The origin of life is one of the most interesting questions in biology. However, it also is one of the most difficult to address. There was no one there to witness the process, therefore we must deduce what happened through other means of inquiry. To address how life originated, we can perform experiments and examine organisms that live today. To address where this may have happened, we can examine existing organisms and perform experiments. Finally, to address when life first arose, we can look to the fossil record, as well as use the information from molecular clocks.

The universe formed about 10-15 billion years ago in a process known as The Big Bang. Earth formed 4.5 billion years ago. How do we know this date so precisely? There are no rocks on earth that are this old; the oldest rocks date to about 4 billion years ago (Figure 2.22). However, there are moon rocks, collected on manned missions to the moon, that have been dated to 4.5 billion years. Because it is assumed that the Earth and the moon formed at approximately the same time, these rocks provide information on the age of the Earth.

A Single Origin of Life

At some point between 4 billion and 3.5 billion years ago, Earth was transformed from a non-living system to a living system. This time is constrained by the cooling of the Earth's crust. There was probably no constant surface water before about 4 billion years. The fossil record indicates that by 3.5 billion years there were prokaryotes on Earth. The evidence suggests that all life today had a single common ancestor, or a single origin. The most compelling argument for this is that all organisms use the same triplets of nucleotides to code for amino acids. This universality of the genetic code across all known organisms indicates a single common ancestor.

Early Conditions on Earth

The early Earth was very different from the planet we a familiar with. The surface would have been a chaotic place, with strong volcanic activity. Volcanoes today produce large amounts of water vapor and carbon dioxide, and it is assumed that the same would have been true early in Earth's history. Earth also would have experienced heavy bombardment by bolides, extraterrestrial objects such as asteroids and meteorites. While those impact craters are no longer apparent today, we only have to look at the surface of the moon to see evidence that bolide impacts were common in the early solar system. The atmosphere of early Earth also would have been very different. It would have been composed primarily of nitrogen, water vapor, carbon dioxide, with trace amounts of carbon monoxide, hydrogen and methane. Perhaps most importantly, there would have been little or no free oxygen. Therefore, the earliest forms of life were most likely anaerobic, that is they did not require oxygen for their cellular metabolism. Energy sources on the early earth would have been lightning, which is produced during volcanic eruptions, and sunlight. While the sun was weaker at that time, there was no protective ozone layer in the atmosphere; therefore more ultraviolet (UV) radiation would have struck Earth's surface.

The Miller-Urey Experiments

In 1953, Stanley Miller, then a graduate student, and Harold Urey, devised an experiment that would cast new light on the origin of life. They set up an apparatus that simulated the atmosphere of early Earth. They then applied energy, in the form of an electric spark, and collected the chemicals that resulted from this reaction. They were able to synthesize a number of organic molecules from inorganic components, including some amino acids. Their experiments have come under scrutiny in recent years because the "atmosphere" that they used was more H rich than the early Earth's atmosphere is now believed to be. But, if this experiment is run using a less reducing atmosphere, biological molecules or their precursors are synthesized. Formaldehyde (H₂CO) is produced, and sugars can be derived from this molecule. Nucleotide bases can be synthesized from HCN, which is derived from methane. Finally, some molecules, such as phosphate, could come from the weathering of rock. But, it is clear that these experiments have shown that biotic compounds can be synthesized from abiotic compounds.

Where did life originate?

This question of where life originated is very difficult to address. Darwin spoke of a warm little pond as home for the first life forms. The support for this is that organic molecules can be synthesized from a simulated early atmosphere. It has been suggested that these compounds could rain down into pools, where further biochemical reactions would then occur. However, there are a number of arguments against this scenario. Remember, in our discussion of polymerization, that many of those reactions are a dehydration synthesis, with the loss of a water molecule.

Some biochemists argue that these reactions would have been unlikely to occur spontaneously in the water. Also, the surface of the early earth was most likely repeatedly sterilized by bolide impacts. As mentioned above, we can see impact craters on the surface of the moon. Some are very large. The Mare Imbrium crater was caused by a bolide about 70 miles in diameter.

Recently, some researchers have argued that life may have originated at deep-sea hydrothermal vents. Today, these vents house a diverse array of organisms whose ecosystem is based on chemosynthesis, rather than photosynthesis. Proponents of this theory point to DNA sequence information from organisms that live today. The data shows that many of the most divergent (oldest) lineages within the bacteria and archaea are thermophiles, species that live in very hot environments. However, this also could be the result of a selective event early in the history of life.

Finally, some researchers have suggested that earth was "seeded" by complex organic molecules from space. This theory is called panspermia. In 1969, a meteorite fell near Murchison, Australia. When that meteorite was examined, it was found to carry many organic chemicals, including amino acids, and in proportions similar to those generated by the Miller and Urey experiments. Most recently, it has been suggested that a Martian meteorite collected in Antarctica contains evidence that life existed on that planet. Obviously, this question will continue to be debated for many years to come.

How did the first macromolecules and cells form?

If polymerization in an aqueous environment does not seem likely, how did the first macromolecules form? It has been shown that polymers can be created in the laboratory without biological catalysts. In these experiments, dilute solutions of monomers, such as amino acids or nucleotides, are placed on a hot, mineral substrate, such as clay or sand. The water vaporizes and short polymers will form. It also has been shown that macromolecules will aggregate in solution. Microspheres, which are small droplets composed of protenoids (short polymers of amino acids), have been synthesized in the lab. These do show some properties of cells, including selective permeability.

How did early life transmit genetic information?

For many years, scientists were puzzled by a seeming paradox. DNA carries genetic information and the information is used to direct the synthesis of proteins within a cell. However, in order for this message to be transcribed from the DNA, or for the DNA to replicate, proteins are required. This leads to the question of which came first -- DNA or proteins?

In the 1980s, it was discovered that RNA, in addition to carrying genetic information, could, as "ribozymes", catalyze some basic reactions. This raised the possibility that perhaps the early earth was an RNA World, with RNA both carrying genetic information and catalyzing chemical reactions. This also is supported by the many functions of RNA within the cell; it is integral to all aspects of protein synthesis, including the formation of the peptide bond.

When did life originate?

The first prokaryotic fossils date to approximately 3.5 billion years ago. Many of these earliest fossils are microfossils, requiring a microscope to detect them. There are fossils of structures that look very much like present day stromatolites. These stromatolites are formed by mats of cyanobacteria, and are only found in very saline marine environments from which grazing animals are excluded. There are some chemical fossils dated to 3.8 billion years that suggest biological activity because they are enriched in the isotope of carbon, C12, that is used by living organisms. However, because there are relatively few rocks older than 3.5 billion years, fossils of the very earliest organisms may never be found. But, from the available evidence, it is likely that life originated and diversified within a 500 million year period; that earliest life was probably prokaryotic, and anaerobic. Aerobic respiration and the eukaryotic cell arose about 1 billion years later. In the next lesson, we turn our attention to cell structure, focusing on the eukaryotic cell.