fertilized egg divides? At what point do a group of cells become a baby? Even trying to define life is fraught with difficulties. NASA provides an encompassing definition of “life [as] a self-sustained chemical system capable of undergoing Darwinian evolution.”28
Following science’s effective method of reducing a problem to the smallest discrete component, scientists have focused on the simplest expression of life in unicellular organisms. Simple cells contain four key types of complex molecules: proteins, nucleic acids (DNA and RNA), sugars (polysaccharides), and lipids. Each of these biomolecules is a polymer with a specific cell function: proteins perform the cellular reactions, nucleic acids code for the organization and replication of the cell, sugars trigger recognition events, and lipids form the core component of cell membranes.
DNA is often grandly called “the blueprint for life.” But despite the remarkable code embedded within DNA’s structure, DNA does not come close to the NASA definition of life. Rather, DNA behaves like many other polymers with an overall molecular motion caused by individual vibrations of the constituent atoms. Over time individual DNA molecules will move and bind to various receptors. However, the binding and recognition stems from the attraction between specific types of atoms rather than the inherent “mind” of DNA. The search for the beginning of life must, therefore, look not to smaller atomic entities but to larger structures of which DNA occupies just one critical role among many.
DNA contains an incredible amount of information. Tightly stuffed into cells, DNA would stretch to about 6 feet if drawn out from a human cell and unwound. Virtually all living organisms use DNA for storing the biological code; the chemical composition of the double helix varies between individuals and species but is universal for life on earth. How DNA came to be on earth 3.5 billion years ago is not known, but the common sequences between very diverse organisms provide an independent witness for DNA being crucial in life’s beginning.
Living Cells
Scientists have sought to find the key components of life from the earliest studies of biology and chemistry. Historically, people believed that there was a “life force” inherent in living systems that was not present in inorganic materials such as rocks and minerals. Few people believe in a vital life force any more, but, at the same time, the essential ingredients for life to appear and reproduce have also not yet been found.
Protocells represent the link between the synthesis of macromolecules and the appearance of the first living cells. Encapsulating all of the cellular components into an organized protocell is the biological equivalent of a quantum jump in understanding. The centrality of living cells has stimulated much research on the transition from cellular components to the formation of life. No other machine is known to completely assemble itself, creating one of the most daunting challenges in biochemistry. Self-replication in artificial systems is contingent on understanding this self-assembly. Understanding the evolutionary transition to replicating cells offers the potential to understand what life really is.
Cells are the central monomeric unit on which all life is based. Cells use close to a million different components and processes allowing them to function internally, to move, to signal to and find other cells, and to coalesce into multi-cellular organisms. Cells are truly remarkable nanoscale manipulators. Viewing cells gives the impression of a factory running by remote control because cells contain an enormous number of feedback loops to ensure that the right components are present in the cell. From an evolutionary perspective, the first cells would have a much simpler number of components that were later able to add additional levels of complexity.
A lipid-based cell wall provides a robust compartment capable of recognizing and excluding foreign invaders while providing a safe passage for cell metabolites. Localized within the cell are smaller entities that provide the energy for the cell (mitochondria) to synthesize proteins (Golgi), and form a central cognitive system (nucleolus). Efforts to mimic simple cells have identified reactions that can be performed inside cell walls, although these are controlled by the inherent reactivity of the chemicals rather than by a central cognitive system. Complementing this build-it-yourself strategy is the construction of artificial cells by inserting a minimal set of enzymes, nucleic acids, and cell metabolites to bring the cell to life. Simple processing by strings of RNA is possible but a great divide exists between simple chemical reactions and a cell capable of the three defining characteristics; metabolism, self-reproduction, and evolution. At the heart of the dilemma is the paradox of life: the cell components, the membrane proteins, RNA, and DNA are all interdependent. The cell wall and membrane encapsulate these key molecules in a safe environment which in turn requires proteins, DNA, and RNA for their synthesis. How cells became self-replicating is one of the most incomprehensible processes in biology.
An enormous gulf exists between simple and artificial cells and the simplest cellular organism. Genetic experiments aimed at determining how many genes are required in the simplest cell indicate that about 250 genes are minimally required for cell function. For a simple bacterium the genome consists of around 106 nucleotides, representing one DNA sequence out of a possible 102.4 million. The chance of randomly forming the genome is vanishingly small. Once the construction of the first living cell through synthetic assembly is achieved, if the endeavor is even possible, this will only provide a shadowy, though monumental, contribution to understanding the origin of life.
Perhaps the most striking aspect of the evolution of life on the earth is that it happened so fast. Scientists have suggested that life may be almost as old as the earth with an origin that may have virtually coincided with the birth of the planet. As an example, the population of organic walled microstructures from the Swaziland System, South Africa, found in 1977, was identified as the morphological remains of primitive prokaryotes. The rocks were dated as 3.4 billion years old, relatively close to the age of earth at 4.5 billion years. Despite dramatic advances in molecular biology, there is still no agreement in how life first began. Where and how life began is one of science’s great mysteries. Guesses range from life being a spectacularly successful accident to being the expected outcome of a universe primed for life.
Living cells are the most complex small systems in the universe. Specialized molecules work in concert, seamlessly conveying messages to ensure that the cell performs exactly the right function within the living organism. Most perplexing is the lack of an intelligent agent controlling the cell; life is sustained and replicated by individual organisms themselves. How the first single-celled organisms came into being is a puzzle which science has been trying to unravel. At the root of the problem is a philosophical issue: from where did life’s instructed complexity come? Organisms literally have a life of their own.
Information has to come from somewhere. DNA is full of information for protein synthesis, some of which forms the machinery to make and repair DNA. Random mutation can give rise to new sequences of potential information, requiring some screening process to weed out the beneficial mutations. That screening process is again another information source. Where did all the information come from in the beginning, and how did the protein-DNA symbiosis come into being? DNA is the cell’s software which delivers the message for protein synthesis on the cell’s main-frame. The origin of this information-rich system is currently unknown. Perhaps the information was primed into the universe from the beginning, although how this was done remains highly speculative.
The classic view of life’s origin is through a series of key events, each building on prior levels of complexity. Empty vesicles, like oil droplets, encapsulated primitive biomolecules whose chemistry powered the cell’s energy needs. During the progressive development, the enzymatic production of DNA was adopted, improved, and became an intimate part of cellular programming. The origin of many key steps in the development of life is as yet unidentified. Some argue that science will eventually be able to discover exactly how self-replicating, complex organisms first came into being. Others argue that life is too complex to understand completely and that while dramatic advances will continue, understanding life’s origin will always be elusive. Both are philosophical speculations.
Conclusion
