The instant you come across someone talking about "a horse didn't become a bear" or some other such silliness, you may as well move on because the instant they utter that (or similar) remark,
you realize that they don't get that one aspect of evolution is that Archea and other prokaryotes are what evolved into bears, horses, birds, etc. You have two choices with those folks:
This is exactly what I've been saying.
You claim that all creatures evolved from the same common ancestor, yet when challenged to explain how, this is how you avoid it.
You can ask for that proof until your are blue in the face and perhaps you'll find a fool or three who will absolve you and other theists of their burden of proof with regard to the assertion "God exists." I, however, am not among "the three." You see,
when two parties are in a discussion, and one asserts a claim that the other disputes, the one who asserts has a burden of proof to justify or substantiate that claim. (Cargile, James (January 1997). "On the Burden of Proof". Philosophy (Cambridge University Press) 72 (279): 59–83.)
Equally important to note is that science does not present evolution with the assertion that "because life evolved, it's thus clear that God does not exist." Consequently, whether one can show ever stinking link from one prokaryote right up to today's house cats, for example, is irrelevant to the proof of whether (a) God exists. It's irrelevant because in the debate of whether God exists, the initial assertion is "a god exists." In the atheist line of argument, "no god exists" is the conclusion at which they arrive when they examine the arguments supporting the assertion/premise "a god exists." In other words, what atheists say is, "we do not find that the theist arguments in support of the assertion 'there is a god' are sufficiently convincing for us to accept the assertion as true."
Furthermore, atheists, the smart ones at least, realize that no matter how comprehensive becomes, or incomplete remains, science's ability to establish the physical and genetic links/steps that got us from prokaryotes to, say, house cats, the fact is that even a 100% complete set of links would not disprove or prove that one or several supernatural beings exist. What such an accomplishment would do is show beyond a shadow of a doubt that evolution is exactly what got us from "primordial soup" to today's cats, dogs, people, etc.
When it comes to establishing whether one or several supernatural beings exist, let alone what "it" did or didn't do re: the state of the Earth as we observe it today or in the past, the burden of proof rests with the theists. And when attempting to support that assertion, theists do not get to utilize irrational lines of argument to make their case (although nearly all of us try to).
When you ask, in the context of the topic presently at hand, "how did prokaryotes evolve into the critters and plants we see today?", the implication of your doing so is this: science cannot provide a 100% complete picture of how evolved each life form we see today; therefore the assertion "a god exists" has not been disproven. Because it has not been disproven, it must be true." I would think that most high schoolers can see the illogic of that line, yet more than a few grown people put it forth. Perhaps they have forgotten what they learned in school about logic?