Evolution is the change over time of populations of organisms, is it how we account for the great biodiversity on our planet. Abiogenesis on the other hand, is how life came from non-life. It is not uncommon for a theist who doesn't believe in evolution to claim that it is illogical to accept the theory of evolution until we have a complete picture of abiogenesis as well. The argument being that without a beginning, it doesn't matter what else we know, we can't accept the theory. This argument is rubbish, and I aim to demonstrate this with a hypothetical. (I originally made this argument earlier this year, but I can't find it, I assume that means it was in the comments section of someone else's blog)
Suppose there was an alien race who had some scientists watching our planet since it formed. They are meticulous record keepers and are interested in how life could evolve. They were able to record every life form and all of their offspring in immense family trees. For our first exercise, let's assume that in addition to every life form, they were able to observe how the simplest life forms were able to form from non-life. Although the aliens have kept their presence secret all this time, tomorrow they decide that it is time to make contact and give us access to this database. We now have access to the complete lineage of every living thing that ever lived on this planet. If you wanted you could follow a line from yourself all the way to an original single celled life form on this planet. We could also, of course, take any particular organism in the past and look forward at all of it's descendants, you could see which lines went extinct, and which moved forward all the way to current day. Let's assume after examining this evidence, we discover that evolution happened according to the way our current scientists think it happened (naturally we will be wrong on some details here and there, but the overall picture is correct).
In this situation, we have complete information. Anyone who is even remotely reasonable must at this point accept that evolution happened. We were able to connect organisms from today all the way back in time as far as you wish to go, and we can even see how the first organisms arose out of non-life. But was the abiogenesis part of this really necessary? Let's change this up ever so slightly and see what happens.
Suppose we have the same aliens who are observing our planet, but their ridiculously awesome technology is slightly weaker than described above. It will be unable to detect abiogenesis this time, in fact, it is only able to reliably detect a life form if it is of a certain size, let's say at least 1000 cells. Other than that, the situation is exactly as previously described. You can look at the database and track your ancestors all the way back to a being that consists of 1000 cells. You can look at the gradual change, you can see how within a few generations anywhere along the chain your ancestors look about the same, but if you take a long view you can see the gradual changes produce one species after another.
In our second situation, since we have zero information about abiogenesis, would it be reasonable for us to dismiss the idea of evolution? Of course not! We have proof of change over time, we have speciation events, we have a full family tree of every life form on this planet of at least 1000 cells. To claim the lack of information about the start invalidates the rest would be dishonest in the extreme.
Of course, in reality we don't have any where near this much information about the history of organisms on the planet, but that's not really the point. The point is that I want to decouple the idea of abiogenesis and evolution. It is possible to have enough information to know that evolution happened without knowing how abiogenesis worked, this example proves this assertion. And pretty much every reputable scientist that works in an area that relates to evolution will state that evolution has been proven. If you try to counter such a statement by stating we don't know how life came from non-life, you are changing the subject. Understanding abiogenesis is not necessary to an understanding of evolution