Wednesday, June 17, 2009

From an evolutionary perspective

Stuttering has a genetic component. First, homozygotic twins are more likely to stutter both than heterozygotic twins. Second, several locations on chromosomes are correlated to stuttering. These two sources of evidence in combination with research from other disorders allow two conclusions. First, there is not a single gene for stuttering, but many different ones or combinations of. (For example, deafness is associated to tens of different genes.) Second, genes are not a sufficient condition to develop stuttering in all people: if you have the genes, you could be fluent. And if you dont have the genes, you could develop stuttering. However, the likelihood of being in the stuttering group is influenced by the genes. For the non-genetics group, it is likely that a neurological incident (possibly enhanced by environmental stress) kickstarts stuttering. Examples would be an infection or a brain trauma, whose likelihood could also be related to non-speech related genes. Such genes would probably not show up in correlation studies as their signals are too weak and multi-factorial.

Due to its genetic component, stuttering is a candidate for evolutionary adaptation. About 1% of the population stutters, and did not recover naturally from the sample of 3-5% of children who temporarily stutter. It is interesting to look at the past history of stuttering, and ask whether this ratio was stable throughout the evolution of speech in humans, or whether in fact the proportion of the stuttering population was greater in early humans. If the proportion was indeed greater, there must have been evolutionary pressure that reduced the proportion to the 1% of today. (A gender perspective would be even more interesting, as women are 4 times less likely to develop stuttering. This could suggest that they were under greater evolutionary pressure, however this might not be directly linked to speech but more general pressure that favours stable and fast maturity of females for the survival of tribes.) The question is whether this scenario makes sense theoretically and fits all known data, and if it makes sense, whether nature took this course, and whether experiments can be devised to check predictions.

The mechanism of exactly how and why evolutionary pressures made humans evolve speech unlike in their closest relatives the apes is not clear. It is reasonable to assume that humans started out like the apes of today in that they used different sounds with different intonation to communicate mental states. The crucial aspect that gave advantages to early humans is probably the emergence of rudimentary grammar that allowed early humans to combine those tens of sounds representing general concepts into thousands of different sentence combinations. However, grammar requires extra brain resources/modules. So instead of a mental concept being associated to one sound and going to the motor cortex, it now took a detour to a new region that combines words into sentences to better represent and communicate a mental concept. This ability to create sentences imposed great pressure on the brain and at the same time more greater fine motor control of the muscles is needed. A reasonble guess is that the brain divided up the pathway of making sounds into two: one old for sounds (how we speak), and one more recent for automatic speech where the early human could focus on what to say (and also on how to combine words) as opposed to how (intonation) he would say it. The social and natural smile might be an analogy.

It is reasonable to assume that nature favours fluent speakers. This is especially true as early humans became more and more organised in tribes, and social skills and communication skills became important, and a lack thereof a distinguishing feature from others. The evolutionary pressure only started when fkuency became important. So it might well correlate with emergence of culture. (Another example would be genes for dyslexia, that could only have been exposed to evolutionary pressure when writing became important, unless dyslexic have other defficiencies.)

It is possible to at least in principle falsify the theory. My theory is that early humans had more genes creating unstable speech, stuttering,and these are being selected out by evolutionary pressures. So the theory predicts that a higher proportion of early humans carry stuttering genes. So a comparison between a sample of modern humans and early humans would reveal sample differences. In practise, such a test is not possible yet as old DNA degrades relatively fast, unless preserved under special conditions. The promising news is that new techniques are being developed to extract ancient DNA. For example, such techniques have been used in Neanderthals with an age of tens of thousands of years. So there will probably be a DNA bank at some point in the future on ancient DNA which could be used.


Anonymous said...

Well, what if the genes that ultimately cause stuttering have nothing whatsoever to do with speech? I mean, maybe 90% percent of the population may carry these genes, but it is only when there is a very specific environmental trigger that the stutter can develop. This is a valid question because nobody has pinpointed any "stuttering gene". In other words, to me, your evolutionary perspective is just science fiction until they actually identify what the genetic component really is.

But I have a question for you. I read an old sci-fi book recently called "Last and First Men" written by Olaf Stapledon in 1931. In that book, there is a race of martians who communicate not by speech, but by electromagnetic fields. They live side-by-side with humans, and eventually a new race of human is born. These new humans are able to communicate with each other directly from brain to brain by radio communication ... a kind of radiotelepathy. My question: if, in 100 years time, scientists are able to develop a device that would allow radiotelepathy between people, do you think that stutterers would still stutter in this mode of communication?

Tom Weidig said...

But even if stuttering only occurs in 10% of the carriers, the genes have something to do with stuttering! They will be selected out but more slowly.

Lets assume mating success:

stutterers: 49%
non-stutterers: 51%

and then

non-carriers: 51%
carriers: 90% * 51% + 10% * 49%

As I said many times before, there will not be a stuttering gene as such, except possibly in stuttering families where everyone stutters. More likely 10s of gene combinations that increase likelihood.

You should also not forget that selection on speech MUST have happened! Else we would not be able to speak!

Anonymous said...

I knew that you would make this kind of argument, and I was hoping you wouldn't, because it is very simplistic.
Let's say there is one gene which *indirectly* causes stuttering, but only under certain specific environmental circumstances. And let's say that this gene is present in 90% of humans (stutterers and non-stutterers). For this gene to be present in such a large percentage of humanity, it must confer some evolutionary advantage. So, this hypothetical gene confers an evolutionary advantage, but it occasionally also causes stuttering, which may or may not stop the carrier from breeding. In that case, your argument would be simplistic (to say the least). I can raise this argument against your theory because nobody has yet found any specific gene(s) responsible for stuttering.

The search continues.

Tom Weidig said...

Being a simple mechanism does not make it wrong. And it might well only be a first approximation to a more complex mechanism.

How do you think that you are what you are? Why can you speak? Evolution and genes does work. We have moved from no speech primate to speech primate. So there was pressure on speech genes that led to inferior mating success.

Yes, but you need to look at the 10% with an alternative gene, they will have better mating/survival chances. Maybe the Neanderthals had this 90% gene, and where are they now? They are gone.

As I said many times before twin studies, stuttering families, and other research show genetics influences.

Anonymous said...

You and your twin studies. The only thing twin studies tell us with any certainty is that there is a genetic influence AND an environmental influence. It says nothing about what the genetic influence is and it says nothing about what the environmental factors are.
My point in the previous post was very much valid but you failed to address it. The genetic influence may well be a gene (or genes) unrelated to speech, that confers an *advantage* to the carrier but, on rare occasions under certain circumstances (e.g when specific environmental factors are present) also cause stuttering. Such a gene would NOT be selected out of the gene pool because it confers an advantage.

I find it astounding that you can talk about the evolutionary perspective of stuttering, when nobody knows the genes responsible.

I look forward to Tom2020, because Tom2009 still has a lot of bugs.

Tom Weidig said...

>> The genetic influence may well be a gene (or genes) unrelated to speech, that confers an *advantage* to the carrier but, on rare occasions under certain circumstances (e.g when specific environmental factors are present) also cause stuttering. Such a gene would NOT be selected out of the gene pool because it confers an advantage.

I am talking about the past genes mostly.

Your special case might apply for some of the stuttering genes today, and provide a counter force against disadvantage of stuttering.


a) it is very unlikely that ALL genes are like this. In deafness they found 10s of genes. You are constructing a special case that is not representative.

b) I am talking about genes from the last 100'000s of years. It is impossible that all genes are like this. We could never ever have had the development of speech itself! Again I ask you how did we evolve speech? There must have been 100s of fluency genes that carried an advantage.

c) Even today it might happen that a new gene mutation occurs that conveys the advantage of the stuttering gene but not the stuttering, and it would be selected for.

What I agree with you is that it might well be that after 100'000s of years of evolution it is more or very difficult to fine-tune speech fluency as the selection pressure is too slow and the genes have other benefits, but again my argument is one from the last 100'000s of years.

Finally, I am brainstorming about evolution. I am not saying it is like this.

Anonymous said...

It may not be such a special case. A lot of people believe that the amygdala has a role to play in the development of stuttering. You may not believe this, but it doesn't make it incorrect.
Basically, the amygdala is part of the very ancient limbic system of the brain, and is responsible for fear conditioning and also discrimination between the hundreds of facial expressions. A very active/sensitive amygdala confers a survival advantage, but some people believe that it may also trigger the development of stuttering under particular circumstances during speech development. A gene for an active amygdala is totally unrelated to speech, it gives an evolutionary advantage to the carrier, and some say that it might cause stuttering.

I know you're brainstorming, but people normally brainstorm when they are backed up by solid facts. We still do not know the genetic component of stuttering, so talking about an "evolutionary perspective" is meaningless.

Anonymous said...

The found a stuttering gene chromosome 12 I think. There is a family in Cameroon where 48 out of 106 stuttered.