Diversity of Life

Diversity of Life

Introduction

Before we can ask “How did life evolve?” we must first ask “What is life?” Unfortunately, this deceptively simple question has no simple answer; a clear, unambiguous definition of life is elusive. Viruses, for instance, are not alive in the conventional sense but have some properties of living systems. Nevertheless, we can formulate a working definition of life with which most biologists would agree: A living organism is a self-contained, organized entity that uses matter and energy from its environment to grow, maintain itself, and replicate itself with heritable variation (Lazcano 1994a).

Discussion

Paleontology, together with geochemistry and planetology, delineates the timing of life's appearance on Earth. The oldest accepted fossils are approximately 3.5 billion years old. They include several different bacteria-like forms, implying that life arose and began to diversify even earlier. In fact, there is indirect evidence that life was present 3.85 billion years ago, the age of the oldest surviving sedimentary rocks-implying that the Earth had a stable crust and liquid water at this time. The oldest rocks in the solar system-certain meteorites-are about 4.6 billion years old, and the consolidation of the proto-Earth was largely complete about 4.5 billion years ago. From the ages of cratered lunar regions, we know that until about 3.9 billion years ago, massive impacts were frequent enough to melt the Earth's crust and evaporate its oceans, sterilizing most or all of the Earth for several thousand years (Chyba 1993). This leaves, at most, a few hundred million years for life to have originated-not much time, from a cosmic perspective.

In the late nineteenth century, Charles Darwin speculated about life arising in “some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present. …” (Darwin 1887). Modern theories of the origin of life, however, began ...