I'm still fascinated by evolution and embryology, even if former Georgia congressman Paul Broun called the sciences "lies straight from the pit of Hell." He doesn't speak for all Georgians.
Apparently, almost all animals pass through a watershed moment early in their life called gastrulation. Typically, before gastrulation, an embryo consists of a hollow ball of cells, the blastula, the wall of which is one cell thick. During gastrulation, the ball indents to form a cup with two layers, and the opening of the cup closes to form a small hole called the blastopore.
For many animals, including mammals, reptiles, birds, and such, that small pore becomes the anus, which is used to expel wastes from the body cavity (the inside of the cup) and a mouth later develops on the opposite side. However, most other animals, including the great variety of insects, spiders, crustaceans, segmented worms, and molluscs, including the slug that crawled across a wall of my house yesterday, do it differently. These animals, which are far more numerous than the first bunch, use the blastopore as a mouth, and then later develop an anus on the opposite site or somewhere else on their body plan. All these creatures are collectively called protostomes, which means "mouth first," and we humans and other mammals, reptiles, birds, etc. are called deuterostomes, which means "mouth second." All living protostomes descended from the first "mouth first" animal and all living deuterostomes descended from the first "mouth second" creature.
So the common ancestor between man and slug is a creature that's neither a protostome nor a deuterostome and lived before animals started going through gastrulation. That common ancestor was probably something similar to certain of the current flatworms, those small worms living in or near the ocean that lack a body cavity (no gastrulation) and an anus. The protostomes and deuterostomes did not evolve from those living flatworms, the Acoela and the Nemertodermatida, but all of us deuterostomes, the protostomes, and the living flatworms all descended from a common ancestor that was probably very similar to the present Acoela and the Nemertodermatida.
Two other things I've learned that make this interesting, at least to me. The flatworms most of us are at least passingly familiar with, the tapeworms, flukes, and such, don't have body cavities or an anus, but are still considered protostomes. Natural selection has actually caused them to lose the body cavities and anuses they've inherited in order to fill some niche, just as some mammals (whales and dolphins) have returned to the sea and at least superficially resemble fishes, but are still mammals. So that common ancestor of man and slug was not quite like a tapeworm or our usual idea of what a flatworm looks like, but actually something stranger still.
The Acoela, a tiny creature more representative of the common ancestor, is so simple that some species let algae inhabit parts of their bodies and therefore benefit from photosynthesis to supplement their diet. As Richard Dawkins describes them (I'm indebted to Dawkins for most of my understanding about all this), the Acoela can be seen living in colonies on the beaches of Brittany, where they appear as a green slime due to all the algae. They crowd the surface as much as they can to give the algae as much sunlight to work with as possible, but as one approaches, the green slime suddenly disappears by retreating into the sand.
My point in all of this is that the more we examine and the more we learn about life on this planet, the more connected we realize we are and the more alike we realize we are, despite the profound differences we see at first.