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Ancient DNA appears to be at most "several hundred thousand years" old.
Do we have an aDNA sample which we can reasonably confidently attribute to a species which has at least two modern descendants?
PS. This question is much less interesting than I originally thought: there are modern species which are proven ancestors of other multiple modern species (lake Victoria Cichlids).
Assuming you mean by "modern descendants" "two different species that are also different from the ancient DNA", then to the best of my knowledge, no; the large vertebrates (bear, horse, humans, mammoths) that have been focused on for the most ancient DNA samples have common ancestors that are outside the range of ancient DNA isolation. I'm not very familiar with plant ancient DNA; maize DNA from a few thousand years ago is the oldest I'm aware of, but I might easily have missed some.
However, it's important to point out that the way this question is phrased makes it merely trivia, with no scientific significance.
Assuming (from the poster's other questions) that he believes this is somehow relevant to questions about speciation, I would point to modern genomes from ongoing speciation. For example, Heliconius butterflies are speciating, and there's some work looking at the genomic components of that speciation:
Stickleback fish also show a number of examples of incipient evolution, and again genome studies have been done:
Since, as I say, the question as phrased is simply asking for scientifically unimportant trivia, I'll leave it that as far as guessing as to what the actual point is.
DNA is "lego-like" structure from "lego"-blocks
ancient DNA is just a name of more or less long lego DNA sequence
let's say someone in the future finds a box of lego block but does not find any of lego-blocks built toys of our times
or finds sequences only of a few blocks
what would it mean?
another thing - lego blocks are made from a pretty sturdy material (actually like DNA). However, DNA is not the only molecule that can be a template for the protein assembly - RNA can do similar job. However, RNA is extremely fragile and degrades in minutes, and therefore cannot be found in fossils. Have you seen a cake made from lego blocks made from dough and cream? How long such cake will last?
If you don't find a dough lego, can you say that it never existed?
ps i agree with these words from the first answer "However, it's important to point out that the way this question is phrased makes it merely trivia, with no scientific significance."
"Cats have 90% of homologous genes with humans, 82% with dogs, 80% with cows, 79% with chimpanzees, 69% with rats and 67% with mice"; so why not with an ancient ancestor of them we would not find the genetic similarity? $\endgroup$ – Always Confused Dec 21 '16 at 14:30