People tend to think that DNA passes down unchanged from generation to generation. But this isn't true. DNA goes through lots of changes before that bouncing baby inherits it.
Most of these changes involve mixing and matching what is already there in a process called recombination
. But every generation, a few new changes pop up too.
Now the fact that this happens isn't news. It is common enough that scientists have given it one of their awful names--de novo
. What is news is that scientists are finally coming up with a rough idea of how many new mutations happen in each generation.
The most recent study
puts the number of new mutations somewhere between 30 and 50. Previous estimates had been around 100-200 new DNA changes per generation.
These older estimates involved some guesswork as scientists couldn't look at a family's entire set of DNA. This is what is different in this study.
Scientists looked at the DNA of two families that each consisted of mother, father, and child. What they found was that one child had 35 new mutations and the other had 49. This compares favorably with a study
done in 2010 that looked at a mother, father and two children and found 70 new mutations.
Figuring this out was not an easy process. They had to look at the over 6 billion letters of DNA for each person over and over again (22 times!) to rule out any technical mistakes. Then they had to figure out which changes happened between generations and which just happened in some of the child's cells or for technical lab reasons.
This is many fewer than scientists previously thought. And if this result holds up, it will affect scientists' estimates of how long human evolution took.
To figure out how long it has been since two species shared a common ancestor, scientists compare their DNA. They figure out how many changes there are and then divide that by the number of changes per generation.
Previous results suggested that humans and chimpanzees shared a common ancestor around 5 million years ago. Using the new numbers, it looks like 7 million is a more likely number. This second number is actually consistent with more recent fossil evidence. But it isn't set in stone either.
This was a very small study that looked at two children and four parents. Many more studies will need to be done before the 30-50 number becomes official. And before we nail down how long it has been since chimpanzees and humans shared a common ancestor.