Quick facial thinking
I have always found it remarkable that the average person can identify probably thousands of individuals by face “Face Smarts,” (SN: 10/6/12, p. 20) and perhaps hundreds by voice, as well as some just by their gait. Clearly such identification at a distance must have been a crucial survival advantage during our evolution; this unfortunately suggests to me that the larger threat to earlier humans was not lions or tigers and the like, but rather other members of our own species.
Peter Benson, North Oaks, Minn.

Timing human history

No home for Homo sapiens” (SN: 10/20/12, p. 9) says that the Khoisan separated from the rest of humanity “at least 100,000 years ago.” For much (if not all) of that time, the Khoisan have had contact with other humans, and we can assume there has often been some interbreeding. If the Khoisan genome has incorporated sufficient DNA from other branches of the human family, the date of the split would look more recent than it actually was. Do the researchers have an earliest plausible date for the split? Is that date early enough that the split could have happened when Homo sapiens was first emerging, rather than after our species was well-established? And finally, if the split might be that ancient, could the Khoisan and the rest of us have been on the way to becoming separate species had it not been for adventurous sex?
Tim Cliffe, Emmitsburg, Md.

The researchers do not offer an earliest possible date for the Khoisan’s split from other populations, although 100,000 years is in line with other estimates. They do acknowledge that Khoisan mating with other populations could cause underestimates of when the Khoisan actually diverged. The estimated sizes of past populations also affect divergence dates. —Erin Wayman

Test for Alzheimer’s idea
Alzheimer’s protein could help MS” (SN: 9/22/12, p. 14) is very interesting, but it would be difficult to do experiments in people — at least before we have some indication that it might work. One approach would be to compare the prevalence of MS in people with and without Alzheimer’s. If it’s lower in people with Alzheimer’s, that might be an indication that the idea is worth pursuing in people.
Ted Grinthal, Berkeley Heights, N.J.

Don’t try at home
“Dip your finger in water and then quickly dip it in molten lead…” This opening sentence in “Bubble-free boiling” (SN: 10/6/12, p. 16) makes me cringe. It’s scary to think of inexperienced people doing this on their own.
Bob Eramia, via e-mail