Re: Why did organisms first make the transition to land?

My curt reply to your question is that organisms made the transition to 
land because they could.  This sounds glib, but really it is correct.  
Many answers to your question might imply that organisms recognized that 
the land environment was better for them than aquatic environments.  On 
some level, this is true.  The land environment is better in some ways, 
but the organisms didn't know that.  Moreover, there were some obstacles 
that needed to be overcome, and the organisms had no idea what 
they were or how to overcome them.

In general, organisms tend to live where they are relatively well adapted, 
and they don't adapt to new environments merely because they recognize a 
need to do so.  For colonization of a new environment to take place, 
organisms need to have the appropriate characteristics that allow them to 
live there.  These characteristics are the result of mutations and genetic 
recombination, both of which occur more or less randomly.  Then these 
characteristics are chosen for by natural selection, which doesn't occur 
randomly.  The best way to think of these new characteristics, in my 
opinion, is as preadaptations.  In other words, the organisms evolved some 
characteristic that was used for something, but was eventually taken over 
by some other use.  These preadaptations are similar to what are often 
called exaptations (Gould and Vrba, 1982).

Now, let's look at how this evolution could have occurred.
A couple aspects of land environments are definitely more conducive for 
life than aquatic environments.  At the time organisms moved to land, it 
was biologically barren.  Therefore, organisms that could live there had no 
competition for resources or habitats.  This was a big advantage.  
Additionally, organisms that respire had a much larger supply of oxygen, 
because its concentration in water is significantly lower than in the air. 
 In my mind, these are the biggest advantages to living on land.
However, the organisms were adapted to living in aquatic environments.  
Water is approximately the same density as living organisms.  Therefore, 
they had little need for structures that physically support themselves.  
Think about seaweeds.  If you take them out of the water, they collapse.  
The gills of many organisms are also water supported.  Thus, these had to 
change if organisms were to live on land.  Additionally, organisms that 
live in aquatic environments have no need for water-conserving mechanisms. 
 Thus, if they were to move to the land, they needed some of these to 
evolve, also.  Moreover, they had little need for mechanisms to conserve 
water.  Algae dry up quickly in open air, as do many aquatic animals.
The actual evolution onto land occurred in several stages and was not 
overnight.  Organisms acquired traits that adapted them to land one at a 
time, as random mutations.  For example, some algae probably evolved a 
mechanism that secreted waxy substances on their surfaces.  This helped 
them avoid dessication.  Consequently, these plants could have lived at the 
edge of a lake or other water-way that was prone to periodic drying.  If 
the wax became thicker, it could have helped support them, and allowed them 
to grow larger.  Thus, these particular mutations were retained in these 
organisms, and pre-adapted them to living in a land environment.  Once 
these plants became adapted to the land environment, they had a whole range 
of specific land areas that they could colonize as they diversified.
The evolution of animals to land environments is similar.  Some of them had 
characteristics that acted as preadaptations to the land.  As animals took 
advantage of these preadaptations, they colonized the land.  One good 
example of a possible preadaptation in vertebrates is the swim-bladder in 
fish.  This structure is used in bony fishes to control buoyancy.  The fish 
fills it with gas to help it float, and empties it to help sink.  It is 
sometimes considered to have evolved into the lungs of vertebrates.  
Another preadaptation are the lobes in lobe-finned fishes.  These are 
muscular, bony lobes that go out into their fins.  These fins extended and 
evolved to become legs.  
It's necessary to close by noting that what I said about HOW evolution to 
land could have occurred for plants and animals (called an evolutionary 
scenario) are hypotheses, and as such, they are subject to modification.  
The plant scenario and the lobe-finned fish scenarios are pretty well 
accepted currently, as is the lobe-finned fish.  I'm not sure how well 
accepted the swim-bladder scenario is.  Tests of these hypotheses come from 
a variety of sources, including phylogeny studies (using molecular as well 
as morphological data), paleontology, biomechanics, and development.

References:

Futyuma, D.J.  1998.  Evolutionary Biology, 3rd ed.

Gould, S.J. and E.S. Vrba.  1982.  Exaptation-- a missing term in the 
science of form.  Paleobiology 8:4-15.  

Gray, J. and W. Shear. 1992.  Early life on land.  American Scientist 
80:444-456.

Ridley, M.  1996.  Evolution, 2nd ed.

Strickberger, M.  1996.  Evolution, 2nd ed.