Earth's Early History

• Biology
• 10th Grade

Earth's Early History

Page 72

Essential Question: What do scientists hypothesize about early Earth & the origin of life?

Intro to Earth

• After the big bang (13.7 billion years ago), pieces of cosmic debris were flying throughout the universe. 
• As cosmic debris collided, large planetary bodies were formed.  
• Gravitational forces caused these planetary bodies to begin orbiting larger objects, such as stars, causing the planets to round out.  

How did Earth form?

• Earth formed from the collision of cosmic debris.
• Early Earth was incredibly volcanic, suffered many collisions, and had little to no oxygen & no liquid water. 
• Around 4.2 billion years ago, the surface of Earth cooled and atmospheric water vapor cooled.  It fell as rain and formed our oceans.

Early Earth's Atmosphere

• Early Earth's atmosphere contained almost no oxygen.  It was mainly carbon dioxide, water vapor, and nitrogen.
• At that time, the sky was pinkish-orange and a taking a breath would be toxic enough to kill you.

So, how did life form?

• Many basic building blocks (such as amino acids) can be found on modern meteorites and comets.  
• Scientists hypothesize that many organic building blocks, such as amino acids, were found on early Earth as well.

Miller-Urey Experiment

• In 1953, two scientists name Miller & Urey wanted to test if early Earth's atmosphere and the organic molecules we know were present could spark life.
• The Miller-Urey experiment supported the idea that organic compounds necessary for life could have arisen from non-living compounds found on Earth when energy was applied.

Review question: What do amino acids combine to make (hint: which macromolecule are amino acids the monomer of)?

1. Carbohydrates
2. Proteins
3. Lipids
4. Nucleic acids

Review Question (again!): Which macromolecule makes up cell membranes?

1. Carbohydrates
2. Proteins
3. Lipids
4. Nucleic Acids

From amino acids to cells...

• Just like amino acids, simple lipids called "fatty acids" could be formed from non-living compounds on early Earth.
• These fatty acids could have assembled to form the first membrane-enclosed "protocells."
• Just like the Miller-Urey experiment, protocells have also been generated in labs by simulating early Earth's conditions.

The RNA Hypothesis

• Because RNA is less complex than DNA and is used directly in the production of proteins, it is hypothesized that early protocells carried their genetic information on RNA.
• DNA evolved later on.

                  Atmospheric Changes

• The first prokaryotic cells in the fossil record are from 3.5 billion years ago.
• Roughly 2.2 billion years ago, photosynthetic structures appeared within these early bacteria.  They began converting Earth's carbon dioxide to oxygen.  At this time, the ozone layer formed & the sky changed to blue.
  

The Rise of the Eukaryotes

• The first eukaryotic cells in the fossil record are from 2.1 billion years ago.
• Early eukaryotic cells most likely formed from the joining together of smaller prokaryotic cells, called endosymbiosis.
• Lots of evidence, including the fact that the mitochondria has its own DNA and are roughly the same size as a prokaryotic bacteria, support the endosymbiotic theory.