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10.14: Evolution of Simple Cells

Difficulty Level: At Grade Created by: CK-12
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Who was the ancestor to us all (and I really mean us ALL)?

If we trace all the evolutionary lineages (humans, sponges, slime molds, etc.) back, at some point there would be one organism that is the ancestor to all of the others. This organism is referred to as LUCA, which stands for the "Last Universal Common Ancestor." LUCA lived 3.5 to 3.8 billion years ago.

Simple Cells Evolve

Simple organic molecules such as proteins and nucleic acids eventually became complex organic substances. Scientists think that the organic molecules adhered to clay minerals, which provided the structure needed for these substances to organize. The clays, along with their metal cations, catalyzed the chemical reactions that caused the molecules to form polymers. The first RNA fragments could also have come together on ancient clays.

E. coli, a primitive prokaryote that may resemble the earliest cells

E. coli ( Escherichia coli ) is a primitive prokaryote that may resemble the earliest cells.

For an organic molecule to become a cell, it must be able to separate itself from its environment. To enclose the molecule, a lipid membrane grew around the organic material. Eventually the molecules could synthesize their own organic material and replicate themselves. These became the first cells.

Prokaryotes

The earliest cells were prokaryotes ( Figure above ). Although prokaryotes have a cell membrane, they lack a cell nucleus and other organelles. Without a nucleus, RNA was loose within the cell. Over time the cells became more complex.

A diagram of a bacterium

A diagram of a bacterium.

LUCA was a prokaryote but differed from the first living cells because its genetic code was based on DNA. The oldest fossils are tiny microbe-like objects that are 3.5 billion years old. Evidence for bacteria, the first single-celled life forms, goes back 3.5 billion years ( Figure above ).

To learn more about LUCA’s characteristics, see Wikipedia: http://en.wikipedia.org/wiki/Last_universal_ancestor .

This animation begins with the Big Bang, which will be discussed in the chapter Beyond the Solar System, and goes through the history of life on Earth: http://www.johnkyrk.com/evolution.html .

Photosynthesis

The earliest life forms did not have the ability to photosynthesize. Without photosynthesis what did the earliest cells eat? Most likely they absorbed the nutrients that floated around in the organic soup that surrounded them. After hundreds of millions of years, these nutrients would have become less abundant.

Sometime around 3 billion years ago (about 1.5 billion years after Earth formed!), photosynthesis began. Photosynthesis allowed organisms to use sunlight and inorganic molecules, such as carbon dioxide and water, to create chemical energy that they could use for food. To photosynthesize, a cell needs chloroplasts ( Figure below ).

Chloroplasts are visible in these cells found within a moss

Chloroplasts are visible in these cells found within a moss.

Importance of Photosynthesis

In what two ways did photosynthesis make the planet much more favorable for life?

1. Photosynthesis allowed organisms to create food energy so that they did not need to rely on nutrients floating around in the environment. Photosynthesizing organisms could also become food for other organisms.

2. A byproduct of photosynthesis is oxygen. When photosynthesis evolved, all of a sudden oxygen was present in large amounts in the atmosphere. For organisms used to an anaerobic environment, the gas was toxic, and many organisms died out.

Cyanobacteria

What were these organisms that completely changed the progression of life on Earth by changing the atmosphere from anaerobic to aerobic? The oldest known fossils that are from organisms known to photosynthesize are cyanobacteria . Cyanobacteria were present by 2.8 billion years ago, and some may have been around as far back as 3.5 billion years.

Cyanobacteria were the dominant life forms in the Archean. Why would such a primitive life-form have been dominant in the Precambrian? Many cyanobacteria lived in reef-like structures known as stromatolites ( Figure below ). Stromatolites continued on into the Cambrian but their numbers declined.

These rocks in Glacier National Park, Montana may contain some of the oldest fossil microbes on Earth

These rocks in Glacier National Park, Montana may contain some of the oldest fossil microbes on Earth.

Modern cyanobacteria are also called blue-green algae. These organisms may consist of a single or many cells and they are found in many different environments ( Figure below ). Even now cyanobacteria account for 20% to 30% of photosynthesis on Earth.

A large bloom of cyanobacteria

A large bloom of cyanobacteria is harmful to this lake.

Summary

  • A prokaryote has a cell membrane but otherwise organelles are loose within the cell.
  • Photosynthesis allows organisms to produce food energy with oxygen as a by-product.
  • Cyanobacteria, which are still around today, were the earliest known photosynthesizing organisms.

Practice

Use this resource to answer the questions that follow.

http://www.youtube.com/watch?v=uU00tg98Jjw

1. What are cyanobacteria?

2. When did cyanobacteria dominate the Earth?

3. How does a prokaryotic cell differ from a eukaryotic cell?

4. What are cyanobacteria responsible for?

5. Where are cyanobacteria found?

6. What are stromatolites?

7. What is a heterocyst? What does it do?

8. What is an akinete? What is its function?

Review

1. Why does there need to be a LUCA and how can scientists learn about it?

2. Why was the development of photosynthesis so important to the evolution of life?

3. What is the role of modern, primitive photosynthesizing organisms?

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Feb 24, 2012

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