World Builders™                                                                    Session Four  --  Microbiology             

The Evolution of Eukaryotes    

The presence of oxygen in the atmosphere made larger, more complex cells possible.

 

    The earliest cells were tiny prokaryotes, and for more than two billion years they were the only life forms on earth.  These cells were anaerobic, which means that oxygen was poisonous to them. These cells were well adapted to living with the early atmosphere, which was probably a mixture of nitrogen, carbon dioxide, and water vapor.  Because they lived in the water, they were sheltered from the ultraviolet radiation that came from the sun.   

     Some of these prokaryotes began to capture the sun's energy using photosynthesis.  The waste products of photosynthesis are oxygen and water.  The photosynthesizing  prokaryotes released oxygen, which dissolved in the water.  There was iron dissolved in the water,as well, and for millions of years the iron combined with the oxygen and so did not escape into the atmosphere.  Geologists still see evidence of this in the banded iron formations, which are very old rocks with alternating  pale and rust-colored stripes.

     The blue-green cyanobacteria were especially notable for their oxygen production.  They still thrive, virtually unaltered, and release an important part of the oxygen produced on earth today.

An Important Environmental Change

    After two billion years of prokaryote life, the oxygen concentration in the atmosphere reached about 2% of its present concentration in our atmosphere today.  (Our present oxygen concentration is about 21%, so this works out to 0.42% in the ancient atmosphere.  We wouldn't even notice that 0.42%: we would just die in about 5 minutes!)  This wasn't very much oxygen, but its appearance coincides with the emergence of the eukaryotes.  This was a gigantic step forward in the development of life on earth.  Not all eukaryotes use oxygen, but being able to use oxygen increased the diversity possible for eukaryotes.

Life Forms Responds to Change

      Environmental change creates new opportunities and presents new problems.    For the prokaryotes there was a problem: oxygen was poisonous to many of them. However, cell chemistry using oxygen can be more efficient and productive.  This represented an opportunity that some of the eukaryotes grasped, although they had problems of their own to solve. 

     Eukaryotic cells are microscopic in size, but they have about ten times the diameter of  prokaryotes.  Back on the circle math page we could see that when you want to know the surface of the sphere, you square the radius.  This means that the eukaryote cell has about (10*10) 100 times the surface area of a prokaryote.  That sounds pretty good!  But wait a minute.  To get the volume of a sphere, you cube the radius.  That would mean that the eukaryote had (10*10*10) a thousand times the volume but only a hundred times the surface area of a prokaryote.  Getting oxygen and chemicals to all the parts inside the cell became a very challenging problem.  

     Eukaryotes are much more complex inside than prokaryotes.  They contain intricately folded membranes that separate the contents of the cell and direct the flow of materials within it.  The DNA, the instructions to the cell of how to reproduce and maintain itself, changed from a single slender loop into pairs of chromosomes that were kept together in the nucleus of the cell.  More DNA was needed because the larger, more complex cell needed more instructions.  However, "Today's solution becomes tomorrow's problem"  (Viau's Law).  The eukaryotes needed to develop a way to divide up the chromosomes when the cell divided.  This led to a complex, beautiful, dance-like process that duplicated and divided the chromosomes, providing each cell with a full compliment of DNA .  This process is called mitosis.

     Eukaryotes also took some prokayrotes into their own cells. These prokaryotes live in a symbiotic relationship with the host cell.  The organisms making up the cell work together and help each other.  After so many millions of years of living together, the pieces of the cell could not survive alone now.  The prokaryotes have become endosymbionts, life forms living within a host organism.  They still have their own DNA, and divide by binary fission just as they did when they were wild and free.  Now, the interior of the eukaryotic cell is a very comfortable environment for them.  The eukaryotic cell protects them from contact with oxygen, which is poisonous to prokaryotes, and provides a stable environment with water and the necessary chemicals for their life processes and reproduction.  These organelles perform tasks that help their host cell

     Here is a diagram of a eukaryote cell compared with a prokaryote cell.  I have not put in all the parts, just a few parts to show you some of the progress that was made when the eukaryotes formed.  Scientists have studied the chemical activity that goes on inside the cell; the chemistry is interesting but complex.  Let's start simply.  



See a comparison between eukaryotes and prokaryotes


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© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at eviau@earthlink.net .
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