So clearly the smaller, and simpler creatures that have all had a major test run on this planet are the most predominant. They may not be able to think or build spaceships, but they have no reason to. Us mammals and lizards are larger and by the fact that we are so much fewer and consume so much more resources makes us bigger candidates for extinction. We are also more fragil then these other creatures. Oh, the sacrifices we lifeforms have made for superior intelligence.