Evolution May Take Longer Than Previously Believed

Each of Us Only Has 30-50 New Mutations
June 15, 2011 A new study suggests that humans gain 30-50 new DNA changes (or mutations) each generation. This is lower than was previously thought. The result suggests that humans and chimpanzees shared a common ancestor 7 rather than 5 million years ago. The study authors were also able to figure out that the mutations can come from mom or dad and that neither always contributes more. This was unexpected as scientists have assumed that dads would be the main drivers of new mutations. It is important to keep in mind that this is a very small study...the DNA of only two families were studied. To nail down these conclusions, many more families will need to be studied. And they will be able to as the price of looking at DNA continues to drop.
Slowing Evolution Down
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.

More Information

Looks like seven million years
separates these two.
Mutations Come from Mom and Dad
Now that DNA sequencing is becoming more affordable, scientists will be able to do more of it to answer some fundamental questions about mutations. For example, they can figure out if mom or dad is behind most of these new DNA changes. In this study, researchers found that one child got 92% of his or her mutations from dad and the other child got 36% from dad. This isn't what they expected. For a long time, it has been assumed that dads would contribute more to the mutation rate than moms. This is because men make sperm all their lives while women are born with their eggs. DNA has to be copied each time a new cell is made. And every time DNA is copied, there is a chance for a mistake to creep in. So as men crank out new sperm, they are slowly building up DNA mutations. This is why older fathers are more likely to have children with various genetic diseases. These include schizophrenia, dwarfism, autism, and many others. Unfortunately this study was too small (two families) to give conclusive results as to whether older dads contribute more mutations or whether dads in general do. All it can say is that dad is not always the culprit behind DNA mutations. Culprit may be too harsh a word. Yes mutations are responsible for many genetic diseases. But they aren't all bad. Most people would rather be human than a slug or a boll weevil. And they can thank mutations for that. Mutations are responsible for all of the wonderful diversity we see around us. What a dull world it would be without them.
New mutations don't just
come from dad.