Letters to the editor

European family ties are knotty
I have trouble understanding “Europeans are one big family” (SN: 6/15/13, p. 8). It says that every person living in Europe today shares a common set of ancestors. First, what does “set” mean? “Set” implies there are certain common characteristics of the members, but people living in Europe 1,000 years ago had only one thing in common: living in Europe. Second, I interpret the article to mean that everybody now living in Europe is descended from people who lived in Europe 1,000 years ago. But this cannot possibly be true. Immigration has been more or less continuous, has it not? The ancestors of many present-day Europeans lived in Africa and Asia a lot more recently than 1,000 years ago.  
Fredric Blum, Merion, Pa.

The reader is right that recent immigrants to Europe might not share ancestors in this time range. Study author Graham Coop notes, however, that these immigrants (and everyone in the world) are probably related to one another over the past 3,000 years or so. The conclusion of the study, though, is that every person who lived in Europe 1,000 years ago and left descendants is an ancestor of anyone with European ancestry today. In other words, everyone with any roots in Europe is related to the same set of ancestors. We used the word “set” to define a specific group of people: all those living in Europe 1,000 years ago who left descendants. It might seem that a Larsson in Sweden and a Rossi in Italy would trace back to different groups of people 1,000 years ago, but actually they would not. The concept is a little mind-boggling! —Meghan Rosen

Counting cracks
The article “Cracked glass tells tales” (SN: 6/1/13, p. 15) reminded me of a high school science fair project I did five years ago — except I impacted Skittles candies with a drop weight instead of firing at glass squares. As it turns out, Skittles are essentially sugar glass, so they crack radially when impacted at high speed. The number of cracks increases with speed, just as the researchers found for glass. Thanks for bringing back fond memories!
Daniel Duncan, St. Charles, Mo.

A chart accompanying the story “Mystery meteorite” (SN: 6/29/13, p. 20) reported that there are 177 confirmed meteorites from the moon. To clarify, there are 177 specimens of lunar meteorites. Many of them are paired with other specimens, meaning that the individual rocks were found close together and are probably part of the same fall. All 177 specimens are thought to derive from approximately 85 lunar meteorites.