Whether the proponents of hell or heaven theories finally convince their rivals of the most plausible scenario for the origin of the first replicating structures, it is clear that the origin of life is not a simple issue. One problem is the definition of life itself. From the ancient Greeks up through the early nineteenth century, people from European cultures believed that living things possessed an elan vital, or vital spirit—a quality that sets them apart from dead things and nonliving things such as minerals or water. Organic molecules, in fact, were thought to differ from other molecules because of the presence of this spirit. This view was gradually abandoned in science when more detailed study on the structure and functioning of living things repeatedly failed to discover any evidence for such an elan vital, and when it was realized that organic molecules could be synthesized from inorganic chemicals. vitalistic ways of thinking persist in some East Asian philosophies, such as in the concept of chi, but they have been abandoned in Western science for lack of evidence and because they do not lead to a better understanding of nature.
How, then, can we define life? According to one commonly used scientific definition, if something is living, it is able to acquire and use energy, and to reproduce. The simplest living things today are primitive bacteria, enclosed by a membrane and not containing very many moving parts. But they can take in and use energy, and they can reproduce by division. Even this definition is fuzzy, though: what about viruses? viruses, microscopic entities dwarfed by tiny bacteria, are hardly more than hereditary material in a packet—a protein shell. Are they alive? Well, they reproduce. They sort of use energy, in the sense that they take over a cell's machinery to duplicate their own hereditary material. But they can also form crystals, which no living thing can do, so biologists are divided over whether viruses are living or not. They tend to be treated as a separate special category.
If life itself is difficult to define, you can see why explaining its origin is also going to be difficult. Different researchers stress different components of the definition of life: some stress replication and others stress energy capture. Regardless, the first cell would have been more primitive than the most primitive bacterium known today, which itself is the end result of a long series of events: no scientist thinks that something like a modern bacterium popped into being with all its components present and functioning! Something simpler would have preceded it that would not have had all of its characteristics. A simple bacterium is alive: it takes in energy that enables it to function, and it reproduces (in particular, it duplicates itself through division). We recognize that a bacterium can do these things because the components that process the energy and allow the bacterium to divide are enclosed within a membrane; we can recognize a bacterium as an entity, as a cell that has several components that, in a sense, cooperate. But what if there were a single structure that was not enclosed by a membrane but that nonetheless could conduct a primitive metabolism? Would we consider it alive? It is beginning very much to look like the origin of life was not a sudden event, but a continuum of events producing structures that, early in the sequence, we would agree are not alive, and at the end of the sequence, we would agree are alive, with a lot of iffy stuff in the middle.
We know that virtually all life on Earth today is based on DNA, or deoxyribonucleic acid. This is a chainlike molecule that directs the construction of proteins and enzymes, which in turn direct the assembly of creatures composed of one cell or of trillions. A DNA molecule instructs cellular structures to link amino acids in a particular order to form a particular protein or enzyme. It also is the material of heredity, as it is passed from generation to generation. The structure of DNA is rather simple, considering all it does. A DNA molecule that codes for amino acids uses a "language" of four letters— A (adenine), T (thymine), C (cytosine), and G (guanine)—which, combined three at a time, determine the amino-acid order of a particular protein. For example, CCA codes for the amino acid proline and AGU for the amino acid serine. The exception to the generalization that all life is based on DNA is viruses, which can be composed of strands of RNA, another chainlike molecule that is quite similar to DNA. Like DNA, RNA is based on A, C, and G, but it uses uracil (U) rather than thymine.
The origin of DNA and proteins is thus of considerable interest to origin-of-life researchers, and many researchers approach the origin of life from the position that the replication function of life came first. How did the components of RNA and DNA assemble into these structures? One theory is that clay or calcium carbonate— both latticelike structures—could have provided a foundation upon which primitive chainlike molecules formed (Hazen, Filley, and Goodfriend 2001). Because RNA has one strand rather than two strands like DNA, some scientists are building theory around the possibility of a simpler RNA-based organic world that preceded our current DNA world (Joyce 1991; Lewis 1997), and very recently there has been speculation that an even simpler but related chainlike molecule, peptide nucleic acid (PNA), preceded the evolution of RNA (Nelson, Levy, and Miller 2000). Where did RNA or PNA come from? In a series of experiments combining chemicals available on early Earth, scientists have been able to synthesize purines and pyrimidines, which form the backbones of DNA and RNA (Miller 1992), but synthesizing complete RNA or DNA is extraordinarily difficult.
After a replicating structure evolved (whether it started out as PNA or RNA or DNA or something else), the structure had to acquire other bits of machinery to process energy and perform other tasks. Some researchers, the so-called metabolismfirst investigators (Shapiro 2007), are looking at the generation of energy as the key element in the origin of life. In this scenario, replication is secondary to the ability to acquire energy.
Finally, this replicating and energy-using structure had to be enclosed in a membrane, and the origin of membranes is another area of research into the origin of life. A major component of membranes are lipids, which are arranged in layers. Precursors of lipids, layered structures themselves, apparently form spontaneously, and models are being developed to link some of these primitive compounds to simple membranes capable of enclosing the metabolizing and reproducing structures that characterize a cell (Deamer, Dworkin, Sandford, Bernstein, and Allamandola 2002). The origin of life is a complex but active research area with many interesting avenues of investigation, though there is not yet consensus among researchers on the sequence of events that led to the emergence of living things. But at some point in Earth's early history, perhaps as early as 3.8 billion years ago but definitely by 3.5 billion years ago, life in the form of simple single-celled organisms appeared. Once life originated, biological evolution became possible.
This is a point worth elaborating on. Although some people confuse the origin of life with evolution, the two are conceptually separate. Biological evolution is defined as the descent of living things from ancestors from which they differ. Evolution kicks in after there is something, like a replicating structure, to evolve. So the origin of life preceded evolution, and is conceptually distinct from it. Regardless of how the first replicating molecule appeared, we see in the subsequent historical record the gradual appearance of more complex living things, and many variations on the many themes of life. Predictably, we know much more about biological evolution than about the origin of life.
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