Carlos E. Semino - The Never-Ending Story of Life A Brief Journey through Concepts of Biology

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For humankind, the most irreducible idea is the concept of life itself. Life is the principal driving force behind many disciplines, including philosophy, literature, biological sciences, medicine, mythology, and theology, but no matter how much we know about it, it will always remain our greatest mystery. Although some scientists study the origins of life to examine the group of pre-biotical, biochemical reactions that may have spontaneously created it, this book begins from the premise that life is a factthat it is everywhere; that it takes infinite forms; and, most importantly, that it is intrinsically self-perpetuating. Rather than exploring how the first living forms emerged in our universe The never-ending story life begins with our first primordial cell ancestor and tells the story of lifes journeyhow it began when that first cell diversified into many other cell types and organisms, and how it has continued until the present day.

This small book is designed to explain complex ideas in biology in a simple way, but not simplistically, with a special emphasis in taking care of the language as well as illustrations that work for all possible readers. Thus, the central concept that this compact but essential book illuminates for biology students and nonbiologists alike is the fact that life is essentially an infinite process, transmitted from generation to generation. Curiously, the strategy that life uses to survive is based on a simple mechanism: it generates a vast amount of highly diverse organisms so that every ecological niche is inhabited by specialized living forms. The remarkable heterogeneity of these organisms ensures that a great proportion of them will be able to adapt to the constantly changing environment. Through this process, life can take infinite biological forms, from simple cells to complex animals, fungi, and plants. Each of these individual organisms has a lifespan and must generate descendants (i.e., offspring) that will be different from their ancestors, because living organisms have intrinsic mechanisms to generate genetic diversity. This diversity is generated when a cell divides to produce two new ones, because genetic changes (i.e., mutations) occur independently during the division process, producing daughter cells that are not exactly identical to the original cell. During this process, life transmits itself to the next generation simply by splitting into two and passing from one cell into the two new ones.

Thus, the book explains how life is passed on during every cell division . It never jumps from one organism to another; instead, it flows on to the next generation. This is a very important concept, because regardless of whether an organism is unicellular or multicellularwhether it is a small bacterium or a whalelife flows exclusively through the process of cell division. This is why cells are the most evolutionary form of life and the smallest building block capable of passing it on. This may be hard to understand at first, but when we consider that even the most complex animals and the largest trees almost all rely on unicellular cells ( gametes ) to transfer life to the next generation, it becomes clear that cell division is the essential mechanism of lifes self-perpetuation. This is also true in some cases where complex structurespieces of tissues rather than single cellsare used to generate more organisms, as in the processes of animal fragmentation and vegetative propagation in plants, where life splits up to continue self-propagating.

In all of these processes, life is immortal , but organisms are not. Each organism has a lifespanhours, days, weeks, or years, depending on the casebut in the end, it must die. The reason for this is simple: organisms have to generate diversity, because this is the only natural way for it to create genetically distinct descendants, and in most multicellular organisms, this process naturally leads to death. Cell division drives growth and development, maintains tissue and organ homeostasis, executes the bodys regeneration and repair, and generates gametes , but it also promotes aging and death. In other words, each organism has an intrinsic program that eventually leads to death. Thus, death is an essential part of life, and each individual organism is a carrier of life that can pass it on to the next generation before dying, thereby continuing with the never-ending story of life.

I would like to thank the many people who have contributed to the organization, editing, revision, drawings, and ideas in this book. In particular, I extend my most sincere thanks to Nausika Betriu, Claire Jarrosson Moral, Alba Costales, Oriol Quintana Rubio, Gustavo Tiscornia, Mariana Resnicoff, Joel Paul, and John Shakespear.