If histones change too slowly to provide an accurate calibration of the molecular clock, then which molecules do change at the correct rate—and how do we know that they do? The answer to these questions for most evolutionary biologists usually runs something like this. We already know that the animal phyla evolved from a common ancestor and we also know roughly when they did; therefore, we must reject studies based on histone sequences because the conclusions of these studies would contradict that date. But do we really know these things, and if so how? Assumptions about the window of time in
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