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The origin of life on Earth is a highly curious thing. In fact, many scientists have dedicated their entire lives to finding out how life came to be on Earth. There are a few key experiments that we will look at in order to gain an understanding of how scientists have best hypothesized how life started on Earth, but let's first take a trip back in time - about four billion years ago.
Primitive Earth was very different than the way things are now. There were probably many oceans and seas with many hot vents at the bottom of these waters and quite a bit of volcanic activity on land. The atmosphere most likely contained water, methane, ammonia, and hydrogen, unlike our current atmosphere, which is mostly nitrogen and oxygen.
Now that we have an idea of what Earth may have looked like, let's take a look at what steps scientists have hypothesized led to early life. It is agreed by scientists that there are four main stages to how life came from non-living things.
The first step is that small organic molecules - such as amino acids that make proteins and nucleotides that make DNA - were made. While these organic molecules are found in living things, they aren't actually living things themselves, but are really just specific combinations of elements.
The second step is that these small organic molecules joined together to form larger molecules. The small molecules are called monomers since they are made of just one unit. However, when they join together, they create polymers that have many repeating units. You may be able to remember this because of the prefixes. 'Mono' means 'one' - like in the words monorail and monocle - while 'poly' means 'many' - like in polygon and polymorph. You can also think of it as putting paper clips together in a long chain. Each individual paperclip is a monomer, but the entire long chain of paperclips is a polymer.
The third step of early life on Earth is when things start to get a little tricky. The polymers that were formed from the monomers grouped together to form protobionts. Protobionts are very important to understanding early life. The name protobionts literally means 'early form of life,' but they are basically small droplets with membranes that are able to maintain a stable internal environment. They are similar to the cells with which we are familiar in that they can reproduce, metabolize, and even respond to their environments. Many experiments have shown that these pre-cell structures can spontaneously form.
The fourth step is that these simple protobionts evolved to pass on genetic information. Protobionts are capable of replicating - that is, they can make new protobionts. However, cells, which are the basic unit of life, are unique in that they can reproduce and pass on genetic information from one generation to the next, metabolize matter and energy, and can evolve. These simple cells were created from complex molecules that were created from simple molecules, then continued to evolve into a wide variety of life forms.
Now that we know the basic steps hypothesized to go from non-living chemicals to life, you may be asking yourself how this all happened. While we don't have a complete record of what actually happened, based on evidence and experimentation, scientists have agreed upon a few things.
The first widely accepted idea was proposed by a Russian chemist in the 1920s. A.I. Oparin proposed that the Earth's early atmosphere was very reactive and, along with lightning and UV radiation, was able to reduce substances. Now, when chemists talk of reducing substances, they don't mean making them smaller like when we reduce our debt. To chemists, reduction means adding electrons to molecules.
Along with this highly reactive atmosphere, Oparin thought that the early oceans contained an organically rich solution. This solution containing many essential elements and compounds is commonly referred to as a primordial soup. Based on this, we generally consider Oparin's hypothesis to be that early life on Earth formed through a series of reactions that made simple compounds gradually more complex.
While Oparin's hypothesis was widely accepted, he didn't actually test the idea. This came later in the 1950s when two men, Stanley Miller and Harold Urey, created a contraption to test the idea of a reducing atmosphere and nutrient-rich oceans creating life. While this may sound like a simple task, the design and implementation of the Miller-Urey experiment were very tedious.
First, the scientists needed to decide how to create a contraption that simulates the conditions of early Earth while keeping it self-contained. They decided on a structure that looks like the picture to the right. We can see that there are areas for the simulated atmosphere and for the nutrient-rich primordial soup. There were several sealed valves included in their design in order to allow for the placement of gases for the atmosphere as well as places to collect the gases and substances created.
Second, Miller and Urey had to decide what to put into their contraption. Based on evidence and speculation, they decided to include hydrogen (H2), methane (CH4), ammonia (NH4), and water vapor (H2O) for the atmosphere. They didn't include pure oxygen as it is agreed that early Earth didn't have much oxygen. They included hydrogen as it is the simplest element. Methane contained the life-essential element carbon and is a common product of things like volcanic eruptions, which were probably common on early Earth. The ammonia is also a common product of volcanic eruptions and contains nitrogen, which is essential for proteins and DNA. The water vapor provided the element oxygen, which we require for life.
Along with these four substances, Miller and Urey decided to use sparks to simulate lightning. Remember that Oparin proposed that lightning and UV radiation provided the energy needed for these simple substances to react and form more complex compounds.
The most important aspect of the Miller-Urey experiment is the results. While there were no living cells produced in their experiments, they did find organic molecules. Specifically, amino acids and oily hydrocarbons were formed. Amino acids are the building blocks of proteins, while hydrocarbons contain mostly hydrogen and carbon. Hydrocarbons, such as petroleum, are generally used for energy. The results of the Miller-Urey experiment indicate that the conditions hypothesized by Oparin may indeed have produced early forms of life.
Additional experiments with slight variations of the conditions of early Earth resulted in the production of nucleic acids, carbohydrates, and lipids. Again, while these substances aren't living things themselves, they are essential to life: nucleic acids contain our genetic information, carbohydrates - such as sugars and starches - are a main source of energy, and lipids are fats.
Scientists generally agree on a few key steps regarding the formation of life from non-living things on early Earth. We first went over these four essential steps before exploring the work of Oparin and the Miller-Urey experiment.
Early Earth was a very different place than it is today. In order for life to have formed, scientists accept that several essential steps must have occurred. First, small organic molecules - such as amino acids, which make proteins, and nucleotides, which make DNA - were made. Second, these small monomers combined to form larger and more complex polymers. Third, these polymers were grouped together, and protobionts were formed. Remember that protobionts are small droplets with membranes that are able to maintain a stable internal environment and are basically precursors to cells. The fourth step is that these simple protobionts evolved to pass on genetic information. The replicating molecules that passed on genetic information were the first cells, which then evolved into countless other forms of life.
In the 1920s, Oparin hypothesized the conditions of early Earth needed to create early life. The Oparin hypothesis basically says that early life on Earth formed through a series of reactions that made simple compounds gradually more complex. Oparin said that the atmosphere was reducing, meaning that it added electrons to atoms. The energy needed to do this was provided by lightning and UV radiation. Also, essential on early Earth was the organic-rich oceans, which are commonly called the primordial soup.
In the 1950s, Miller and Urey tested the Oparin hypothesis. They created a device to see if life can be created from non-living things. In their device, they used sparks to simulate lightning and created an atmosphere containing hydrogen, methane, ammonia, and water vapor to resemble that of early Earth. The Miller-Urey experiment resulted in the creation of organic molecules, such as amino acids and hydrocarbons.
While we weren't there when life was formed on early Earth, we continue to learn more about the history of life on Earth through evidence and experimentation.
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Back To CourseCLEP Biology: Study Guide & Test Prep
24 chapters | 224 lessons