Just wanted to say that although Dawkins obviously has a major downer on religion, not all of his works are about that.
Anyway - to the common ancestor - without overcomplicating it:
The closer two species are genetically, the more recently their common ancestor was around. For instance, whatever species humans and gorillas split from was a lot more recent than, say, humans and horses. Presumably horses and zebras was relatively recent, though zebras acquired stripes - believed to be camouflage.
Throughout nature there is this balance needed for survival between being prominent enough to attract a mate and being hidden enough to hide from predators. This is why some creatures change colour or form when attracting a mate - because the rest of the time they need to be camouflaged. There were even observations that some creatures that were otherwise identical, stood out a lot more in areas where there had been no predators for a while whereas in areas where there were predators, their colourings blended in better with the background.
One of the most remarkable things I read was about the evolution of bacteria in a lab. As they are so tiny, they change form far more quickly.so it is evolution in microcosm. Unfortunately, the details of that lengthy experiment are in a Dawkins book so may be a closed door. It was a remarkable piece of research though.
I'm so glad that you are exploring.
