requiring the body to go directly and to turn left or right, and in the air, where the 3D locomotion is similar to that found in water, and to overcome gravity, large surface wings counterbalance the lack of hydrostatic pressure [ 88 ], and, most pronouncedly in bigger and heavier animals like birds, their dorsoventral polarity also produces a lifting force — similarly to aircraft wings.

Importantly, the adaptation of locomotor systems to life on land had most probably been preceded by the evolution of benthic locomotion, which also requires a 2D movement, very similar to that required on land, and which, most probably, also goes together with dorsoventral polarisation. This is a clear example of the influence of physical forces on overall body symmetry and shape. Thus, since the link between locomotion and bilaterality seems to be evident, it can be argued that bilateral symmetry is optimised for physical forces in locomotion in the macroscopic world, i.

Other potential ultimate factors which favour bilaterality remain to be discovered. It could also be asked whether the body-scale bilaterality present in non-moving sea anemones or slowly moving taxa mussels confers evolutionary advantages, is due to phylogenetic inertia, is an admixture of the two, or is the product of currently unknown factors; however, this type of analysis would require detailed, taxon-focused investigations, which would go beyond the limits of the present paper.

What about the ultimate causes of radial body plans? The function of the overall radiality of cnidarians and echinoderms is explained by their sessile, drifting or slowly moving lifestyle [e. The ordering of body parts according to this symmetry offers the ability to react to environmental forces in every direction with the same efficiency [ 1 , 88 ].

Interestingly, a recent study has reported that following the amputation of a variable number of arms, the ephyra larvae of the jellyfish Aurelia aurita regenerate their radial symmetry, rearranging the remaining body parts without restoring the missing arms [ 91 ]. The process, called symmetrisation, is completed regardless of the number of arms lost, and without any obvious global organiser in the body: it is driven by muscular contractions, pointing out both the importance of mechanical forces as proximate form-shaping effects and the need to restore radial body symmetry [ 91 ].

According to the manoeuvrability hypothesis [ 88 ], however, the radial body form cannot allow such a fast and precise locomotion as the bilateral, as is clearly observable in nature e. The convergence to the cylindrical form of endoparasites and burrowing worms — other groups of animals with radial external symmetry — has been proposed as the logical consequence of the fact that they live in a very dense substrate where locomotion favours body plans whose cross section area is minimised [ 88 ]; consequently, the cylindrical symmetry is optimised for their specific lifestyle and is shaped by physical forces.

The decoupling of the external radial symmetry and the internal bilateral structuring of burrowing and endoparasitic worms [ 88 ] underscores the flexible use of symmetrical anatomical patterns in response to functional and physical requirements [ 1 , 2 ]. Thus, it can be stated that the indirect cause of this symmetry, too, is to conform to the physical environment; i.

The idea that symmetry is mainly shaped by physical forces, has deep roots in time; however, with the advent of modern molecular biology, the molecular approach has taken the leading role in science. Since the publication of his book, numerous experiments have led to the same conclusion, as listed in previous sections of this essay. Nowadays, the time might have come to re-evoke the old, common sense logic, and re-synthesise knowledge on animal symmetry, explained not only by molecular factors but also by mechanical forces.

In summary, I think that instead of treating animal symmetry in general terms as, for want of something better, a combination of developmental canalisation and historical contingency, a more mechanistic view should be adopted. In this concept, the whole story of animal symmetry is fragmentary, and the pieces of the mosaic are not held together by any coherent explanatory concept.

Interestingly, however, the examples of symmetrical patterns of biological structures that turn out to be logically reasonable are justified by physical-type explanations. Disentangling the question of what types of constraints, and to what extent, act on shaping the evolution of animal form, is an attractive problem. However, it seems that exact solutions to this puzzle do not exist in principle, given that we have neither the methods to analyse them in detail, nor any process which could serve as a control situation.

Thus, any answer has to be necessarily speculative. The main types of constraints acting in evolution are classified into two main groups [ 92 ]. First, the mechanical-architectural and the functional constraints stem from structural-functional limitations and physical laws, and they only allow the formation of a subset of the theoretical morphospace.

Second, the developmental and the genetic constraints originate from the non-random production of variants [ 92 ]. The analysis of the different involvement of these diverse constraint types in shaping morphological properties can be fruitful on minor time- and taxonomical scales, such as across orders or families.

However, trying to explain symmetry across the whole of documented animal evolution only by developmental and genetic constraints, seems to be insufficient and misleading. This is also because symmetry is a basic property of the organisation of matter, and genetic and developmental constraints can only come into existence after mechanical-architectural and functional constraints have delineated the basic geometric features of biological structures.

Regarding functional constraints, it has been shown that not all conserved phenotypes are the fruit of convergent evolution constrained by functional necessity; they may simply be frozen combinations on a local optimum of the fitness landscape, limited by unpassable valleys in the genotype space [ 93 ]. This most probably does not hold for symmetry, which frames every phenotype in animal evolution. I propose a flexible concept of symmetry in which simple physical laws, through function, determine which of the symmetries will be expressed from an animal genome that encodes both of them.

In such a mechanistic view, one does not treat as exceptional and incongruent such phenomena as why it is that an endoparasitic animal can have internal tetraradiality and a cylindrical external shape despite being a free-moving animal [ 94 ], or why the bilateral spine distribution of a sea urchin can be explained by the improved defensive function it confers on the animal, and not by efficient locomotion [ 95 ].

But the evolution of motile, modular mega-organisms may be a different story. In contrast to this view, I propose a unifying frame of thinking, according to which, the symmetries present in the diverse organisational levels of the animal body are mainly shaped by physical effects and, in this way, by functionality; thus, their appearance in animal evolution is inevitable.

Since overall spherical symmetry is suboptimal for the body plan of a macroscopic animal that has to deal with gravity and the physical challenges imposed by locomotion such as drag; [ 88 ] , it is only radial and bilateral symmetry which can be deployed when constructing its body. It seems to be obvious that a profound inertia caused by the genetic canalisation of development is characteristic of the evolution of body plans, but, regarding only symmetry as a basic and omnipresent feature of body plans, I emphasise its physically determined character: speaking in terms of geological time, it seems very improbable that the explanation of the symmetry of the body plan or that of minor anatomical structures such as biological tubes should invoke developmental and genetic constraints.

Bearing in mind i that symmetry is a ubiquitous feature of biological structures in every level of individual and infra-individual organisation, and also considering ii the limited number of practically possible symmetry types, iii the physical environment of Earth, iv the enormous amount of time for any potential change in the symmetry of body and transport systems, and v the capability of the animal genome to build both radial and bilateral symmetries, the idea of the determination of symmetry by physical forces further bolsters the concept that both radial and bilateral symmetries are necessary products of animal evolution [ 2 , 88 ].

Thus, in my considered opinion, if the tape of life [ 96 ] was rewound and started again, the many detailed architectural patterns of animal body plans would probably differ from the actual patterns, but the basic symmetries characterising body plans and the many anatomical structures would be identical to those that we find today. Hopefully, our picture of animal symmetry will be further clarified when we will eventually be able to identify the ultimate causes behind the very origin of either radial or bilateral symmetry, long-sought answers to fundamental problems in evolutionary biology.

That said, there is very little concrete, let alone quantitative, argument here as how, specifically, physical factors produce symmetry. Furthermore, the previous work from the author Ref. Regrettably, I do not have the impression that direct and direct causes, and biological and physical factors are disentangled here in a satisfactory manner.

I believe the paper would gain a lot from a more specific description of the way physical factors shape symmetry. The best thing would be to provide actual estimates even ballpark ones of the effects of the forces involved.

I realize that this is a tall order but any approximation woudl be valuable. I am grateful to Dr. Koonin for undertaking the review. I also admit that the paper lacks specific descriptions as to the precise extent physical factors determine symmetrical patterns in the animal body. However, please let me first underline that this hypothesis paper tries to give a general framework for thinking about symmetry, and not to offer exact explanations for individual cases for the specific animal taxa.

Furthermore, to be able to give even approximate numbers for these intervals, the concrete values of the forces involved should be individually measured and published as research articles , which, I think, exceeds the scope of the present paper.

However, I am open to conducting further investigations; in this case, please, give more specific details on how to proceed.

The paper is rich in original ideas, therefore it is worth for discussion and thus, also for publication, although I cannot agree with some of its basic ideas. As a consequence, I suggest a careful revision of the paper but I am also waiting for the objections of the Author in his answers on my criticism.

I think there are two basic flaws of the paper. The first is more philosophic, the second more phylogenetic incl. In details, e. These are insufficiently disentangled in the paper.

First of all I would like to thank Dr. Varga for having undertaken the work of reviewing the manuscript. Please let me note first, that according to the logic presented in the essay, both the whole body and the infra-individual level structures act as biological entities reacting to the forces of their environment. Furthermore, both are built on the basis of genetic programs, which follow a linear order of activation. Naturally, the core of the genetic programs — i. However, in no way does this imply that the whole body should not conform to physical factors, and that the mechanistic view could not also be adopted for the general bauplan.

This means that even though in the case of minor anatomical structures the physical forces may much more easily be identified as the causa efficiens, both the body-level and the infraindividual symmetries can well be constrained by causa finalis, even if, for example, for the bilaterally symmetrical body this is not so obvious at first sight , which means the aim of both is to fit the physical environment.

I think, this basic problem remained unsolved and also undiscussed in the paper. As my answers below will try to highlight, the rejection of this theory is not necessary. What I propose only requires a shift away from the view that sees the whole of morphological evolution as the manifestation of genetic programs passing from generation to generation. In this aspect, it is mainly, or only, the genetic information which constrains the individual bodies so that they develop in a specific order, and it is only mutations and other — also stochastically acting — genetic effects which produce the variability on which natural selection operates.

Simply put, evolution of form springs from genetic processes. This is also true but is only one side of the picture. I think that even if genetic processes do have their own laws, the organisms in which the genetic programs are manifested have to fit physical effects, otherwise non-conforming forms will be ruled out from evolution.

Thus, morphological evolution has to follow genetic processes, but genetic processes have to follow physical effects. Development, in itself, is mainly a strict manifestation of a genetic algorithm, and even the direct action of physical forces is largely hidden: intricate and meticulous experiments are necessary to see how physical effects work during development — but now this has also been widely acknowledged, as it is evident from the many works listed in the paper.

Furthermore, I suggest that, in terms of the evolution of symmetry, they are the guiding factors. If I am right in perceiving the reasons behind the objections, their main source was that several of the statements I made were inaccurately formulated, and sometimes not clearly defined, either e. I have tried to make them more precise, and so I hope now the message is more effectively conveyed to the readers.

Thank you for pointing this out, the sentence was not accurate. You are right: the sentence summarises two ideas coming from two different sources.

The distinction between whole body symmetry and regional level symmetry is dealt with later in the Introduction section; please also see my answers which follow below. It might seem that I tried to reject the statement cited above, but I did not.

Conversely, this notion supports my view. If the changes in animal form are due to changes in the GRNs, then it is important to study the fundamental and general properties of the operation of GRNs.

And since these are mosaic both in terms of their evolutionary history and their functioning, it may be inferred that there is no essential and compulsory hierarchy between the diverse GRN modules from which the body is built up, in terms of symmetry. For example, it is not mandatory that every part of the body should be bilaterally symmetrical only because the basic organisation of the whole body follows that order, governed by the first activated GRN subcircuits.

Later activated circuits may express another, different symmetry type if that serves the animal. In the sense of biological organisation, the formation of the diverse body parts is hierarchical. The genetic program, itself, is also hierarchically organised in the sense that the order of kernels and the outer shells of the GRNs cannot be changed or mixed.

However, the GRN subcircuits are separate from each other, and their activation follows a linear path. In this linear code, the subunits are not, of course, independent from each other, but have quite a clear autonomy: what is happening in the later operating subcircuits is not directly influenced by the previous subcircuits. Thus, considering only the symmetry of the diverse structures, there is no evidence to claim that all symmetrical patterns must follow the firstly established, i.

Now the sentence has been completed and reads:. Row 50ff: In this view, it can be said that the overall symmetry of the body plan is not the symmetry of the animal, since the symmetries of minor body parts also have to be taken into account when speaking about body plan symmetry. In this statement the nested hierarchy of the body organisation is completely forgotten.

The basic problem is the modular organisation, i. However, I think this, in itself, does not contradict the results of the modelling reported by Frederick W. Cummings , Int. Surely, the Author is right that physical environment must shape the morphogenetic processes. You are right to observe that this part of the text only deals with the regional level effects of physical forces, and its aim is to highlight the fact that genes and morphogenes cannot be sufficient to explain morphogenetic events.

Asymmetrisation can thus always be present when symmetry is not constrained by locomotion, or by physical forces in general, so it does not necessarily have to be under a direct influence of physical forces; what allows asymmetrisation to develop is rather the absence or reduced importance of the effect of physical forces regarding the given structure.

The phylogenetically most important event is, however, the basic divergence between radial and spiral cleavage — latter occurring in triploblastic animals only! Thank you for pointing this out, my phrasing was confusing here. I would like to highlight the emergence of the blastula as a spherically symmetrical structure, to emphasise that the symmetry of the blastula stage is the symmetry from which the body symmetry forms, and that there is no sense in speaking about preceding phenomena such as yolk distribution and cleavage.

By referring to the uneven yolk distribution I wanted to point to the importance of the interaction between the environment and the external layer of a biological structure, but I admit that the formulation of the whole idea was obscure and misleading. The next constraint of bilateralisation is the formation of mesoderm and coelom both in phylogeny and ontogeny! These facts remain unexplained in the paper! The bilaterisation is a general trend, often connected with secondary asymmetrisation — e.

I am sorry for the wording, which may have led to misunderstandings. Row Second, the appearance of a single cell stage — the egg — in the life cycle of multicellular organisms has been proposed as a necessary step in evolution since it increases the evolvability of the organism, and also reduces the probability of intraorganismal cell-cell conflict [ 95 ].

Thus, the egg itself is not inevitably necessary for the multicellular organism because many cells should and could develop only from a single cell, but is rather a versatile adaptive tool for evolvability and for the exploration of a diversity of life strategies. I am afraid I do not understand why this would be a misunderstanding.

As argued by various authors e. Nature ; ; Newman SA. However, while I was writing the answer to the concern raised by Dr. Manuel please see below , whose objection referred to another part of this subsection, I had to admit that the whole argumentation on early embryonic events does not essentially affect the main line of thinking of the article either in a supportive or a contradictive sense , and so it should be left out of the text.

In fact, about 99 percent of animals have bilateral or two-sided symmetry, says my friend Erica Crespi. Birds might have a hard time flying with one wing. Frogs might hop in circles. Bilateral or two-sided symmetry in the body, like having an even number of legs and arms, can help you move around.

It turns out two-sided symmetry is just one kind of symmetry we see in nature, Crespi says. Take the starfish. That is, until it goes through a natural process called metamorphosis which completely changes its body shape. And why are E. Standen thinks it's likely because the symmetry that surrounds us in the real world guides human creativity and imagination. What's more, if we consider that the laws of physics that govern our locomotion are constant, we may actually be correctly predicting that symmetry would also be advantageous for aliens navigating in our shared universe and that they would hence also exhibit bilateral symmetry.

Social Sharing. Quirks and Quarks The reason why most animals are symmetrical has to do with their locomotion. Triploblasts that do not develop a coelom are called acoelomates: their mesoderm region is completely filled with tissue.

Flatworms in the phylum Platyhelminthes are acoelomates. Eucoelomates or coelomates have a true coelom that arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This coelomic cavity represents a fluid-filled space that lies between the visceral organs and the body wall.

It houses the digestive system, kidneys, reproductive organs, and heart, and it contains the circulatory system. The epithelial membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates.

The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility and improved animal motility. The coelom also provides cushioning and shock absorption for the major organ systems, while allowing organs to move freely for optimal development and placement.

The pseudocoelomates have a coelom derived partly from mesoderm and partly from endoderm. Although still functional, these are considered false coeloms.

The phylum Nematoda roundworms is an example of a pseudocoelomate. Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in their early embryonic development. These two groups are separated based on which opening of the digestive cavity develops first: mouth protostomes or anus deuterostomes. Early embryonic development in eucoelomates : Eucoelomates can be divided into two groups based on their early embryonic development.

In protostomes, part of the mesoderm separates to form the coelom in a process called schizocoely. In deuterostomes, the mesoderm pinches off to form the coelom in a process called enterocoely. The coelom of most protostomes is formed through a process called schizocoely, when a solid mass of the mesoderm splits apart and forms the hollow opening of the coelom.

Deuterostomes differ in that their coelom forms through a process called enterocoely, when the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse to form the mesoderm, which then gives rise to the coelom.