#17 - Samples from the floor of a rock shelter in

[CristinaCP]

I also had the same confusion about the stimulus, where I thought that the critics were hypothesizing about ALL of the samples, not just the bottom layers, and therefore didn't think [A] actually weakened their conclusion. Was my error that I didn't properly infer that the skeptics are not also hypothesizing about the uppermost layers (because the uppermost layers date to the present, and therefore could have been contaminated by the "old carbon")?

[Jeff Wren]

Hi Cristina,

The only reason that the skeptics suggest that the samples could have been contaminated by dissolved "old carbon" is that they don't believe that humans were in North America at that oldest date of the sample (approximately 19,650 years ago).

Basically, the skeptics think to themselves, "Well, those old samples couldn't have been due to humans because humans weren't even in North America at that time, so maybe it was old carbon that contaminated the samples. While the skeptics never specify that the old carbon contaminated only the bottom layer, since that is the date that they take issue with, the explanation of old carbon is brought up to explain that puzzling fact. In other words, the skeptics don't have a problem with the more recent dates given because those are consistent with what is known about human migration.