If you take a look at the Lenski paper[1], it says: > The potentiating mutations are not known to confer any phenotype amenable to screening, so there is no si

If you take a look at the Lenski paper[1], it says: > The potentiating mutations are not known to confer any phenotype amenable to screening, so there is no simple way to distinguish between potentiated and non-potentiated clones. Now, that isn't conclusive evidence that there was no benefit on its own, but it doesn't sound like there's any evidence that it did provide some kind of greater fitness. It sounds like all evidence shows that the original mutation didn't provide any particular phenotypic change on its own, it was only after the later mutation that it had any measurable effect. Now, with dinosaurs it's much harder to tell. We obviously can't do any kind of controlled experiments, or observe them directly. But we can offer up a few potential explanations. One is that some mutations can occur without either helping or harming, but later on another mutation can lead to something useful. Or there may be benefits conferred by proto-feathers. Remember, in addition to their similarity to down, proto-feathers are somewhat similar to hair, and may provide many of the benefits that hair provides: warmth, colors which allow either catching the attention of mates or blending in to the background, better sense of touch (you can feel something before your skin brushes against it), better sensation of the wind, and protection from things that may scrape or cut you. According to Wikipedia[2], the insulation and display (colorful feathers for attracting mates) hypotheses seem to be the more probable. Obviously, we're not going to solve this by speculating here on HN, and it may be that the reason can never be conclusively determined because you can't go back in time to run controlled experiments, but based on the evidence we have those theories sound reasonably plausible. [1]: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3461117/ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3461117/ [2]: https://en.wikipedia.org/wiki/Feathered_dinosaur#Primitive_feather_types https://en.wikipedia.org/wiki/Feathered_dinosaur#Primitive_f...