Chances Are Our Hearing Didn't Evolve "To Do" Anything

[Jud]

Perhaps you've read here in the past my objections when people start talking about how our hearing evolved to detect tigers that were about to pounce on us and that is why we are really good at hearing transients (or locating sounds, or whatever) and wondered what I was on about.

 

Here's a link to a good article by an excellent scientist about the pitfalls of the "pan-adaptationist paradigm" (in other words, thinking all aspects of human biology are the way they are for "survival of the fittest" reasons): https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-016-0338-2

[Abtr]

Actually, the article suggests that human evolution in relatively small populations with little evolutionary pressure allowed for much of our genome to be non-functional DNA (so called junk DNA). This may have resulted in a more effective preservation of functional genes and many not immediately functional changes that were only later 'exapted' (recruited) for complex functional adaptations in a genetically 'noisy' but highly creative evolutionary regime. In this context, our hearing must be a complex functional adaptation and (obviously) not some non-functional sensory modality..

[Kal Rubinson]

It also co-evolved with speech which would have placed a different and greater value on hearing in terms both of survival and reproduction.

[Jud]

It also co-evolved with speech....

 

How do you know?

 

 

Edit: The actual scientific evidence suggests against it. In a Guardian article summarizing a genetic study of gorillas -

 

Studying the gorilla genome suggests that the divergence of gorillas from the common ancestor of humans and chimpanzees happened around 10 million years ago. Humans and chimpanzees last shared a common ancestor around 6 million years ago. Eastern and western gorillas split some time in the last million years.One curious find was the evolution of genes associated with hearing, which seem very similar between humans and gorillas. "Scientists had suggested that the rapid evolution of human hearing genes was linked to the evolution of language," said Chris Tyler-Smith, senior author from the Wellcome Trust Sanger Institute. "Our results cast doubt on this, as hearing genes have evolved in gorillas at a similar rate to those in humans."

[Jud]

Actually, the article suggests that human evolution in relatively small populations with little evolutionary pressure allowed for much of our genome to be non-functional DNA (so called junk DNA). This may have resulted in a more effective preservation of functional genes

 

 

Relatively small populations, yes. "Little evolutionary pressure" is a misunderstanding of the article. To explain in a little more detail:

 

- Non-functional DNA has an evolutionary cost, but a very small one.

 

- Such small evolutionary costs are selected against only in circumstances of very, very small "sampling error," which is what genetic drift is: "error" in the sense that survival and reproduction are imperfectly linked to evolutionary fitness, and stochastic and contingent (random) factors have greater chance to operate. Thus tall, strong, smart, handsome Uncle Harry dies in a car accident before he has kids.

 

Think of polling in advance of voting - those polled reflect those voting only imperfectly, which is sampling error. The larger the effective population size, the smaller the "sampling error" (just like the larger the number polled versus those actually voting the smaller the sampling error, until once you get to the actual vote there is zero sampling error), and the more that even small evolutionary fitness costs are taken into account. So bacteria, with huge effective population sizes, evidence extremely strong selection effects, or to put it another way, there's very little genetic drift or "junk" DNA.

 

Thus "more effective preservation of functional genes" with small effective population sizes is also mistaken, because in fact (remember Uncle Harry), with small population sizes there is actually a greater chance some random event will cause functional genes *not* to be preserved. Thus when huge numbers of dinosaurs are killed off and only a small remnant is left, the DNA that formerly made T. Rex now codes for - chickens.

 

 

and many not immediately functional changes that were only later 'exapted' (recruited) for complex functional adaptations in a genetically 'noisy' but highly creative evolutionary regime.

 

 

Quite few initially non-functional changes are exapted and become functional.

 

 

In this context, our hearing must be a complex functional adaptation and (obviously) not some non-functional sensory modality..

 

 

No. We evolved from common ancestors with animals like bats, dogs, whales, etc., so our hearing has actually become much *less* keen over evolutionary time. You have taken a paper that explicitly cautions against finding "the best of all possible worlds" in our genes and turned it into the exact opposite of what was intended.

[Ralf11]

It cannot be said that "our" hearing has become less keen over evolutionary time. Humans and bats have common ancestors but they are pretty far back and their hearing is unknown.

 

Hearing is known in insects, and may have antecedents in aquatic forebears like early chordates much less amphibians & reptiles, some of which are our evolutionary relatives.

 

Hearing in general certainly did not co-evolve with speech, and biologists mean something different when they use the term co-evolve...

 

Some aspects of human hearing could have evolved in concert with human speech but those would most likely be mental structures involved in perception, so I will let the cognitive psychologists deal with that.

[Kal Rubinson]

Edit: The actual scientific evidence suggests against it. In a Guardian article summarizing a genetic study of gorillas -

Yup but high similarity is not an equivalence. Such similarities between apes and humans are typical but it does not mean that there were no significant change in neural functions related to hearing while speech and language evolved.

[Ralf11]

phun with physiology...

[Jud]

It cannot be said that "our" hearing has become less keen over evolutionary time. Humans and bats have common ancestors but they are pretty far back and their hearing is unknown.

 

Yes, a better way of saying it would be that over both the relatively short (great apes) and somewhat longer (less closely related mammalian) evolutionary term, there is no overall direction of progress toward keener hearing.

Hearing in general certainly did not co-evolve with speech, and biologists mean something different when they use the term co-evolve...

 

Some aspects of human hearing could have evolved in concert with human speech but those would most likely be mental structures involved in perception, so I will let the cognitive psychologists deal with that.

 

 

The key phrase as far as I'm concerned being "could have" - so far there is no physiological or genetic evidence for it, as there is for selection on genes involved in speech (e.g., FOXP2).

[Jud]

Yup but high similarity is not an equivalence. Such similarities between apes and humans are typical but it does not mean that there were no significant change in neural functions related to hearing while speech and language evolved.

 

So your original thesis was that speech and hearing co-evolved, and the actual scientific evidence that there was strong selection on speech genes while there was no similar selection on genes governing hearing means to you there was a "significant change in neural functions related to hearing"? So the scientists who conclude otherwise have just got it wrong?

[Ralf11]

I know it's convenient but "co-evolved" means an interaction between two different species - like humans and dogs, flowering plants and pollinators, or men and women...

[Kal Rubinson]

So your original thesis was that speech and hearing co-evolved, and the actual scientific evidence that there was strong selection on speech genes while there was no similar selection on genes governing hearing means to you there was a "significant change in neural functions related to hearing"? So the scientists who conclude otherwise have just got it wrong?

Not at all. What I am saying is that neural changes associated with language development affect both.

[wgscott]

A lot of so-called junk DNA codes for regulatory RNAs.

[Ralf11]

Not at all. What I am saying is that neural changes associated with language development affect both.

 

what supports that assertion?

[Jud]

A lot of so-called junk DNA codes for regulatory RNAs.

 

But folks who know anything rather than inaccurately tossing words around know regulatory DNA ain't junk. They also know "a lot" is a very relative term, and that all non-junk, including regulatory sequences, amounts to ~9% of the human genome.

 

 

Sent from my iPhone using Computer Audiophile

[Jud]

Not at all. What I am saying is that neural changes associated with language development affect both.

 

It's a plausible idea for which there's absolutely no evidence, Kal. That's the thing about the paper I cited, it so clearly explains why in humans and other non-microscopic life forms drift rather than adaptation is the default explanation for genetic change.

 

 

Sent from my iPhone using Computer Audiophile

[mansr]

So your original thesis was that speech and hearing co-evolved, and the actual scientific evidence that there was strong selection on speech genes while there was no similar selection on genes governing hearing means to you there was a "significant change in neural functions related to hearing"? So the scientists who conclude otherwise have just got it wrong?

If scientists can't even understand a straight piece of wire, how could they possibly be trusted on something as complex as genetics?

[Jud]

If scientists can't even understand a straight piece of wire, how could they possibly be trusted on something as complex as genetics?

 

 

(OK, OK, so I do want to insert here that it's not the properties of a length of wire by itself we're interested in, any more than thoroughly understanding the specs of a capacitor will allow us to design a complex circuit including capacitors without any mistakes - though it certainly helps. At least one very good engineer who likes to do a lot of measuring - yes, I know that's redundant - Bruno Putzeys, has measured effects of cables as part of audio systems that he says are significant. And at least according to Bruno, the measurements show neither audiophiles nor skeptics were entirely correct.)

[Kal Rubinson]

what supports that assertion?

The neurology of language processing includes both hearing and speech (and other things).

[sphinxsix]

If scientists can't even understand a straight piece of wire, how could they possibly be trusted on something as complex as genetics?

Hah!

 

You've just made me reconsider my willingness to trust them with the audiophile DNA modification.

 

As for the evolution of hearing - there are many examples (even here on CA) that maybe it hasn't evolved that much.

[Jud]

The neurology of language processing includes both hearing and speech (and other things).

That's absolutely true, which is what makes it a plausible hypothesis. But when scientists checked out the genes involved in hearing to see whether they saw the same signs of selection that we see in genes related to speech, they weren't there. So it's a plausible hypothesis refuted by the evidence.

 

It may be useful to establish more of a context for discussion.

 

When we compare genomes, we see some broad categories of DNA sequence:

 

- The two broadest categories are sequences that mutate at the background rate and sequences that are under selection.

 

- In humans the background rate is (very) approximately 130 mutations per generation. Sequences that are under selection mutate faster or slower than the background rate. Mutation at the background rate is in effect a signal that a sequence (at least as a particular sequence, rather than as "bulk DNA") isn't important to the survival or reproduction of the population. About 90-91% of the human genome falls into this category.

 

- Sequences that are under selection are either conserved, in which case they mutate slower than the background, or new mutations, in which case they are changing faster than the background.

 

- An example of conserved sequences are the Hox genes, responsible for animal body development and configuration. Your Hox genes are reasonably similar to those of worms and fruit flies, meaning they have been conserved over hundreds of millions of years of evolutionary time. This is a signal that these DNA sequences are essential for survival and reproduction of the population.

 

- New mutations can be either advantageous or disadvantageous. If advantageous, a mutation will become "fixed" in the population (i.e., show up in all members) more rapidly than would be the case with the background mutation rate. If disadvantageous, a mutation will disappear from the population more rapidly than would be the case with the background mutation rate. Sickle cell trait is an example of both. In African populations affected by the malaria parasite, it was advantageous and rapidly moved to fixation. Away from the malaria parasite it is a disadvantageous mutation because it can cause sickle cell anemia. In the absence of medical treatment it would be disappearing rapidly from populations with African ancestry that are no longer so widely affected by malaria.

 

- Advantageous new mutations are not the only way new DNA sequences move to fixation in populations. Much more common in non-microscopic life forms is fixation due to drift, the "sampling error" I discussed in an earlier comment. All of the human genome that is not under selection - the 90-91% figure I mentioned above - is the result of fixation due to drift, so-called "neutral (or near-neutral) evolution." Thus neutral evolution is very much the rule, and adaptation - presence in the genome due to selection - is the exception.

 

- Therefore, whenever we look at some aspect of the human genome, we are dealing with a high probability that the sequence under consideration is there simply because of drift - random "sampling error" - and not because it was advantageous. So in order to prove any hypothesis that some aspect of our makeup exists because it was advantageous to our survival, we must show it is part of a sequence that is under favorable selection, and not just part of the usual background mutation level.

 

The gorilla study I referred to earlier showed genes involved in speech moved rapidly to fixation in the human population, a clear signal that the capability of speech was advantageous to human survival and reproduction.

 

Genes involved in hearing did not display this level of rapid change. Of course this doesn't mean hearing isn't important. As you point out, you can't have one (speech) without the other (hearing). What it does show is that the level of hearing acuity reached by the great apes (not particularly high compared to other mammals) did not require any "boost" in capability for humans. So forget about our developing keen hearing in order to hear a tiger snap a nearby twig (or for that matter being able to smell them - our sense of smell is no great shakes in comparison to other mammals' either). What our genes tell us is that being able to communicate with each other in an especially sophisticated way by means of speech was one of the great advantages in the evolution and survival of modern humans.

[jabbr]

A lot of so-called junk DNA codes for regulatory RNAs.

 

Or site for epigenetics ... or sites involved in the structure or supercoils and 3D structure of chromosomes themselves.

[jabbr]

If scientists can't even understand a straight piece of wire, how could they possibly be trusted on something as complex as genetics?

 

Scientists are very seriously trying to study genetics.

[Jud]

Or site for epigenetics ... or sites involved in the structure or supercoils and 3D structure of chromosomes themselves.

 

Epigenetics has received a lot of publicity, but the effects of epigenetic changes may last at most in rare cases for perhaps 20-odd generations. That sounds like a lot, but it is not close to evolutionary time scales.

[jabbr]

Epigenetics has received a lot of publicity, but the effects of epigenetic changes may last at most in rare cases for perhaps 20-odd generations. That sounds like a lot, but it is not close to evolutionary time scales.

 

So called "junk" DNA or non coding regions have effects that go beyond the more straightforward specification of the amino acids in a protein. They can be involved in the regulation of gene activity and the structure of the entire chromosome itself. https://www.ncbi.nlm.nih.gov/m/pubmed/20300217/