Sir

Your News Feature “Skeleton keys” (Nature 433, 454–456; 2005) contains two statements that, together, are misleading. First, it is stated that “Clovis people had made it to the southwestern United States by 11,500 years ago”, and second, that Kennewick man is “a 9,000-year-old human skeleton”.

So Kennewick man lived 2,500 years after Clovis? No, in fact, he lived some 4,000 years after (see M. A. Jobling, M. E. Hurles and C. Tyler-Smith Human Evolutionary Genetics, Garland Science, New York, 2004).

The confusion arises because the Clovis date is in radiocarbon years, whereas the Kennewick date appears to be somewhere between radiocarbon and regular calendar years: 8,400 radiocarbon or 9,300–9,500 calendar (see http://www.cr.nps.gov/aad/kennewick/c14memo.htm).

Radiocarbon years differ from calendar years because the amount of 14C in the atmosphere has varied considerably in the past. So the quoted Clovis date of 11,500 years corresponds to approximately 13,500 calendar years.

Please be consistent, and stick to calendar dates so that non-specialists can understand.