Re: Why didn't mammals evolve with lungs and gills?

It’s all a matter of cost ratios:

For this, your answer is in your question – i.e. larger more complex 
creatures means that they have more of an investment in ‘complex’ 
structures.  Mammals are pretty complex organisms, but they still are only 
able to invest 100% of their resources.  100% of those resources are 
already divided into making feet, hair, lungs, etc.  To develop both lungs 
and gills, there would be a higher maintenance cost – something that might 
not be offset by greater access to resources.

A second way of looking at the question is through evolution.  When the 
first lunged animals evolved, there were more unexploited resources on 
land than in the water.  In streamlining natural selection, those 
organisms that only had lungs did no worse than those organisms that had 
both.  Indeed, they did better (as evidenced by the current lack of gills 
in adult mammals).  Why did lunged animals do better?  Because the 
resources they needed to maintain gills were not offset by a greater 
availability of food.

Now if you take my logic and turn it around, you’ll get that if life 
evolved on land first, then we would probably have gills and no lungs!  
(For the logical conclusion, we only have to turn to Hollywood and Kevin 
Costner in Waterworld – the mutant with gills was favored when there were 
more available resources in the water than on the land.)

Here is an interesting link, yes, it is about snails, but it gives you an 
idea about the cost of having both lungs and gills:
 
http://www.biosci.ohio-state.edu/~zoology/eeob405/lab_5_q01.html