

Did Neurons Evolve Twice? - digital55
https://www.quantamagazine.org/20150325-did-neurons-evolve-twice/

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sago
Ugh, I hate reporting about evolutionary biology. This isn't an egregious
example, but the 'it challenges the idea that complexity increases over time',
'it calls into question the origin of the brain', 'forces scientists to
question how animals got their start', 'evolution debate', etc, gives me the
creeps.

In a country where more than 50% of the population thinks that evolution is
bullshit, it doesn't help to try to phrase scientific disagreements to make it
sound like it is some fundamental problem being argued about, instead of the
placement of one phylogenetic branch.

Now, the science itself is interesting. The DNA sequencing is quite weak
evidence, from what I can see (its been 20 years since I did my PhD, though),
but the radical difference in biochemistry is a good sign that, if they didn't
evolve twice from scratch, at least the common ancestor probably used the
cells that later became neurons in a fundamentally different way.

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Jedd
My initial reaction too - though the phrase that triggered it for me was
'challenges the deeply entrenched idea that evolution progresses steadily
forward'.

I am clueless about the identity of the demographic that holds this deeply
entrenched idea, other than the vaguely labelled 'breathtakingly bewildered'.

When you say 'in a country where more than 50%' ... are you referring to the
country that you're currently in, or the one I'm in, or the one where you
assume most people who are reading this article are in, by the way?

~~~
Elrac
He's most likely referring to the US. Depending on who does the polling and
how, about half the population thinks the Theory of Evolution is a bad joke or
a scientific conspiracy or something.

The opposing side is known as "Creationists." They like to characterize
scientifically informed people as "Evolutionists" but of course that's more
misinformation. One of the various tracks of Creationist argumentation is that
scientists make the "unjustified assumption that change progresses at a more-
or-less constant rate" or, "that the rate of change today is the same as the
rate of change way back when." They like to claim we can't know the decay rate
of radioactive carbon (carbon dating, y'know) was the same a few thousand
years ago (when the Earth was created) as it is today.

I mention all this because this red herring about steady evolutionary progress
is one of the arguments of Creationists, and so it's annoying to see those
words used in that context in a science-y article.

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hayd
> Because they are so complicated, they are unlikely to have evolved twice.

I don't get this argument at all. It seems obvious that most of the time an
organism gains a property it immediately dies (or fails to pass it to its
offspring). They have lots of tries, they're not special.

So: "Common ancestor" my arse. They insist on drawing these diagrams as trees
when they are much more complicated graphs.

~~~
happimess
I think that you misunderstand what the article means by "evolved twice."
Neurons certainly did not appear out of whole cloth in one generation, and the
process involved many false starts and dead ends.

The question is, do all currently living creatures with neurons trace back to
a common ancestor who also had neurons, or are there several neuron-having
lineages alive today, each of which traces back to a different earliest-
neuron-having ancestor. In the latter case, each of those neuron-having
ancestors would have shared a common ancestor, but one that did not have
neurons.

The lineage of all life on earth certainly creates a more complicated graph
than a tree, but at the time scale needed to create a novel feature like
neurons, it certainly looks like a tree. To say otherwise, you'd have to point
to a case where genetic information passed laterally between species, rather
than vertically from parent to child. This probably has happened [1], but is
not so common as to make all tree-shaped charts misleading.

[1][http://en.wikipedia.org/wiki/Symbiogenesis](http://en.wikipedia.org/wiki/Symbiogenesis)

~~~
im2w1l
>point to a case where genetic information passed laterally between species

[http://en.wikipedia.org/wiki/Horizontal_gene_transfer](http://en.wikipedia.org/wiki/Horizontal_gene_transfer)

~~~
happimess
Thanks, that's a more apt link than symbiogenesis.

However, it doesn't change the fact that, when viewed from 10,000 feet, the
tree of life looks more like a tree than a web. Especially with regards to the
question "did neurons evolve twice," very little is lost by ignoring that
sometimes cousins marry, and sometimes a virus leaves a chunk of genetic
material in its host.

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storgendibal
So if neurons did evolve twice, it is unlikely they are the Great Filter.
Hopefully it is still behind us.

~~~
Udo
Don't believe people when they say rubbish like this:

    
    
      Because they are so complicated, they are unlikely 
      to have evolved twice.
    

It's not like that complexity has spontaneously come into existence, it's the
result of normal evolutionary processes which give rise to complexity of all
sorts all the time. If you ask biologists about this, most would agree there
is nothing inherently improbable about the evolution of any known cell type.

Of course that doesn't mean those processes wouldn't produce a completely
different organism on another world, _but_ chances are this organism would
have to solve similar problems as the ones here on Earth. The need to process
information and react to it in some way is not unique to our world in any way.

 _> So if neurons did evolve twice_

Looking at _all_ the systems organisms are using to carry information (not
just neurons, but all kinds of chemical signaling) suggests that these systems
perform a very basic and ubiquitous role in all ecosystems. Even if the
assertion about neurons _was_ true (which it most certainly isn't), claiming
all of our biological information-passing systems as special and "unlikely to
evolve twice" is stretching an implausible proposition even further.

 _> it is unlikely they are the Great Filter_

It's unlikely anything _within_ biology is the Great Filter, but the thing at
the _beginning_ of biology might be a candidate: it's unclear how probable the
formation of a simple cell from non-biological material is. My personal hunch
is that the Great Filter might really be a combination of factors, both local
to us and global, and abiogenesis _might_ be one of those that globally lower
the odds for life significantly.

~~~
storgendibal
Thanks for the detailed, informative reply!

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infinity
_Because they are so complicated, they are unlikely to have evolved twice._

This reminds me of the beginning of the babel fish argument for the non-
existence of god by Douglas Adams:

 _Now it is such a bizarrely improbable coincidence that anything so
mindbogglingly useful could have evolved purely by chance ..._

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raverbashing
Wings are complicated and they evolved multiple times (insects, birds, bats)

Neurons (or at least proto-neurons) are not extremely complicated, at least
simple enough so that they can "do something" and then evolve.

(Not sure how/if they were able to evolve without anything to control like
muscles or at least some motor mechanism, but even bacteria does that)

~~~
jug
Eyes (complex, image forming ones) are also believed to have evolved
independently. Nature got it more right with the squids -- their eyes got
wired so they lack a blind spot! Actually there are many cases:
[http://en.wikipedia.org/wiki/Convergent_evolution](http://en.wikipedia.org/wiki/Convergent_evolution)

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koberstein
Pretty sure everything has evolved infinitely.

