The First Life Forms

PROKARYOTES

Proposed sequence of events for the development of cells (draw sequence boxes)

1. Small organic molecules, such as amino acids and nitrogenous bases, were formed by abiotic events or factors (volcanos, lightning, meteroites, deep-sea vents).

2. Macromolecules were formed by the combination of the small organic molecules.

3. Protocells or protobionts, systems that are considered to have been precursors to prokaryotic cells, were formed from macromolecules. Protocells had bubble-like membranes that separated internal materials from the external environment.

4. Certain molecules, perhaps ones similar to modern RNA, developed the ability to self-replication and created the process of inheritance.

• Between 3.9–2.5 billion years ago, cells resembling prokaryotes appear. These first organisms are chemoautotrophs: they use carbon dioxide as a carbon source and oxidize inorganic materials to extract energy.

• They are found near deep sea vents where many inorganic materials being ejected Ex. sulfur dioxide, Iron, magnesium, calcium, nitrogen

A black smoker in the Atlantic Ocean providing energy and nutrients

• These first chemotrophs were anaerobic since there was still no oxygen present on earth.

• They appeared approximately 3.5 billion years ago and are classified as prokaryotes.

Chemoautotrophs     - Energy from oxidation of inorganic substances (e.g. NH4, and S)     - CO2 is the carbon source

        Example:  Sulfolobus, Beggiatoa

• Between 3.3 and 2.7 billion years ago the first phototrophs (used sunlight to produce energy through the process of photosynthesis) evolved.

• These bacteria, known as cyanobacteria, helped to raise the oxygen levels on the earth so that more complex life could begin.

One of the most independent organisms on earth: Cyanobacteria (Anabaena)

• Cyanobacteria get their name from the bluish pigment phycocyanin, which they use to capture light for photosynthesis.

• They also contain chlorophyll, the same photosynthetic pigment that plants use.

• They play an important role in the development of Eukaryotes

Cyanobacteria

Cyanobacteria: Gloeothece (top left), Nostoc (top right), Calothrix (bottom left), Fischerella (bottom right)

Stromatolites are formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria.

Stromatolites in Sharks Fish Bay, Australia

Prokaryotes come in several forms – rods (bacilli), spherical (cocci), and spiral shapes

Heliobacter: Causes stomach ulcers

Prokaryote

Evolution of Eukaryotes

• Evidence supports the idea that eukaryotic cells are actually the descendents of separate prokaryotic cells that joined together in a symbiotic union.

• In the late 1960’s Lynn Margulis was studying the structure of cells and she noticed that mitochondria looked remarkably like bacteria.

• With the improvement of genetic tools and methods, scientists were able to compare genes from different species.

• Both mitochondria and chloroplasts contain their own DNA.

When the genes from these structures were compared to bacteria it was found that the DNA in chloroplasts resembled that of cyanobacteria, while the DNA within mitochondria resembles that of a group of bacteria that causes typhus (Rickettsia bacteria)

• With all of this information scientists were increasingly convinced that mitochondria and chloroplasts lived in permanent symbiosis within the cells of other bacteria through the process of endosymbiosis .

• Margulis spent several years trying to prove that symbiosis was a major force in the evolution of eukaryotic cells. She published her argument in her book:

The Origin of Eukaryotic Cells

MITOCHONDRION

CHLOROPLAST