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The Origin of Life on Earth: Theories and Explanations

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  • 0:05 Primitive Earth
  • 0:50 Stages of Early Life on Earth
  • 3:04 The Oparin Hypothesis
  • 4:07 The Miller-Urey Experiment
  • 6:49 Lesson Summary
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Lesson Transcript
Instructor: Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

Ever wonder how living things came from non-living things? We will explore what conditions on early Earth may have created life from non-living items.

Primitive Earth

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.

The atmosphere of primitive Earth did not contain oxygen and nitrogen, as it does now.
Earth primitive atmosphere

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.

Stages of Early Life on Earth

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.

The Oparin Hypothesis

Russian chemist A.I. Oparin
Origin of Life on Earth Oparin Hypothesis

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.

The Miller-Urey Experiment

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.

The Miller-Urey experiment involved creating a self-contained atmosphere.
Miller-Urey Experiment

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.

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