The Drunken Philosopher #6: Something about Doubles

A thought just occurred to me, which has given life to a new theory.
I was pondering the existence of Pangaea, the supercontinent of old which contained all of Earth's current landmass. I got to thinking, "how in the world does a planet of such size exist with such an uneven mass distribution?" Then I considered the (nigh indetectable) slowing of our planet's rotation, and the phenomenon of tectonic shift. I then came to one conclusion:
Perhaps this planet's entire surface used to be covered by water. Once, a couple billion or so years ago, the Earth was actually a binary planet system. Something happened which caused the orbit of the less fortunate (and undoubtedly smaller) of the aforementioned planets to decay, gradually being pulled inextricably into the larger planet's gravity well. As we know, as an orbiting object's path becomes gradually tighter, the rate of completing said orbit becomes faster and faster (in which case the distance travelled decreases at a higher rate than the speed of the object; otherwise it meets at the precise escape velocity for that distance out from the gravity well, stabilizing the orbital distance). The rotational speed of the planet may well have been augmented from having hit at such a speed, the moon creating a "spin" from which the Earth is just now recovering. The largest part of the mass of the smashed down into the crust is what became the first landmass to stick out of the water: Pangaea... while the debris from the impact flung out into orbit from the smaller planet being torn to bits pre-impact would have formed a ring which orbited the Earth until it coalesced to form a smaller version of what it once was, which we now know as the moon. The phenomenon of tectonic shift (the mechanism by which the continents are now shifting about) is just the planet's means of re-distributing this formerly uneven mass to something more uniform and spherical.
Now imagine, if you will, an Earth which was never hit by its smaller half. It would have been a planet with somewhat lower gravity, covered in water (or perhaps ice, as the volcanic activity responsible for the first "greenhouse" layer which caused the initial warming of the atmosphere was in turn the result of the tectonic movement caused by the aforementioned impact... but let's be optimistic here and assume that our planet would have borne the capacity to support life regardless). A huge moon looms in the sky, more than large enough to (frequently) block out the sun... while the idea of a full lunar eclipse is laughable. Running with the assumption of life existing on the planet, without land providing a wider variety of environments for life to adapt to, birds and most insects, probably mammals as well would never have reason to have existed. The most advanced of creatures are most likely to be apex predators such as sharks (or their equivalent), intelligent encephalopods like octopi.
tl;dr: In an alternate universe, we are squid people.