From single cells to multicellular organisms

In the process leading up to multicellular organisms, eukaryotic cells had to form.  A suggested mechanism is below.  It is thought that a progenitor cell may have got larger than other cells.  To maintain SA:vol ratios it folded its membranes, some of which then enclosed the DNA (forming a nucleus) and the endoplasmic reticulum and Golgi apparatus.


From here we move into something called endosymbiosis (endocytosis - to engulf something, often for nutrients, symbiosis - a long term interaction between 2 things, often to the point where one can survive without the other).

It is thought that the progenitor cell engulfed a heterotrophic bacteria and over time it degenerated into a mitochondria.  These cells were to become heterotrophic eukaryotes.  One of these also engulfed a photosynthetic prokaryote.  This photosynthetic prokaryote degenerated into a chloroplast and the entire cell was the ancestral plant cell.

Evidence to support this idea includes:

1. The fact that mitochondria and chloroplast divide independently of each other
2. The DNA of mitochondria and chloroplasts is circular (like bacteria) and has many common bacterial sequences

A link to describe the process in more detail is here.


Now that we have cells that are structurally similar to modern ones the next step is to get them to work cooperatively.  The levels of cooperation were thought to be 

One example is colonial algae,  Volvox.  It is broken into 2 cell types; cells for swimming (outer shell) and ones for reproduction (small green spheres)



In the case of the Portuguese Man O War (bluebottles)there are a number of additional cell types which are working cooperatively to assist the survival or the organism.  Just to clarify the labelling here, the Gonozooids (reproductive structures) are the light blue ones near the polyp while the Gastrozooids are the red/brown ones just below.

From here it is possible to see further increases in cooperation between cells, and therefore increasing complexity of the organisms that possess them.

The Urey Miller Experiment

In 1953, Stanley Miller, a PhD student, proposed to his supervisor Harold Urey, an experiment to test if the chemosynthetic origins of life were possible under the conditions of what early Earth was like.

This would in part answer the chicken and the egg question about whether it was possible for the early Earth to produce the chemicals needed to sustain life, before life itself actually got going on Earth.

Energy source and some of the gases that Miller proposed to use.           Note NO OXYGEN.

Miller then set up his experiment as follows:



Over a number of days the apparatus showed orange brown materials sticking to the glass and in solution.  Chemical analysis showed the following:


The final group of materials include amino acids (used to make proteins). Other substances include metabolites that certain types of cells can get energy out of.  


The findings were significant for a number of reasons:

1. The early Earth could not have had oxygen present (supported by other geological evidence)
2. The early Earth had conditions that could have allowed for the generation of molecules that would sustain life
3. Other variations of the experiment showed that molecules that could be used in DNA and RNA could be produced.

So this experiment supports the chemosynthetic theory, but because we weren't there to observe it, it is NOT proven.

Chemosynthesis versus Panspermia

There are 2 theories that are proposed to try and explain the origings of life.  These are:

1. Chemosynthetic origins - The chemicals of life were made on Earth.  Once made, these eventually combined to make living organisms.
2. Panspermia - The chemicals of life and/or life was seeded from asteroids and meteorites from space.

Lets start with chemosynthesis.

Looking at the Earth 3.8 billion years ago, it was thought to be highly volcanic, anoxic and had liquid water.  Lightning storms were thought to be frequent around erupting volcanoes (as they are today).  The surface of the planet was being bombarded with radiation as there was no ozone layer formed. The atmosphere was thought to be made up of simple molecules.

It was proposed that the energy of any of the above sources could have been used to break and reform chemical bond, thus turning the simple molecules of the atmosphere into more complex ones.  There will be some description about the experiments involved in this later.

Panspermia describes the idea that complex molecules form in space.  Many dust clouds in space contain carbon, nitrogen and water (ice crystals) in them. In addition asteroids and comets contain these elements as well.  Energized by radiation from the stars the chemical bonds can be rearranged to form complex materials some of which are found on Earth.

The conditions thought to occur in deep space have been modeled in Earth labs and so this theory is plausible as well.  If we find life of a similar chemical make up on other Earth-like planets, it may be that Panspermia becomes the preferred model.  Given what we currently know, it appears that the chemosynthetic origins of life is still favoured. The experiment that got the ball rolling on this was the Urey-Miller experiment performed in 1953. This is the subject of the next post.

Life on Earth - Conditions of Early Earth

This module looks at the development of life on Earth and the conditions that may have spawned  life.  To do this we need to see what the initial conditions of life on Earth were like.

4.5 billion years ago to 3.8 billion years ago.

At this time the Earth has no liquid water, and no stable crust.  The planet was still subject to heavy asteroid bombardment.  Because of the heat on the surface, any gases present would have escaped into space.  


3.8 - 3.45 billion years ago

By this time the Earth had cooled to the point where stable crust had formed and the temperature had cooled to the point that liquid water covered most of the surface of the planet.  Volcanic activity was still violent and frequent, but the gases being released from them were beginning to create an atmosphere.

This atmosphere was thought to be rich in methane, ammonia, carbon dioxide, carbon monoxide and possibly cyanide.  No free oxygen was present at this stage.  Hydrogen may have been formed from the spitting of water by UV radiation but the oxygen would have reacted quickly.  The lack of oxygen makes this atmosphere anoxic.

A number of small volcanic island were thought to have been forming at this time.  At the edges there may have been hot mud springs.  These springs may have been the place where life arose.  But before life can start, the complex molecules for life need to be created.  This is a topic for another post.