What evolutionary pressure(s) selected for this asymmetrical configuration and size of the two human lungs? Is this lung asymmetry also present in lower mammals, such as dogs, cats and mice?
As a complement of what was said, other mammals also present this asymmetry based on the same principals of development. In early stages of development humans and other mammals are quite similar. But on late stages of development different mammalian species have organ structures with the same function, but with small or big differences, adapted to the needs and purpose . As an example, dogs have 4 lobes in the right lung (more room and less interference with heartbeat) and 2 on the left (although the cranial lobe is divided in cranial and caudal portion).
Hope I could add something to this discussion.
I agree with previous answers when considering than lesser left lungs in mammals intend to provide room for a left-sided heart. However, while previous answers are correct, I think that they focus on efficient or "proximal" physiological causes.
But let me try a different reasoning, more focused on the evolutionary origin and sense of this assimetry. Or "ultimate" causes if you prefer.
Mammalian current aortas derive from branchial arches, which are symmetric from their very origin. However, the evolution of asymmetrical specialization of the fourth arch made it possible to completely avoid blood mixing in endothermic amniota. Blood mixing can be completely avoided in other animals, such as crocodiles, however these ectothermic vertebrates may find some physiological advantages in blood mixing under certain conditions. But endothermic animals will always need high levels of oxygen supply, because of their increased methabolism.
So the fourth arch becomes totally asymmetric both in birds and mammals, by convergent evolution. By the way, this asymmetry happens differently in birds and mammals, retaining the right side of the arch in birds and the left side in mammals.
So the retaining of the left side of the fourth branchial arch in mammals as the only systemic aorta makes it more efficient to locate heart slightly to the left, which ultimately explains why left lungs are lesser than right ones in mammals.
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