Re: Why all apes including humans do not have tails

On 01/03/2024 05:31, DB Cates wrote:

On 2024-02-29 1:17 PM, Bob Casanova wrote:

On Thu, 29 Feb 2024 08:05:05 -0800, the following appeared
in talk.origins, posted by erik simpson
<eastside.erik@gmail.com>:
>

On 2/29/24 3:55 AM, RonO wrote:

On 2/28/2024 5:41 PM, erik simpson wrote:

On 2/28/24 3:21 PM, RonO wrote:

It turns out that the common ancestor that between gibbons and the
great apes had an ALU transposon jump into the intron between exon 6
and exon 7 of the TBXT gene.  There was already an transposon between
exon 5 and exon 6.  Monkeys and apes have the ALU insertion in the
intron between exon 5 and exon 6, but the apes have the second ALU
insertion in the intron between exons 6 and 7.  So it turns out that
apes still have the exon 6 sequence in the TBXT gene, but the two ALU
transposon sequences form a stem loop structure in the RNA transcript
that messes up processing so exon 6 is skipped and exon 5 is stuck to
exon 7 in the final ape mRNA.  So part of what makes us human is due
to a transposon insertion mutation into the TBXT gene.
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The insertion happened in the common ancestor of all extant apes, and
has been retained by the extant ape lineages.
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https://www.nature.com/articles/s41586-024-07095-8
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The article is open access.
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Ron Okimoto
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Another effect of this modification is also "Moreover, mice expressing
the exon-skipped Tbxt isoform develop neural tube defects, a condition
that affects approximately 1 in 1,000 neonates in humans10. Thus,
tail-loss evolution may have been associated with an adaptive cost of
the potential for neural tube defects, which continue to affect human
health today."
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Evidently, the advantages of losing the tail outweigh the disadvantage
of the neural tube defects.
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What were the advantages?
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Some other simian lineages have lost their tails, but what is the
advantage?
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Apes did become brachiators, but other simian lineages did not, and some
simian lineages that adopted a similar lifestyle for supporting
themselves in the trees, actually developed prehensile tails as a fifth
limb for supporting themselves hanging from branches.
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For birds there was a selective advantage in terms of weight, and the
tailbones degenerated and fused into a small nub.  The tail was not
lost, and birds still have a nub that they call a pygostyle that still
supports the muscles that control the tail movements and so the feathers
associated with the tail.
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Ron Okimoto
>

I believe the article mentions that bipedalism is speculated to have
made bipedalism easier.
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No causal link there... ;-)

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That could be a just-so story, but mutations
that are adopted and fixed within a population must have advantages that
outweigh potential advantages.
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Indubitably.

>

Really? Drift is out?
 

I don't know if drift is ever out but is it particularly plausible in the case of tail loss, something that seems really rare in tetrapods? Like, what lineages actually lost their tails - like, really lost, not "reduced" or "replaced by a non-bony appendage that serves a taily function": frogs, apes, manx cats... bears are maybe on their way... who else?
Not to mention the article suggests tail loss could be associated with neural tube defects, which would definitely make drift much less likely.

Aside from posture I can't think of what
the advantages might be.  Pants are easier? (Note to literalists: I
don't seriously suggest that.)