The complexity and diversity of living things has challenged thinkers for thousands of years. Has the earth always been filled with living things? Were there living things that have ceased to exist? Did living thing originate on the earth? Do new living things originate today? As our understanding of the laws of nature and the nature of living things have developed, the way we approach these questions has changed. However, the basic questions remain the same: How did life originate? and How and why does it change?

In earlier times, no one ever doubted that life originated from nonliving things. The Greeks, Romans, Chinese, and many other ancient peoples believed that maggots arose from decaying meat; mice developed from wheat stored in dark, damp places; lice formed from sweat; and frogs originated from damp mud. This theory was widely believed until the seventeenth century (figure 20.1); however, there were some who doubted it. These people subscribed to an opposing concept that life originates only from preexisting life.

One of the earliest challenges to the idea that life can be generated from nonliving matter came in 1668. Francesco Redi, an Italian physician, set up a controlled experiment (figure 20.2). He used two sets of jars that were identical except for one aspect. Both sets of jars contained decaying meat, and both were exposed to the atmosphere; however, one set of jars was covered by gauze, and the other was uncovered. Redi observed that flies settled on the meat in the open jar, but the gauze blocked their access to the covered jars. When maggots appeared on the meat in the uncovered jars but not on the meat in the covered ones, Redi concluded that the maggots arose from the eggs of the flies and were not produced from the decaying meat.