The concept of Evolution is tough for most adults to grasp, so trying to explain it to a five and seven year old isn't exactly easy. They can grasp some of the more specific aspects of it, but the general concept, as a whole, still causes confusion.
Nico once argued with me for days when he asked who the first person was, and I told him there was no first person. Populations evolve, not individuals. He just couldn't wrap his head around that concept. Tonight, Lucas asked me how all the people in the world were born, when in the past there were no mommies and daddies to make babies (i.e. before people evolved). I didn't think I could adequately explain it with out help, so I pulled one of their books out to help.
Our Family Tree: An Evolution Story by Lisa Westberg Peters does a fantastic job of explaining how life evolved, in a simple way for children to understand. We read the book together, stopping along the way to discuss various aspects of evolution. I was rather surprised by the tough questions the boys asked along the way; such as Nico asking how the first cells got into the oceans to begin with.
During our conversation about evolution we also touched on abiogenesis, embryology, climatology, cell theory, just to name a few. The boys couldn't stop talking over each other, eager to discuss the many questions popping into their heads. After we put the book down, we went to youtube.com and watched some videos on how embryo's grow and develop from a single cell, into multicellular organisms.