“Precambrian rabbit[edit]
J. B. S. Haldane famously stated that "fossil rabbits in the Precambrian" would disprove evolution — and this has been a talking point in philosophy of science for some time. This phrase is reported to be a rebuttal to the accusations that evolution is not falsifiable. However, the reality of disproving evolution in this manner is quite complicated. As science is based on an interplay between theory and evidence a single point of data is not enough to completely destroy a theory - just as much as an excellent theory can't win out against overwhelming data. Such a thing as finding fossilised rabbits wouldn't cause scientists to throw the theory of evolution out completely and immediately, so a little more explanation is needed.
First of all, it must be remembered that the fossil record is merely supporting evidence for evolution. This is contrary to the ideas put forward by creationists that state the gaps in the fossil record prove evolution to be false. If the fossil record simply did not exist it would make no difference to the validity of the theory of evolution — indeed, natural selection was initially formulated without the aid of fossil record, and subsequent DNA evidence can stand completely without it. The simple truth is that a single strange fossil would probably not make much difference. In practice, the evidence in the fossil record which supports evolution is so overwhelming that a single fossil would be regarded as curious certainly, but compared to the mountain of evidence in favor of evolution it would probably be regarded as an anomaly while more data was awaited. Imagining the fossil rabbit in the Precambrian as disproving all of natural selection would confuse the specifics of an individual evolutionary pathway with the falsification of the whole theory itself, as mentioned above.
However, the existence of entire groups of anomalous fossils would be a different thing — Haldane did say rabbits after all. Again, in practice an effort would initially be made to fit the new data into the existing framework — this is not cheating but simply the way science works. But still, in principle some quite major revisions to the theory may be needed to explain them. Such a situation would not immediately and conclusively prove a special creation over a naturalistic evolution, however, a key point that creation proponents tend to stubbornly overlook. Eventually, a new theory would develop to include these oddities, but this isn't necessarily a special young Earth creation as this assertion would also require supporting evidence, and lots of it. Perhaps this anomalous group was due to a now extinct second genesis, which would be a remarkable find, but unlikely to disprove evolution outright. Regardless of what it was, this new theory would explain both the evidence we have now and the hypothetical rabbit fossils and would indeed be science fully supported by evidence.”
The above is from: