How Did All This Come to Be?
Origin questions fascinate today’s scientists, just as they have human thinkers since before recorded history. The origin of the universe remains perhaps the greatest cosmological question, with scientists, philosophers, and theologians all staking a claim to the answer. Locally in the cosmos, the origins of our galaxy and solar system are questions of mythic stature that invite broad speculation and foster intense debate. But of all the origin questions, the origin of life is certainly among the most profound. Fortunately, it is also highly amenable to exploration through experimental science, since it is a chemical process that might be duplicated in the laboratory.Two complementary research strategies converge on origin of life questions. The quest to create life in the laboratory began in 1952 when University of Chicago professor Harold Urey and graduate student Stanley Miller devised glassware that sent electric sparks though a primordial atmosphere of methane, hydrogen, and ammonia circulating above warm water. Much to their surprise, in a matter of days the simple solution turned from colorless to pink to red to brown as a rich broth of organic molecules formed. These experiments, which in essence work forward in time from the basic carbon compounds that existed 4.5 billion years ago, when the earth first began to form, reveal that the primitive oceans must have become stocked rather quickly with a variety of relatively complex organic molecules. The earliest oceans and sediments may have grown increasingly concentrated in these organic molecules, for there was no life to gobble up the rich mix.
There is still a tremendous gap between Miller and Urey’s sterile soup of organic molecules and a living cell. But that gap may be narrowed by an alternative research strategy that examines the chemical mechanisms of two of the earth’s most primitive single-celled organisms: mycoplasma and cyanobacteria. The smallest of these, mycoplasma cells, are only about one ten-thousandth of an inch in diameter. The least complex life forms known, these cells depend on their environment to supply many kinds of organic nutrients, including amino acids and nucleic acids. Cyanobacteria, in contrast, are larger and more complex single cells, but they have the ability to survive and reproduce entirely from the most basic ingredients-carbon dioxide, nitrogen, and water, plus a few mineral nutrients.
The structural simplicity of mycoplasma and the chemical simplicity of cyanobacteria can illuminate different aspects of early life. For example, the cellular structures and metabolic pathways by which cells extract energy from sugar are common to all life forms, and must have been present in some fashion in the earliest cells. By paring down metabolism to its most basic chemical reactions, scientists hope to glimpse a plausible sequence of events that might have occurred spontaneously, before the first cell began to reproduce.
The origin of life was a historical event, and many details of that history are still preserved in the chemical structures of cells. Through biochemical studies we can deduce and perhaps reproduce some of the chemical steps associated with that event. But even if someday centuries from now we learn every nuance of the origin of life on our planet, who can predict how many alternate chemical pathways to life may have arisen elsewhere in the cosmos? We can imagine no end to the search for the possible myriad origins of life in the universe.