to take advantage of resources and withstand challenges in the environment. Third, the variations had to be heritable, so that some variants would increase in number under favorable environmental conditions.

No one yet knows which combination of molecules first met these conditions, but researchers have shown how this process might have worked by studying a molecule known as RNA. Researchers recently discovered that some RNA molecules can greatly increase the rate of specific chemical reactions, including the replication of parts of other RNA molecules. If a molecule like RNA could reproduce itself (perhaps with the assistance of other molecules), it could form the basis for a very simple living organism. If such self-replicators were packaged within chemical vesicles or membranes, they might have formed “protocells”— early versions of very simple cells. Changes in these molecules could lead to variants that, for example, replicated more efficiently in a particular environment. In this way, natural selection would begin to operate, creating opportunities for protocells that had advantageous molecular innovations to increase in complexity.

[RNA: Ribonucleic acid. A molecule related to DNA that consists of nucleotide subunits strung together in chains. RNA serves a number of cellular functions, including providing a template for the synthesis of proteins and catalyzing certain biochemical reactions.]

Constructing a plausible hypothesis of life’s origins will require that many questions be answered. Scientists who study the origin of life do not yet know which sets of chemicals could have begun replicating themselves. Even if a living cell could be made in the laboratory from simpler chemicals, it would not prove that nature followed the same pathway billions of years ago on the early Earth. But the principles underlying life’s chemical origins, as well as plausible chemical details of the process, are subject to scientific investigation in the same ways that all other natural phenomena are. The history of science shows that even very difficult questions such as how life originated may become amenable to solution as a result of advances in theory, the development of new instrumentation, and the discovery of new facts.

The fossil record provides extensive evidence documenting the occurrence of evolution.

Early in the 19th century, naturalists observed that fossils occur in a particular order in layers of sedimentary rock. Older materials are deposited more deeply and thus lie closer to the bottom of sedimentary rock than more recently deposited sediments, although older rocks can sometimes lie above younger rocks where large upheavals in the Earth’s crust have taken place.

[Sedimentary: Rocks formed of particles deposited by water, wind, or ice.]

Fossils that closely resemble contemporary organisms appear in relatively young sediments, while fossils that only distantly resemble contemporary organisms occur in older sediments. Based on these observations, many naturalists, including Charles Darwin’s grandfather, proposed that organisms had changed over time. But Darwin and Alfred Russel Wallace were the first



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