PHY - Origins of Life [LESSON]

Origins of Life

Origins of Life

Although no one was around during Earth's early history, there is plenty of scientific evidence to draw conclusions about what might have happened during that time. The earliest evidence of life on Earth comes from fossils about 3.5 billion years old, but of course scientists have wondered, when did life on Earth originate? Experimentation in chemistry, geology, and physics have outlined the characteristics of the early Earth and have developed multiple hypotheses about how the first living things could have occurred.

Flip through the Origins of Life presentation for some of the major points about how life could have originated on Earth.

What is the age of Earth as measured by radiometric dating?

4.6 billion years old!

Geological evidence shows us that when Earth formed approximately 4.6 billion years ago, it was covered with active volcanoes. In addition, with no protective atmosphere, Earth was probably very hot and constantly bombarded with comets and asteroids. Around 4.2 billion years ago, Earth cooled enough for the surface to solidify and for water vapor to condense and fall as rain. This allowed permanent oceans to form. The earliest fossil evidence for life, fossils of large clusters of prokaryotic cells, dates to 3.5 billion years ago. Taken together, this evidence provides a range from 3.9 billion to 3.5 billion years ago in which the first living cells formed.

The transition from inorganic molecules to organic molecules begins on Early Earth, four billion years ago. There are multiple theories to consider. The first theory is that organic molecules were the result of introducing inorganic molecules into the unique conditions on Earth. The inorganic molecules were nucleotides and amino acids. The conditions were low oxygen, high UV radiation, high heat, and large amounts of energy. Another theory is that organic molecules were introduced to Earth by meteorites. Either way, organic molecules began to change the Earth. Small organic compounds formed larger organic compounds, which is the basis of the RNA World Hypothesis. Organic compounds changed the environment of the Earth. Organic compounds also produced oxygen, which inhibits certain chemical reactions. Oxygen also produced the ozone layer, which reduced UV radiation. The addition of oxygen and reduction of UV radiation inhibits the creation of new organic compounds.

By 2.2 billion years ago, photosynthetic bacterial cells became common. These cells used energy from sunlight to produce food, giving off oxygen gas in the process. As oxygen accumulated in the atmosphere, the ozone layer also began to form. Over time, the oxygen levels rose until they reached the levels present today. As the conditions of the atmosphere changed, different types of organisms developed to live in the now oxygen-rich environment.

The fossil record and other clues help scientists study the history and origins of life on Earth. Evidence indicates that all living organisms have evolved from the same early ancestors, simple prokaryotic cells. There are several hypotheses about the origin of life on Earth, each based on supporting evidence from many different fields of science. The RNA world hypothesis helps to explain the evolution of earliest genetic material from RNA to DNA.

Now, watch this History of Life video about the origins of life on Earth and the Miller-Urey experiment.

Theories suggest that some of the first organic molecules formed on Earth may have been amino acids and nucleotides. Amino acids are the small organic molecules that bond together to form proteins. Nucleotides, another type of small organic molecule, bond together to form RNA and DNA molecules. The spontaneous formation of these small organic molecules from nonorganic molecules can no longer happen in today's oxygen-rich atmosphere. Many scientists have attempted to re-create Earth's early atmosphere to determine if these organic molecules could have formed from inorganic molecules present at Earth's beginning.

Miller-Urey Experiment

In the 1950s, Stanley Miller and Harold Urey performed an experiment that was profoundly important in giving weight to speculations about the chemical origin of life on Earth and elsewhere in the universe. They set up an apparatus to create conditions that mimicked those thought to exist on the primitive Earth. They filled a chamber with gases such as methane, ammonia, hydrogen, and water vapor. Then they generated electrical sparks to simulate lightning.

RNA World Hypothesis

So how did RNA evolve on Earth? Scientists think RNA building blocks (nucleotides) emerged in a chaotic soup of molecules on early Earth. These nucleotides bonded together to make the first RNAs. No sooner were they made than they broke down; however, new ones were made in their place. Some RNAs turned out to be more stable than others. These RNA strands grew longer and bonded nucleotides more quickly. Eventually, RNA strands grew faster than they broke down. This was RNA’s chance to start life. RNA is thought to be the first genetic material for a couple of reasons.

All living things reproduce. They copy their genetic information and pass it on to their offspring. And for RNAs to start life, they needed to reproduce too. This is why scientists think that the RNA world took off when an RNA emerged that could make copies of itself. As it did, new self-copying RNAs emerged. Some were better at copying themselves than others. The RNAs competed against each other, and the most successful won out. Some of these molecules are still present in prokaryotic cells today.  For example, the ribozyme is a molecule that can self-cleave and functions as an enzyme and RNA together. Over millions of years, these RNAs multiplied and evolved to create an array of RNA machines. At some stage, DNA and proteins evolved. Proteins began to drive chemical reactions in cells, and DNA—which is more stable than RNA—took the job of storing genetic information.

 

 

Try the Origins activity below for some practice.

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION