What we are only beginning to realize is that these two forms of superhuman intelligence—alien and artificial—may not be so distinct.
The technological developments we are witnessing today may have all happened before, elsewhere in the universe. The transition from biological to synthetic intelligence may be a general pattern, instantiated over and over, throughout the cosmos. The universe’s greatest intelligences may be postbiological, having grown out of civilizations that were once biological. (This is a view I share with Paul Davies, Steven Dick, Martin Rees, and Seth Shostak, among others.)
To judge from the human experience—the only example we have—the transition from biological to postbiological may take only a few hundred years.