Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email. Jennifer Kennedy. Marine Science Expert. Jennifer Kennedy, M. It is important to say that the changeover does not necessarily have to be drag-assisted.
Some radially symmetrical animals, such as jellyfish, use asymmetric contractions of the bell, thus generating asymmetric jet flows to steer. However, the accuracy and the speed of this medusan-type manoeuvring [ 12 ] are much more modest than the drag-based manoeuvring of bilateral pelagic animals.
Bilateral symmetry has also proved to be succesful both on land and in the air. On land, the force-generating role of the drag in water is replaced by gravitation and so by the necessity of leaning on the land. In this regard, locomotion on land is analogous to that on the fluid—solid interface. This locomotion essentially occurs in two dimensions, thus, direction shift on land requires the body to be capable of turning left or right, and so of being supported from the right as well as from the left.
The effectiveness of creeping locomotion has been improved by the evolution of limbs, which, placed on the two sides of the bilateral body, satisfy the above-mentioned condition. For the sake of simplicity, we will not deal with the limbless evolution of snakes and limbless lizards here. Flying, similarly to swimming, requires the animal to create pushing surfaces in the air. The evolution of large-surface wings allowed the animals to locomote in a medium which, compared to water, has a lower density, and as a consequence, is almost completely lacking in the hydrostatic pressure that to a certain extent counterbalances the force of gravity in water.
The combination of bilaterality with the centralisation of the nervous system and cephalisation allowed the evolution of really successful body plans ensuring precise locomotion and rapid information processing. From the principles developed so far it follows that asymmetry or radial symmetry could have evolved only in animals which do not locomote or locomote slowly. Hence it would vary from species to species. We hope that the understanding of the essence of this concept will not be disturbed by the absence of a clearly defined value.
By sacrificing quick locomotion, these animals necessarily become more vulnerable to predators. Thus they have to be well protected e. The radial body symmetry will be ideal for these animals because it confers on the body the ability to react to environmental forces in every direction sessile cnidarians and echinoderms , to be able to catch food around with the same probability cnidarians, ctenophores, echinoderms and to maintain a static position, adhering to the substratum against water currents locomoting echinoderms [ 1 , 10 , 13 ].
There is also another evolutionary situation in which the body has to be externally cylindrical: a burrowing lifestyle. This lifestyle develops when an animal lives and locomotes in a very dense medium: in earth or equivalents and in the body of other organisms. In these media the density is so high that any lateral structures which increase surface area will be disadvantageous.
So the external body form will be the one that assures a minimum cross section and hence a minimum friction per body mass; and at the same time also reduces the vulnerable body surface. This form is cylindrical. Naturally, this does not necessarily mean that externally bilateral burrowing animals cannot exist but, even in this case, their main body form tends to be nearly cylindrical and the surface-augmenting effects of limbs must be counterbalanced by their burrowing or other functions e.
Here it is crucial to note that animal body symmetry is often different externally and internally [ 1 , 3 ]. It is enough to consider that the external side of the animal interacts directly with the environment while the internal side does not. Hence they face very different conditions that may require different symmetries.
Since those animals which are not bilaterally symmetrical are typically sessile or planctonic drifters, while most bilaterals are free locomoting, the association of bilateral symmetry with directed locomotion seems obvious. Beklemishev [ 2 ] pointed out that when the body is asymmetric, as it reaches a certain speed, rectilinear locomotion becomes impossible and the body begins to move in a helical trajectory.
As he explains, the advantage of bilateral symmetry is precisely that the environmental pressures on the two sides of the body are equalized, guaranteeing a rectilinear locomotion. Following this view, the close association between free swimming and bilaterality has also become widespread in textbooks e.
However, it could also be due to the lack of an adequate explanation for this — otherwise widely accepted — relationship that several authors have questioned it. It has been hypothesized that the origin of bilateral symmetry in animals could have been favoured by internal transport, not by directed locomotion [ 19 ].
Based partly on this view, it seemed problematic to couple the tetraradial symmetry and the active locomotion of the endoparasite cnidarian Buddenbrockia , so a further dissociation of symmetry from locomotion has been proposed [ 20 ]. It has also been reported that the bilateral body form [ 21 ] and the bilateral spine distribution [ 22 ] of sea urchin species was connected to efficient body protection, not to efficient locomotion. Based on the concept presented here it can be understood that the cylindrical external form and the internal tetraradiality of Buddenbrockia is not inconsistent with its active locomotion [ 20 ], and that the slow locomotion of a sea urchin does not have to be closely related to its bilateral body form [ 21 ] or its bilateral spine distribution [ 22 ].
Another potential question may emerge if one examines the earliest trace fossils from the Precambrian. These traces are retained horizontal burrowings in the upper layer of the sediment [ 23 , 24 ] and are also attributed to bilaterian animals [ 24 ]. However, this view has been challenged by the discovery of trace maker giant protists [ 25 ], put forward as candidates for the producers of those ancient trails.
Now, according to our hypothesis, it seems easy to reconcile the putative burrowing behaviour and bilaterality in the precambrian animals mentioned above if they really existed considering that the upper layer of the sediment is likely to have a loose structure with low density, hence it does not necessarily require the body burrowing in it to be cylindrical.
It has been suggested that radial and bilateral body plans could have been generated with the same or similar genetic toolkit but with different regulatory networks [ 8 , 26 — 28 ]. This means that most likely there was no genetic barrier to the emergence and evolutionary competition of the two body plans.
Whatever the case, we argue that this competition was strongly determined by the physical laws of locomotion. Here, we do not consider the temporal priority of radial or bilateral symmetry in early animal evolution but see refs. We only state that, from the moment bilateral symmetry arose in macro-animal evolution it represented a potentially enormous selective advantage over other body plans assuring faster changeovers and a more precisely directed locomotion.
This is a key to survival both for prey and for predators. At the system level, these considerations mean animal evolution should be viewed as a strongly channelled process cf. Very probably, other key selective forces also influenced and influence the evolution of basic animal body plans. However, these factors are yet to be explored. We propose one of them — one that may in itself be enough to favour bilateral symmetry against other symmetries.
This is an interesting analysis providing a physical explanation for the maintenance of bilateral symmetry in animal evolution. I find the paper well written, the arguments convincing, and only have a few comments to clarify the discussion. The authors refrain from discussing locomotion in the microscopic world. However, I think that they miss an opportunity here. We know that in the micro world many organisms can navigate very efficiently. They achieved this not by being bilaterally symmetrical, but by using helical swimming and the adjustment of the helical trajectories.
This happens very often in diverse phototactic protist e. Chlamydomonas, dinoflagellates and in the close-to spherical ciliated larvae of bilaterians e. One reason why this is an effective strategy for small organisms but not large ones is, as discussed in the paper, the different Reynolds numbers.
This could imply that bilaterality only evolved once the early metazoans had attained a sufficiently large size. This interesting physical threshold could be discussed in more detail. Importantly, she notes, not all animals are bilaterally symmetrical. Some animals have radial symmetry with four or five axes, like starfish, jellyfish and sea urchins. The only creature on Earth who is not symmetrical in any way is the sponge.
And why are E. Standen thinks it's likely because the symmetry that surrounds us in the real world guides human creativity and imagination. A taxonomic clade, Bilateria, includes animals with bilateral symmetry. These animals also referred to as bilaterians have left and right sides to distinguish them from those with different form of symmetry e. Bilateral symmetry in the plant kingdom is exemplified by the orchid and pea families. Synonym s :.
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