Evolution Topic D: 1

Evolution Topic D: 1 The Origin of Life http://www.youtube.com/watch?v=dYphAH2tKYE&feature=related

Early Earth Conditions Earth formed 4.6bya The cooling crust released, carbon dioxide, nitrogen, methane, hydrogen, water vapor and ammonia. There was no oxygen gas because the metals in the rocks reacted with it to form oxides.

As the crust continued to cool, water vapor condensed to form the oceans. The Earth remained sterile for the next billion years before organic molecules began to form. Oxygen did not begin to appear in the atmosphere for another 1.3 billion years

The Spontaneous Origin of Life Prokaryotic and eukaryotic organisms alive today all have structures based on the cell. Cell structure must have originated from chemicals present on early Earth. For this to happen four essential steps are required.

1. Living things are made of organic molecules, so simple organic molecules such as amino acids, sugars, fatty acids, glycerol and bases must have formed.

2.Organic molecules in living organisms (such as triglycerides, phospholipids. Polypeptides and nucleic acids) are large, so single molecules must have been assembled to make the more complex molecules.

3. All living things reproduce, so molecules must have formed that could replicate themselves and control other chemical reactions. This is the basis of inheritance.

4. Cells have membranes, so he mixtures of these molecules must have been enclosed within membrane-bound vesicles.

Under certain conditions, particular molecules such as small polypeptides or phospholipids mixed in water collect together to from small spheres, called microspheres.

If these microspheres form from larger polypeptides along with other organic molecules, and if they become surrounded by a skin of water, called coacervates. These represented the first membranes.

In early oceans, microspheres and coacervates formed, many would have broken up but some may have contained a mixture of chemicals that increased their survival time. They may have formed internal environments different to their surroundings, some may have contained small molecules of RNA and amino acids. These vesicles would’ve been able to replicate asexually, allowing for natural selection to act on them and evolve.

Origin of Organic Compounds The early Earth was bombarded with debris from space such as comets, meteorites and space dust, which could have contained organic molecules. In September 1969, a meteorite fell in western Australia; 100kgs of material were recovered from it and an analysis showed that is was rich in amino acids. Panspermia Theory- (seeds everywhere)

The water bear an small invertebrate was able to survive outer spaces harsh conditions in an experiment done by NASA. Providing proof that bacteria and other simple creatures could have travelled on a meteorite from Mars.

Urey and Miller Experiment In 1953, Harold Urey and Stanley Miller put together an apparatus and attempted to mimic the conditions thought to have existed on early Earth. They wanted to test the theory that organic molecules could have been produced when simple molecules came together.

The flask contained distilled water, which boiled and condensed, just was it would have billions of years ago. The gas mixture contained methane, ammonia and hydrogen, and the electrodes produced sparks to simulate lightning. After several days, the condensate became brown and contained many organic compounds and amino acids.

Later other scientists used this investigation involving other gases that were present and were able to form adenine, a base found in RNA and DNA, and components of ATP. When they added sand in the mixture complex amino acids were formed.

Where could Organic molecules have formed? Hot springs and thermal vents have been places where scientists can see similar conditions of early Earth. The vents and hot springs contain mineral rich water heated by volcanic activity, containing very high temperatures and pressures forming organic molecules and are teeming with bacteria.

The Endosymbiotic Theory The endosymbiotic theory is an important theory explaining how eukaryotic cells could have developed from simple prokaryotic cells. The theory suggests that some organelles found inside eukaryotes were once free-living prokaryotes.

There is evidence to suggest that prokaryotes were engulfed by larger cells, and were retained inside their membranes. Evidence includes that two organelles, mitochondria and chloroplasts share many characteristics with prokaryotic cells.

Both mitochondria and chloroplasts: Contain ribosomes, which are smaller than those found in other parts of the eukaryotic cells but same size as ones found in bacteria (70s). Contain small circular pieces of DNA similar to bacterial plasmids. Have their own envelope surrounding them, one inner membrane where proteins are synthesized, suggesting they may have used this ability long ago when they were independent. Can replicate themselves by binary fission.

These organelles (chloroplast and mitochondria) are modified bacteria that were taken in by phagocytosis, early in evolution of eukaryotic cells, but not digested. Instead they became useful inclusions. The double outer envelope of the organelles may have originated from the bacterial membrane and the vesicle from the host.

Some of the enclosed bacteria had pigment covering their membrane and used light energy to make organic molecules and release oxygen and became chloroplasts. Others may have been efficient at using oxygen molecules for aerobic energy production and became mitochondria. Aerobic respiration is much more efficient at releasing energy from organic compounds than anaerobic respiration. This increase along with sexual reproduction could of lead to a rapid evolution of life on Earth.

Critics of the theory might argue that, even if prokaryotes were engulfed by larger cells, there is no certainty that they could be passed on to both daughter cells when the cell divided. However, when a cell divides by binary fission each daughter cell contains some cytoplasm from the parent which could contain the engulfed prokaryotes.

Evolution Topic D: 1