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Abstract
BackgroundThe overwhelming majority of animal species exhibit bilateral symmetry. However, the precise evolutionary importance of bilateral symmetry is unknown, although elements of the understanding of the phenomenon have been present within the scientific community for decades.Presentation of the hypothesisHere we show, with very simple physical laws, that locomotion in three-dimensional macro-world space is itself sufficient to explain the maintenance of bilateral symmetry in animal evolution. The ability to change direction, a key element of locomotion, requires the generation of instantaneous “pushing” surfaces, from which the animal can obtain the necessary force to depart in the new direction. We show that bilateral is the only type of symmetry that can maximize this force; thus, an actively locomoting bilateral body can have the maximal manoeuvrability as compared to other symmetry types. This confers an obvious selective advantage on the bilateral animal.Implications of the hypothesisThese considerations imply the view that animal evolution is a highly channelled process, in which bilateral and radial body symmetries seem to be inevitable.ReviewersThis article was reviewed by Gáspár Jékely, L. Aravind and Eugene Koonin.
Highlights
The overwhelming majority of animal species exhibit bilateral symmetry
Even if the slow, cilium-based locomotion on a substrate may explain the generation of bilateral symmetry, it certainly cannot account for its survival over millions of years of animal evolution
Knowing that a rectangular plate has an approximately 50 to 70 % higher drag coefficient than a cylinder [9], we can say that bilateral symmetry offers the evolutionary possibility of increasing F by as much as 50 to 70 % compared to cylindrical symmetry, thanks to the drag coefficient
Summary
Animals show diverse types of symmetry including spherical, cylindrical ( known as perfect radial), radial, biradial, bilateral and asymmetric (for review, see ref. [1]). 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 another evolutionary situation in which the body has to be externally cylindrical: a burrowing lifestyle. 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. 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, it does not necessarily require the body burrowing in it to be cylindrical
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