The egg again, which was laid by the genetic ancestor, which probably came from the same family, but formed the separate genus.
To be fair though, it probably wouldn't have just been one egg. It is more likely that there would have been a sub-species within that family that all started laying eggs with adaptive features, that would compete against the existing species and prevail. The survival of the fittest and all that, and eventually this group became a distinct genus in its own right.