The creationist argument about life's origins can be summed up like this: "If you can't show how something happened, you can't prove that it happened either." This is an equivocation fallacy equating the implications of two different questions: "How did X happen?" and "Did X happen?" Let me use some examples of why this is fallacious thinking (if it wasn't already clear enough).
Scenario #1: Suppose you come home one day and find that your television, DVDs, computer and lawnmower are gone. You see tire tracks in the dirt driveway that are not from your own vehicle. Finally, you had a security camera mounted on your front porch. When you review the footage, you see a strange truck drive into your driveway. Three people get out and then walk out of the camera's view. After fifteen minutes, you see them returning with each of the missing items and loading them into their truck before finally driving away. Problem is, the camera didn't catch them actually entering the house. You can assume that they used the back door or a side door or even the chimney like Santa Claus. Maybe you left the doors unlocked or they picked the locks. You don't know. Ultimately, you can't explain how they got into your house. You can come up with various possible explanations, but you can't prove any of them. So if the creationist logic of "If you can't show how something happened, you can't prove that it happened" is valid, that means that you cannot prove that you were robbed because you cannot show how the robbers got into your house. Just as with evolution relying on the existence of the first living thing in order to happen, the robbery relies on the robbers getting into your house in order to happen.
Scenario #2: Imagine you are one of the first meteorologists to study tornadoes. You go out and observe tornadoes over a matter of years and use your observations to summarize the life cycle of tornadoes: the formation of a funnel cloud, the growing stage, the mature stage and finally the dissipating stage. Then you realize there is something that you don't know: how the funnel cloud forms in the first place. You have various ideas about how it happens, but cannot test or prove any of them. If the creationist logic of "If you can't show how something happened, you can't prove that it happened" is valid, that means you cannot prove that tornadoes form at all or that your life cycle model is correct because you cannot prove how tornadoes form in the first place. Just as with evolution relying on the existence of the first living thing in order to happen, the tornado life cycle relies on the initial formation of a tornado in order to happen.
Scenario #3: You are now an anthropologist studying a newly-discovered tribe on an island in the Pacific Ocean. You study their oral history as well as artifacts from their past, forming a detailed history of how their society has changed over the past 500 years. You can even divide their history up into eras based on the changes of leadership, the invention of new fishing techniques and clothing and the development of new concepts and words. However, you can't get any data on them past the 500-years-ago mark. How the first members of their civilization got to the island or where they came from are a mystery to you. Maybe they came from a neighboring island, or maybe they arrived from mainland South America. Now, if the creationist logic of "If you can't show how something happened, you can't prove that it happened" is valid, that means you have to throw out everything you learned about these people and how they changed over the decades because you can't show where they came from. Ancient tablets and hieroglyphics? Nope. History passed down orally from generation to generation? Nope. It's all meaningless until you can show where they came from. Your studies were obviously for naught, you poor anthropologist.
So what am I saying with all this? That abiogenesis happened even though we can't demonstrate how it happened? No, that's not what I'm saying at all. I'm saying something much more general: that life happened even though we can't show how it happened. Living things exist and this is observable fact. It follows that living things therefore got started somehow. We don't have to know how they came into being to know that they did come into being. That's all that evolution needs in order to work: living things. In conclusion, the prerequisite for evolution (that life began) has been filled.