10.1–10.3: Life on earth most likely originated from non-living materials.

Life on earth arose in a very different environment than we experience today. (Shown here: the formation of volcanic rock in Hawaii.)

In the beginning, there was nothing. Now there is something. That, in a nutshell, describes one of the most important, yet difficult to resolve, questions in science: how did life on earth begin? We open our investigation by describing a few things that we do know and exploring how they help us speculate about the things we do not yet know. First, let’s give a basic definition of what we mean by “life.” Life is defined by the ability to replicate and by the presence of some sort of metabolic activity (the chemical processes by which molecules are acquired and used and energy is transformed in controlled reactions). In the next section we expand on this definition and explore the transition from non-living to living.

Earth formed about 4.5 billion years ago from clouds of dust and gases, and as it very gradually cooled, a crust formed at the surface and condensing water formed the oceans. This probably took several hundred million years. The oldest rocks, found in Canada, are about 3.8 billion years old. And the earliest life forms appeared not long after these first rocks formed: fossilized bacteria-like cells have been found in rocks that are 3.4 billion years old.

From that initial point in earth’s formation, tremendous biodiversity arose—variety and variability among all genes, species, and ecosystems. In this chapter we explore how this biodiversity might have come to be, how we name groups of organisms, and how we determine the relatedness of these groups to one another. We begin by returning to the question of the origin of life on earth.

How did these first organisms arise? Some have suggested that life may have originated elsewhere in the universe and traveled to earth, possibly on a meteor. It is hotly debated, however, whether microbes could even survive the multi-million-year trip to earth in the cold vacuum of space with no protection from ultraviolet and other forms of radiation. Experimental data have been unable to answer this question definitively. The vast majority of scientists believe, instead, that life originated on earth, probably in several distinct phases.

Phase 1: The formation of small molecules containing carbon and hydrogen. The conditions on earth around the time of the origin of life were very different from those of today. In particular, chemical analyses of old rocks reveal that no oxygen gas was present. The atmosphere included large amounts of carbon dioxide, nitrogen, methane, ammonia, hydrogen, and hydrogen sulfide. Most of these molecules were produced by volcanic eruptions. It was this environment that probably served as the cradle of life, or what Darwin called the “warm little pond” (FIGURE 10-1).

Figure 10.1: Darwin’s “warm little pond.” The first life on earth tolerated an atmosphere without oxygen.

Critical to the origin of life was the formation of small molecules containing carbon and hydrogen. As we saw in Chapter 2, carbon readily bonds with hydrogen and with other carbon atoms, creating molecules with a huge variety of forms that interact with each other in reactions that would eventually become the processes of life. There are several plausible scenarios for how these small organic molecules might have formed. The most likely comes from some simple but revealing four-step experiments done in 1953 by a 23-year-old student named Stanley Miller and his advisor, Harold Urey (FIGURE 10-2).

Figure 10.2: A promising first step. The Urey-Miller experiment generated organic molecules from hydrogen, methane, and ammonia.

• 1. They created a model of the “warm little pond” and their best estimate of earth’s early atmosphere: a flask of water with H₂, CH₄ (methane), and NH₃ (ammonia).
• 2. They subjected this mini-world to sparks, to simulate lightning.
• 3. They cooled the atmosphere so that any compounds formed in it would rain back down into the water.
• 4. They waited, and then they examined the contents of the water to see what happened.

They didn’t have to wait long to get exciting results. Within a matter of days—not millions of years or even a few months—they discovered many organic molecules, including five different amino acids, in their primordial sea. (Using more sensitive equipment, recent re-analyses reveal that all 20 amino acids present in living organisms were produced during the course of these experiments.) This was the first demonstration that complex organic molecules could have arisen in earth’s early environment. However, questions remain, such as whether it is reasonable to think that the environment Urey and Miller assumed to exist on the early earth is actually likely to have existed. Still, the experiments are a promising first step, suggesting that complex organic compounds—including amino acids, the primary constituents of proteins and an essential component of living systems—could have been produced from inorganic chemicals and lightning energy in the primitive environment of earth.