DOC,
I think the reason people are going off topic is, at least in part, that you seem to be having an issue with responding to the posts that are on topic.
There are two basic claims in your OP. First, science claims we are all descended from a single bacterium. Second, most atheists are unaware of this.
As for the first claim, it's a matter of interpretation and some speculation. According to the prevailing theories of modern science, we are all indeed descended from a single bacterium, although it is not clear that there is a single ancestral cell that included every single molecule of our ancestral DNA. There may have been some mixing up of DNA fragments, assimilation of free-living cells as organelles, that sort of thing. No one knows, or likely ever will know, the exact mechanism. However, the basic part of your assertion, that there was one sort of organism at one time that is a common ancestor for all current organisms, including all plants, all animals, and all fungi, is generally accepted. Indeed, it is quite possible that there was one specific cell that could be a common ancestor of all multicellular, and most unicellular, life today.
Now, on to claim number two. You claim that very few atheists are aware of that claim. Your assertion is that atheists are ignorant of the claim of modern science, that we are all descended from some extremely simple form of life. On that claim I take issue. I think most people know that. All the ones who have given it any significant thought know that. What's the alternative? There is none, from a scientific standpoint.
I think you have misinterpreted the quibbling such as occurs in this thread for genuine disagreement with the core of the assertion. Many of us note that there is some speculation involved on whether there was a single individual, or a colony, or proto-cells that merged, or many other details that are unknown, but on the core of your assertion, we all agree. We are descended from one celled life.
So here are a couple of on-topic questions.
What makes you think that most atheists are unaware of the claims of modern science with respect to descent from single celled life?
What significance do you think there is to that asserion? Are there philosophical implications? Scientific improbabilities? What is it that makes you think it is important to know? Why do you think it might change someone's thinking if they realized that was what science claims?
It is my opinion that 1) most atheists are indeed aware of the claims that you cite and 2) they don't care. It's not that big of a deal. From a scientific standpoint, it appears to be true, and so there's no point in disputing it. Perhaps as importantly, Hokulele pointed out that, philosophically, there is also no problem. We are related to all living things on the Earth. We aren't just similar, we are quite literally relatives. That doesn't depress me.
Even for believers in God, I don't see an issue. Did one god create man, while a different god created plants? Why would there be a problem for someone believing that God made man by using cyanobacteria and a billion years of mutations? Is it very important to insist that no, we were actually made out of dirt?
I thought though, in some posts, he went a bit too easy on DOC. As I read Meadmaker he doesn't seem to think that DOC is as sly about his intentions as I, personally, think he is. I think DOC knows exactly where he is going with the question of this OP, and that it really has nothing to do with us descending from single cell life and so on. DOC tries to manipulate people into considering his beliefs, plain and simple, he's only very bad at it 
Meadmaker's approach is a good one in any case, he's (she?) saying many wise things.