[shakil]

Any time I read about how some part of the body has no function, I try to remember that our understanding of the body is superficial at best, it was only a 100 years ago when getting your blood sucked by leeches was the standard of care for many conditions. Mother's milk was supposed to contain useless and indigestble substances till it turns out they were important for developing the baby's gut microbiota. Entire organs such as the thymus were considered useless and then we began to understand the critical role they play in our immune systems. Once we consider that even critical muscles waste and atrophy unless used, we have to understand that whatever we retain in our bodies must be absolutely critical to its long-term survival, and there is no such thing as a "junk" component.

[dekhn]

.... you know that leeches are used in modern medicine, right? Not for bloodletting, but many of the contexts that leeches were used for have enough overlap with bloodletting that it's actually best to assume that folks actually did use leeches to good effect.

The core of this ongoing debate is mainly about some regions, like Alu, which appear to truly have no function, and have little to no functional effect. I think scientists could still make a good faith experimental effort at attempting to salvage this idea, see Eddy's proposal about a "neutral genome" here: https://www.cell.com/current-biology/comments/S0960-9822(13)...

The fairest way to describe it is that scientists continually find that more and more DNA that was neglected as having zero functional (no observable, measurable, directed activity that is under evolutionary selection pressure) effect, does indeed affect the fitness of organisms in ways that are not accomodated by existing theories of gene expression or regulation. It seems unlikely, however, that we will ever truly be able to point to every single base pair in teh genome and say "it's under functional pressure", and if that's the case, it's easy to make the argument that some of it is "junk".

However, complex information processing systems like life don't really fall into simple classification, "junk" is really a subjective term, we should focus instead of measurable scientific phenomena like fitness.

[true_religion]

Yes, the main problem with leeches in historical use was that medical theory believed disease was caused by a miasma and that drawing bad blood would cure the cause.

Today, we still use leeches but only for those cases where they can provably affect the cause.

[ksaj]

That reminds me of both tonsils and the appendix - two body parts that are often referred to as "useless" even though they probably do some function we're just not aware of.

As for appendix, it seems to be home for bacteria that can cause dementia but are otherwise useful in the gut. I don't know if that's actually its purpose, but if so, it seems pretty important.

[dekhn]

The appendix is vestigial, not useless. It's fair to say it's vestigial, because you can remove it from large numbers of people and they don't die, which is very different from organs like the liver, heart, or lungs.