Most of the theories of how life on this planet started depends on the concept that at one point in time, the Earth’s environment could only be characterized as a ‘prebiotic soup.’ This atmosphere consisted mostly of water, ammonia, hydrogen and methane. One of the earliest theories suggested that this soup was hit by lightning enough times that the molecules in this atmosphere eventually began to create more complex elements that, over a long period of time, continued to become more complex until life was formed. There are several theories as to just what this process might have looked like.
One theory about how life originated on this planet can be called the extra-terrestrial model. According to this theory, life on earth was introduced by comets crashing into the planet’s surface. If the comets moved fast enough from the origin of life somewhere in space to crash into the earth, the life that existed on them could survive the trip to populate the new earth and develop into new life. Studies have shown that it is possible for comets to carry organic compounds such as amino acids even in the deep cold of space. It has also been determined that there are other places in our own solar system that could support these kinds of compounds. Two examples are Mars and Europa, which is one of the moons of Jupiter. When the comets crashed to earth, it is hypothesized that the complex compounds, or perhaps even small life forms, like small worms, mixed with earth’s natural environment to adapt and change based on the conditions encountered. One problem with this theory is explaining how life forms managed to get on the comets to begin with.
Other theories take a more terrestrial approach such as the ‘genes first’ theory. According to this theory, a specific form of RNA gave birth to life on earth. A seemingly unanswerable puzzle led scientists to the idea that life on earth was formed as the natural processes of catalytic RNA. The puzzle was that life requires DNA for the necessary coding required to create even simple proteins but DNA cannot be formed without catalytic proteins. Trying to answer this question, it was discovered that RNA can also be catalytic and thus provide both the necessary code and the catalyst at the same time. To try to ‘prove’ this theory, scientists tracked it backwards to discover that these catalytic RNA molecules can be formed automatically when RNA nucleotides become activated on the surface of common clay. One of the big questions still to be answered about this theory is how the nucleotides were formed.
A possible answer to this problem is the concept of the metabolism-first approach to how life began on Earth. This theory suggests that iron disulfide, otherwise known as pyrite or fool’s gold, acted as a mineral-based metabolizer. The naturally smooth surfaces of this kind of mineral provided a support for common materials such as carbon dioxide and hydrogen sulfide to mix together in high concentrations and reform into more complex substances including organic compounds. This theory is supported by the fact that both ancient enzymes and modern enzymes still contain traces of iron disulfide. The necessary components for this process to have worked were very available in the environment of early earth, especially when it was discovered that less severe hydrothermal vents were once very active in the planet’s oceans. Today’s ‘black smokers’ are very hot and create an extreme environment unlikely to have been the birthplace of oxygen-utilizing life forms, but the ancient vents were smaller and cooler. The compounds created in these smaller vents would have then been released into the oceans to develop further, perhaps creating the necessary nucleotides for the RNA model.
Another theory builds upon these same ideas but suggests that the compounds developed within the small pockets created between layers of mica. This theory provides the components with the protection and isolated space required for combination and development, including evolution, within an environment that doesn’t require cell membranes to hold everything together as the structure of the mica performed this function. Mica contracts and expands with the heat of the day or the cool of the evening and would have moved with the motion of the tides, providing the necessary energy to encourage the making and breaking of chemical bonds and would have enforced a structure that can still be found in cells today, such as the spacing of component parts.
Works Cited
- “New hypothesis for origin of life proposed.” UC Santa Barbara Press Release. (2007). University of California, Santa Barbara.
- Robinson, R. “Jump-Starting a Cellular World: Investigating the Origin of Life, from Soup to Networks.” PLoS Biology. Vol. 3, N. 11, (2005). Web.
- Shwartz, Mark. “New field of astrobiology has scientists pondering extraterrestrial origin of life.” Stanford Online Report. (2000). Web.