No organism would need to appear.
If an organism is found which has genes from two long divergent animals which do not appear on more recently divergent species, that would be a good evidence against evolution.
For example. You have an ancestor A, decendants B and C, and modern species D, E, F, G, and X, where the evolutionary tree is:
A->B->D
A->B->E
A->C->F
A->C->G
and X is the newly discovered species.
If X shares large numbers of genes with D and F that E and G do not have, that is a problem. Once you start adding many more species into the tree, it makes the odds worse and worse that this is just due to the chance occurance.
Let's put it in another way. Picture that each letter below is a certain type of gene. If your entire set of species known was as follows:
1) AKTNCFH
2) OKTJCWB
3) OKTNCNH
4) OKTJCWH
5) RMTJCYH
6) OKSJCYH
7) RMTJEYH
8) RKTJUYH
You can see from these genes what the phylogenic tree must have been like. Certain genes are very common throughout all of the species - K, T, J, C, and H. The odds of the exact same gene being formed more than once is near zero for any sizable protein. So, if you look at (1), you find that its closest relative appears to be (3), it's next closest appear to be (2) and (4), then (6), then (8) and lastly (5) and (7). (2) and (4) only have one gene different and appear to have diverged recently. (5) and (7) are also very closely related. Etc. By looking at how genes change between species, you can tell how closely related they are, so long as you have enough data points (and there are many datapoints in the real world - millions apon millions of species, with thousands upon thousands of genes).
But what if you found a species that was as follows:
9) RKTNEFB
Whoah! The R branch diverged from the A/O branch back near the beginning of the example. But N developed on the other branch than R! But E developed on the same branch as R! But B developed on the opposite branch as R!
Needless to say, because evolution is real, this doesn't happen. If it did, it would be a huge blow to evolution. There's absolutely no reason it wouldn't happen at least occasionally in a creationist world. Many different genes behave in the exact same manner, so there's no "God did it because it wouldn't work well if you mixed and matched lines!". But it never does. Why would God deliberately create genetics to look like a phylogenic tree? Beats me. Are we back to the "Evil Prankster God" theory?
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"Illuminant light,
illuminate me."
™ Version 4.2
