Origins of Life: Past and Present Ideas.
|
There have been many notions as to the origins of life. One of the oldest, spontaneous generation, has been around since the time of Aristotle. Spontaneous generation is the idea where life could arise from non-life. For example, damp hay and a dark corner could give rise to mice, while mud could give rise to worms, insects and fish. This idea wasn't completely refuted until the mid-1800s, by Louis Pasteur. Life can only come from life. If this is always the case, then where did the first life form come from? Life must have originated somewhere -- it couldn't have always been around. So initially, wouldn't life have originated from non-life? Yes, but not in the same sense as by "spontaneous generation". The distinction between life and non-life is not so clear-cut. Life is made up of molecules and atoms, but so is inanimate matter. The difference is that life is a lot more complex than non-life. During chemical evolution, more complex molecules were generated from simpler substrates. Eventually, molecular pathways and cycles developed where molecules could be synthesized or broken down. These separate pathways, somehow came together to form more elaborate pathways, which ultimately lead to the emergence of life. Life did not simply arise from non-life. It had to develop in steps. First chemical evolution had to develop complex molecules and pathways, and then some form of "proto-life" developed, from which "life" emerged. How life arose on Earth is still somewhat of a mystery. One idea put forth by H.E. Richter and supported by Crick (co-discoverer of the DNA structure), is panspermia. Panspermia is the notion that Earth was seeded by spores that had travelled through space from another planet. This may sound ridiculous at first, however if you realize that matter can be transported to Earth by meteors or IDPs, the idea becomes more plausible. Earth has only been around for about 4.6 Billion years, and there is evidence that life has been on here for at least 3.5 Billion years. That leaves only about one billion years for chemical evolution to give way to biogenesis. So, life could have first originated on a much older planet somewhere else, and then was transported due to some large body collision. There are three things that a spore would face on a trip through space: radiation, extreme cold, and extreme heat. UV radiation could kill off any spores, if they were unprotected. If the spores were protected deep within a meteor they might avoid such damage. Space is very cold (~-220 degrees C, on average). There has been some experiments done where microbes were frozen over a period of many years at a temperature of around -220 degrees C, and some of the spores actually survived. So, it is possible that spores could survive a trip through space if they were hardy enough. Then there is the extreme heat upon entry into a planet's atmosphere and ozone, although on early Earth, this could have been avoided, since there was no atmosphere or ozone initially. If panspermia were true, there is still the question as to how/where life did originate. One of the widely held views as to the origin of life, has been the "primordial soup" theory. This was first developed by a Russian scientist, Oparin, in 1924 (and so, is called "the Oparin Ocean"). Oparin held that the Early Earth had a highly reducing atmosphere, and that organic molecules were synthesized from simple reactions. UV light and/or electric discharge were the primary energy sources that drove such reaction. Eventually, the chemicals reached a critical mass (i.e. the primordial ooze), from which a primitive organism had emerged. A coacervate is simply an aggregation of molecules. Organics can accumulate on a water surface (i.e. "Folsomes scum", as seen in Miller-type experiments). Coacervates can form spontaneously out of solution by physical turbulence (e.g. "shaking" -- this would have been done by the action of waves, during prebiotic times). Oparin believed that some primitive life form emerged from such coacervates. Oparin also held that heterotrophs (cells that eat other cells for food) were the first to arise, while autotrophs (cells that can make their own food from simple compounds) were second. Autotrophs developed because nutrients in the oceans diminished to such an extent that further survival depended on the development of photosynthesis. This is in contrast to what is believed today, where autotrophs were first and heterotrophs were second. In 1980, Woese challenged the old "primordial soup" view, since the current evidence no longer supported Oparin's theory. Instead, Woese held that the early atmosphere was less reducing, and rich in CO2. The early surface was very hot and very dry -- ferrous rock (i.e. iron) dissociated H2O into H2 and O2. As such there were no oceans early on. The atmosphere developed by outgassing of the lithosphere. Meteorites that impacted the planet caused temperature differentials, which caused winds to develop. So, early Earth had a lot of dust storms that brought up various chemicals into the atmosphere. Water droplets in the atmosphere dissolved the chemicals, and allowed simple chemical reactions to occur within. Once the planet cooled enough, the water droplets with concentrated chemicals already had a head start in developing life. Thus, Woese suggested that the first protocells developed in the atmosphere rather than in the oceans. Woese agreed that his theory did not explain how a genetic system could arise, but he argued that researchers accepted the old view out of convenience and that it was more important in developing the possible preliminary steps to the origins of life. There are three key features all life is comprised of: a membrane barrier (in modern times, a lipid-protein membrane), metabolism, and a genetic system. The problem is, since lipid membranes are impermeable to ions and polar molecules, if a membrane (or liposome) developed first, how could the first protocell take in the necessary nutrients for a proto-metabolism to develop? If a liposome did form first, all the necessary chemicals would have had to have been present. So, what are the chances that that could have happened? It seems to be quite unlikely. It is possible that a protocytosol developed first, before encapsulation. A genetic system is very complex -- i.e. many diffferent proteins and RNA molecules are needed to carry out replication, transcription, and translation. Proteins and RNA are made from genes, and it is proteins (enzymes) that are used for their synthesis. So, similarly, how did the first genetic system arise? I will elaborate on these theories in the next section.
|
Get your own Free Home Page