Comment by bbor | Hacker News Reader

Comment by bbor | Hacker News Reader

I saw this on bsky a few days ago and was fascinated, so glad to see it made its way here! I had this question that went unanswered, so hopefully there's some experts/fellow musers here:

Why grow up if you don't need photosynthesis?

From my perusal of The Literature (AKA an amazing Wikipedia article + the paper discussed in this link), we have a few strong theories about the prototaxites:

1. They fed off decaying matter in the soil through a vast network of mycelia/roots, and didn't photosynthesize.

2. They grew large, trunk-like protrusions, but without any branches or leaves (that were fossilized, at least). Like, way, way taller than the bugs, moss, and short weird proto-grass around at the time.

3. These protrusions were regularly burrowed into by aforementioned bugs, which surely was a major health risk.

4. Even before this analysis, they've always been something of an oddball compared to contemporary life -- so everything's on the table, so to speak.

Given those facts, I'm pretty stumped as to what evolutionary pressures might have driven them to grow upwards. At first I considered spore (seed?) dispersal, but mushrooms seem to get along just fine without towering above everything else, and I don't think wind speeds were way lower or anything. Anyone here have any better guesses?

When you feed off decaying matter, hosting a colony of bugs which go out daily, bring back food and leave waste all around your environment sounds like a pretty good strategy when you don't have any way to move to better pastures?

Isn't this a long time before insects and such though?

Wikipedia says:

"evidence of arthropod boreholes in Prototaxites has been found from the early and late Devonian, suggesting the organism survived the stress of boring for many millions of years.[30] Intriguingly, boreholes appeared in Prototaxites long before plants developed a structurally equivalent woody stem, and it is possible that the borers transferred to plants when these evolved"

and for Arthropods:

"The evolutionary ancestry of arthropods dates back to the Cambrian period."

which is about 100 million years before the Devonian

also:

"The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about 420 million years ago in the Silurian period.[82][Note 3] Attercopus fimbriunguis, from 386 million years ago in the Devonian period, bears the earliest known silk-producing spigots"

so there could have been all sorts of weird and wonderful bugs burrowing into Prototaxites I guess

One reason to grow upward is also just the need for oxygen. Mushrooms do this too - they tend to colonise and begin to fruit in high CO2 concentrations, but eventually want to escape it. And you can see the negative side effects of not being able to do this in mushrooms too (if the CO2 is too intense they grow too fast relative to their thickness [trying to reach air] and become nasty and stringy looking)

If I remember correctly, there was this hypothesis that these things were covered by symbiotic algae so the idea is that they grew tall to increase the photosynthesis of their symbiotic organism (so kind of a giant lichen). But I can't find the source anymore, and I think that there isn't much evidence of any photobiont.

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