Is chirality a template of life?
When considering this question, the broadness of the terms involved must first be addressed to best give an answer. “Chirality”, “template” and “life”, can all have more than one meaning depending on context, and hence it is necessary to provide definitions for each term. Chirality refers to any object which cannot be superimposed on its mirror image1. This causes many (the majority of) objects, to be chiral, as without bilateral asymmetry they will have this property. Template refers to anything which serves as a model. Life can be defined as “The condition or attribute of living or being alive; animate existence”1, however, we must still be aware that between organisms, life may be defined differently, as the requirements for life vary between organisms. Most importantly when defining life, it has to be recognised that a certain morphology or structure is necessary for an organism to live. This essay will discuss the relationship between asymmetry and the structure of organisms (due to the significance of structure in enabling life), from various perspectives in order to provide a broad range of supporting evidence.
When we view the morphology of the human body, it is clear that there are many structures that are roughly symmetrical. The central nervous system, skeleton, gross external structure, and to an extent the circulatory and peripheral nervous systems all appear to possess this rough bilateral symmetry. Humans are a good example due to our everyday familiarity with their external structure, however other organisms exhibit this feature as well. (It wouldn't be unlikely for an observer to say these animals were symmetrical, and hence chiral.) On the other end of the scale are single celled organisms, which (exceptionally so when considering the position of the cells organelles and nucleus) rarely appear symmetrical.
Some evidence does suggest that chirality might act as a template for life. Chordates are an example of this. During the gastrulation phase of the embryo (a phase which involves significant Page 1/4
morphogensis, and certainly the formation of a template2, 3), the chordate embryo is bilaterally symmetrical4. This could be used to say chirality is acting as a template, but this is not the complete template as it does not take into account the fact that in order to have life, we must have organs. When organogenesis begins, it quickly becomes clear that there is not a symmetrical template5. The left and right atria of the heart have distinct morphologies4, and the lungs develop three lobes on the right side, but only two on the left6.
In human life, another prime display of asymmetry prevailing as the template is in the brain. The brain is vital for human life. The brain appears symmetrical when its external structure is viewed, but it is often the case that information is only processed in one hemisphere7. The template for the brain allows for neuroplasticity (the ability for neurons to form and break connections with other neurons), we can be certain this is intended model for the brain (template) due to it's presence in normal individuals. Neuroplasticity allows the brain to asymmetrically distribute usage of the brain, giving humans the ability to perform complex tasks8, an ability which humans have since become reliant on (a human could only survive without this if helped by others). Humans can still survive with a single hemisphere8, and even though this is not the template, it is further proof that one of the asymmetry enabling features that the brain has, neuroplasticity, is a life enabling part of the template.
It is also evident that asymmetry is not merely something that can be attributed to being part of the template that enables life in humans or other more derived organisms. There is fossil evidence to show asymmetry in organisms from the Palaeozoic period, with some predators having an asymmetric nervous system, and a tendency to use their right side more9. Due to the presence of this feature in multiple organisms, it is less likely (although not entirely conclusive), that this was an evolutionary advantage, and as a result became part of the organisms template, as they required this feature to live. Page 2/4
To consider all aspects of life, we must consider single celled organisms aswell. This is arguably the strongest evidence that chirality does not have to act as a template for life. Bilateral symmetry has only evolved 550-600 million years ago, when the first “bilateria” (bilateral symmetrical organisms) appear in the fossil record10. As there are many examples of life prior to this (with evidence showing life being around for as much as 3.5 billion years11), and no evidence of bilateral symmetry in them, it is evident that the template for life has been one without chirality more often than not.
All of these points can be used as evidence against the idea that chirality might be used as a template for life. Due to the difficulty in researching it, it is hard to show that it would be impossible for a template for life to be completely chiral; producing an entirely symmetrical organism that could live. However, there is an overwhelming amount of research showing that, not only in humans, but many types of organisms we are aware of, the templates for morphology and structure we encounter for life as we know it are almost never perfectly symmetrical (and little research against this point). It is also demonstrated that this feature of life is not restricted to one area, but in multiple areas (structure in neurology, embryology, evolutionary stages, etc) chirality does not act as a template for life.
References 1. Oxford English Dictionary. 2009. Oxford English Dictionary. [cited 22 March 2009]. 2. Solnica-Krezel L, Labouesse M. Shaping embryos in Barcelona. Nature Cell Biology. 2009; 11:3-6. 3. Hammerschmidt M, Wedlich D. Regulated adhesion as a driving force of gastrulation movements. Development. 2008; 135(22):3625-41. 4. King T, Brown N. Embryonic asymmetry: The left side gets all the best genes. Current Biology. 1999; 9(11):R18-R22. 5. Takano K, Ito Y, Obata S, Oinuma T, Komazaki S, Nakamura H, et al. Heart formation and left-right asymmetry in separated right and left embryos of a newt. International Journal of Page 3/4
Developmental Biology. 2007; 51(4):265-72. 6. Levin M, Mercola M. The compulsion of chirality: toward an understanding of left-right asymmetry. Genes & Development. 1998; (12):763-769. 7. Western. Psychology: Australian and New Zealand edition. John Wiley; 2006. 8. Pulsifer M, Brandt J, Salorio C, Vining E, Carson B, J. F. The cognitive outcome of hemispherectomy in 71 children. Epilepsia. 2004; 45(3):243-54. 9. Babcock L. Asymmetry in the fossil record. European Review. 2005; 13(2):135-143. 10. Mullen L. Astrobiology Magazine [In: Earliest Bilateral Fossil Discovered. June 2004 11. Schopf J, Kudryavtsev A, Agresti D, Wdowiak T, Czaja A. Laser-Raman imagery of Earth's earliest fossils. Nature. 2002; 73(6):416.