This reminds me of a classic trick question that I would ask my biology students: What is the most evolved organism on the planet today? Did you say "humans"? BZZZT! Wrong!

Consider that evolution is the process where by an ecological niche exerts selective pressure on a population and, by doing so, causes certain "best fit" traits to become enriched in that population. It stands to reason then that an organism that has remained in one niche for a long time will have experienced the greatest enrichment of such traits. Now consider the lowly bacteria: most species of bacteria have not moved from their niches in millions or even billions of years. Now consider the viruses that infect these bacteria. All evidence indicates that they've been around even longer!

Do you have a pet? A cat or a dog maybe? How often do they get sick? How often do you get sick? Ever wonder why that is? You see, disease is, as the name implies, a dis-ease experienced by an organism that hasn't achieved a good balance with its environment. The longer a species has to adjust to its environment, the better balance that it can strike with the other organisms, that might function as pathogens or parasites, that share its environment. When it comes to humans, we are probably the least evolved organism on the planet by this measure because we're one of the newest! (Not surprisingly, most of the species which have appeared on the planet more recently than humans are human pathogens.)

Size, as a selected trait, has its own very interesting properties in evolution. If your smaller you ratio of volume to surface area is greater, and therefore you loose more heat. This greater heat loss is made up for by a faster metabolism and warmer core temperature. (Ever taken your cat's temperature?) All evidence indicates that faster metabolisms are also linked to greater frequency of ROSes (reactive oxygen species, not rodents of unusual size ;-) and subsequently a shorter life span. Most organisms that we know that fall into this category compensate for their shorter life span with a greater reproductive rate (litters of 6 or 8 are common for cats, less so for humans).

On the other hand, if you're a large organism then your volume to surface area ratio is low, and you'll do a better job of retaining heat. The flip-side of this, however, is that you require a greater abundance of food and a longer period of development to maturity. Also, you will live longer (fewer ROSes) and so you don't need to reproduce in as large numbers. That's a good thing, because each of your offspring will require a greater investment of resources.

The upshot of all of this is that size really has little to do with "how evolved" an organism is and everything to do with their environment. Larger species tend to dominate when food is plentiful. Larger species also tend to only appear at the tops of food chains, where threats to their few offspring are lessened. All of this, it should go without saying, is a generalization and there are certainly exceptions to the rule.

Really, the only advantage that humans have over any other organism on earth is cognitive ability and a capacity for introspection and curiosity. This is, honestly, what upsets me the most about the whole creationist/ID movement: proponents of this view are essentially asking you to give up that most essential of human traits, the desire to ask questions, and instead accept their "god" or "creator" as an adequate explanation. Instead, I think it is our responsibility to take advantage of this unique capacity that we have and develop some humility. We need to realize that we're the new kid on the block, so to speak, and that while life on the earth will continue with or without us, we have a choice as to how much of an impact we have on our niche. Humans would do well to recognize that we are, so far, a fairly maladapted species. We have a long way to go...