• Adding Time
• How we know what happened when
  • Radiometric dating
  • Stratigraphy
  • Molecular clocks
• Important events in the history of life
  • Origins of life
  • How did life originate?
  • Looking for the oldest fossils
  • Studying the origin of life
  • Human evolution

Studying the origin of life

The origin of life might seem like the ultimate cold case: no one was there to observe it and much of the relevant evidence has been lost in the intervening 3.5 billion years or so. Nonetheless, many separate lines of evidence do shed light on this event, and as biologists continue to investigate these data, they are slowly piecing together a picture of how life originated. Major lines of evidence include DNA, biochemistry, and experiments.

Origins and DNA evidence
Biologists use the DNA sequences of modern organisms to reconstruct the tree of life and to figure out the likely characteristics of the most recent common ancestor of all living things — the "trunk" of the tree of life. In fact, according to some hypotheses, this "most recent common ancestor" may actually be a set of organisms that lived at the same time and were able to swap genes easily. In either case, reconstructing the early branches on the tree of life tells us that this ancestor (or set of ancestors) probably used DNA as its genetic material and performed complex chemical reactions. But what came before it? We know that this last common ancestor must have had ancestors of its own - a long line of forebears forming the root of the tree of life - but to learn about them, we must turn to other lines of evidence.

Another important line of biochemical evidence comes in the form of surprisingly common molecules. As you might expect, many of the chemical reactions occurring in your own cells, in the cells of a fungus, and in a bacterial cell are quite different from one another; however, many of them (such as those that release energy to power cellular work) are exactly the same and rely on the exact same molecules. Because these molecules are widespread and are critically important to all life, they are thought to have arisen very early in the history of life and have been nicknamed "molecular fossils." ATP, adenosine triphosphate (shown below), is one such molecule; it is essential for powering cellular processes and is used by all modern life. Studying ATP and other molecular fossils, has revealed a surprising commonality: many molecular fossils are closely related to nucleic acids, as shown below.

Origins and experimental evidence
Experiments can help scientists figure out how the molecules involved in the RNA world arose. These experiments serve as "proofs of concept" for hypotheses about steps in the origin of life — in other words, if a particular chemical reaction happens in a modern lab under conditions similar to those on early Earth, the same reaction could have happened on early Earth and could have played a role in the origin of life. The 1953 Miller-Urey experiment, for example, simulated early Earth's atmosphere with nothing more than water, hydrogen, ammonia, and methane and an electrical charge standing in for lightning, and produced complex organic compounds like amino acids. Now, scientists have learned more about the environmental and atmospheric conditions on early Earth and no longer think that the conditions used by Miller and Urey were quite right. However, since Miller and Urey, many scientists have performed experiments using more accurate environmental conditions and exploring alternate scenarios for these reactions. These experiments yielded similar results - complex molecules could have formed in the conditions on early Earth.

This experimental approach can also help scientists study the functioning of the RNA world itself. For example, origins biochemist, Andy Ellington, hypothesizes that in the early RNA world, RNA copied itself, not by matching individual units of the molecules (as in modern DNA), but by matching short strings of units — it's a bit like assembling a house from prefabricated walls instead of brick by brick. He is studying this hypothesis by performing experiments to search for molecules that copy themselves like this and to study how they evolve.