This is just a fact. All life that we know of has the same origin. Common descent is a core tenet, and one of the best established facts, of modern biology (https://doi.org/10.1038%2Fnature09014). For there to have been multiple distinct ancestors, we would need to see some relevant differences in the core machinery of life. And the fact is that, despite minor variations, we simply don’t see any. The odds of there being multiple distinct ancestors is astronomical, and would fundamentally impact our understanding of modern biology.
> and b) no organism has drifted in the millions of years of evolution
Organisms have drifted. That’s what evolution is. But evolution plays by rules, it can’t just change things willy-nilly. Changes to the core machinery would presumably either break it outright, or be so strongly detrimental as to be purified away almost immediately. Of course many genetic changes are deleterious to some extent but most effects can either be buffered for a short time because they are minor, or they confer some other advantage. In the core machinery of the cell this is much (!) less tolerated because even tiny chemical inefficiencies would immediately be amplified millionfold. Odds are, the DNA/RNA core machinery of all life sits in a steep local optimum. It’s not necessarily a global optimum but it’s essentially impossible to evolve out of because any individual change, or even a handful of coincidental changes, would leave the organism a lot worse off.