Information is fundamental to the nature of the universe. As the universe expands there are increasing numbers of states that can potentially be adopted and so the potential for increasing information. Intelligent agents are able to distinguish whether potential states are random or contain information encoded through abstract symbolism. The ability to perform abstract thinking is the link between information and intelligence which forms the basis for mathematics, computing, and all forms of communication.
People’s experience is that information comes from intelligent beings. The “Search for Extra-Terrestrial Intelligence” (SETI) involves searching for radio signals with specific patterns that convey information. Life’s existence is predicated on a vast amount of information whose source remains unknown. For some, such as SETI staff, the inability to find other sentient beings simply spurs the search on to different corners of the universe. For religious believers, God is the source of the intelligence in the universe. Each individual makes a free choice, informed by experience and logic, in deciding where the universe’s information comes from.
Replication
The distinction between molecules and living organisms is the information contained in the minimum number of instructions for replication. How the first proteins formed is an unsolved issue in origin-of-life research. Proteins have the remarkable ability to assemble themselves into very specific three-dimensional shapes with exactly the right groups positioned to execute their catalytic function. A protein’s shape not only defines the enzymatic active site but is also changed by interactions with other cell components. Ligand binding changes the protein sufficiently to prevent enzymatic activity and effectively functions as a molecular switch, turning an enzyme on and off. The dual functionality of proteins to manipulate the cell’s atomic building blocks and to provide a feedback mechanism signaling the needs of the cell has been called the second secret of life.
The same replication and feedback loops plague the formation of the first functional RNA and DNA. Polynucleotide strands require linking together one specific geometrically complex sugar with one of four nucleotide bases. Mechanisms continue to be discovered for condensing the monomeric units into the polymers required in functional DNA, with some remarkable consequences. For example, clay particles can provide active surfaces that not only facilitate the monomer condensation but also protect nucleic acids from degradation by a variety of energy sources. Small RNA sequences chosen to be partially self-complementary can self-assemble with a high preference for one monomeric “hand,” providing some encouragement for the origin of RNA sequences having the same sense of handedness.
The same information requirement reoccurs in many of the biomolecules required in cells. Proteins, DNA, fats, and a myriad of cell components are built from smaller components, requiring assembly in very specific sequences for normal cell function. If DNA is the software storing the cell’s information then proteins are the hardware that perform the cell’s functions. Some RNA can catalyze the formation of proteins but the amino acids are not specified during this process in the same way in which cells read DNA and select amino acids for protein synthesis. The fundamental problem lies in achieving replication; DNA codes for RNA that directs protein synthesis that, in addition to acting as the catalytic cell workers, ultimately result in the assembly of DNA.
The Beginning of Life
Exactly what separates living and non-living organisms? Defining the transition from organic molecules to life is enormously difficult because living organisms exhibit such remarkable diversity of complexity. Is a virus “alive”? Is an egg alive immediately after the sperm penetrates the cell wall, or not until the 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