This is a great question. You should ask the principle scientist of the study, Chris Poulsen. He has a twitter account that you can reach out to him through. T

This is a great question. You should ask the principle scientist of the study, Chris Poulsen. He has a twitter account that you can reach out to him through. This is not my field, but my guess would be that 24,000 years ago was chosen because it corresponds to the Last Glacial Maximum. The obvious reason is that it represents the lowest global temperature within a timeframe of normal human activity, and was a recent major turning point of the global climate. Beyond that, I'm not sure why the Last Glacial Maximum was specifically chosen as the starting point but I'm sure someone more versed in climatology may be able to comment on the significance of it.