|MadSci Network: Evolution|
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
Try the links in the MadSci Library for more information on Evolution.