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In the planet I'm working on, strong tidal forces meant early on in evolutionary history most organisms developed a nacre or chitin-like composites with calcium carbonate, even in internal skeletons.

Over time I wonder if some of these creatures could use metals, not as a replacement but rather as a supplement, to their internal structure, like the addition of iron to create 'steel bones', albeit very watered down steel bones.

Hand-waving the chemical issues with breaking down insoluble minerals in water, the 3 main metals I am interested in are Iron, Aluminium, and Titanium-supplemented bones. I'm wondering what biological changes these might and the changes in behaviour and ecological niche that arise from them? Alternatively, if I'm missing a real fun metallic contender do feel free to let me know (it doesn't have to be common to Earth, just to the Universe, so no Platinum bones... for now).

I will beg forgiveness for my lack of chemistry knowledge in this case (not my strongest subject in school), however, I'm not worried about resource/mineral abundance in this case as the planet in question does not have the exact same element distribution as Earth. I'm purely interested in biology and evolution.

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  • $\begingroup$ Small thought, and definitely not a direct answer to your question: In construction, rebar within concrete helps protect against stretching (and maybe as a backup if it cracks?), but not against compression because that's not we found metal is good for (vs weight and costs). If I were world building, and thinking about stronger bones, "metal additives" would make me assume pulling or ductile requirements, and less pushing or compacting, as a major evolutionary force. Strong tidal forces in a low-G world, for example. Otherwise, consider two layers of bones (like reinforced concrete)? $\endgroup$
    – JSmart523
    May 26, 2023 at 16:53
  • $\begingroup$ "Excess iron disturbs the delicate balance between bone resorption and bone formation, resulting in bone weakening. Evidence suggests that both increased bone resorption, and decreased bone formation are involved in the pathological bone-loss in iron overload conditions." ncbi.nlm.nih.gov/pmc/articles/PMC5318432 $\endgroup$
    – Mazura
    May 26, 2023 at 22:36
  • $\begingroup$ "Bone is particularly sensitive to fluctuations in systemic iron levels as both iron deficiency and overload are associated with low bone mineral density and fragility. Recent studies have shown that not only iron itself, but also iron-regulatory proteins that are mutated in hereditary hemochromatosis can control bone mass." sciencedirect.com/science/article/pii/S0037196321000342 $\endgroup$
    – Mazura
    May 26, 2023 at 22:37
  • $\begingroup$ I'm tempted to VTC this quesiton. (a) you're allowed one and only one question. You're asking at least two. (b) It's a prohibited high concept question. We barely understand evolution. Asking how a simple change would effect the evolution of anything is an HSQ - which means it's way too broad, too opinion-based, too hypothetical, and too likely to have all answers with equal value. (c) This Stack has a LOT of my-bones-have-metal questions. Have you searched this Stack? $\endgroup$
    – JBH
    May 27, 2023 at 5:30
  • $\begingroup$ @JBH My apologies, yes I have, I was intending to ask more about behaviour and development and assume the metal-binding properties away. If it is too speculative I would retract the question $\endgroup$
    – Rexotec
    May 27, 2023 at 9:02

3 Answers 3

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Everything you are looking for is right here on Earth?

Limpets are a real life organism that contain what is perhaps the world's overall strongest biological material. Thier teeth are made of an iron composite based crystalline nano-fiber called goethite interwoven in a special protein matrix which together give the tooth an unprecedented combination of both hardness and toughness. Thier teeth have an even higher ultimate tensile strength than spider silk, they are highly resistant to impact and shear, and they are ridged enough to function as teeth... so a similar material could certainly be used to reinforce a skeleton.

If you specifically want chiton that contains iron, gumboot chiton is an iron rich chiton used in the teeth of meatloaf mollusks. Not quite as tough, but still a very impressively strong material.

Why isn't goethite more common in Earth based animals?

Bones are more than just a a framework for moving around. The minerals in our bones are designed to be reclaimable during times of malnutrition, pregnancy, and healing to help keep our bodies going. Calcium is an extremely important micro-nutrient used by animals in over 100 different biological processes, but is a relatively rare element in nature making it especially important for animals to stockpile. While iron is also useful in many biological processes, it is very common in nature; so, it is much easier to get on an as-needed basis.

Without calcified bones, starvation and cellular malformation would be a much bigger cause of death than bone fractures. Calcium is also much less dense than iron which is important for any animal that plans to walk, swim, or fly. So, there is a big evolutionary pressure to favor a weaker calcium hardened bone or chitin than a stronger base material like iron.

Would strong tidal forces cause iron bones?

Probably not. Iron bones would make you more dense which means aquatic life would be less able to float. Such animals would be more in danger of being dragged along the seafloor by the tide instead of swimming over the dangerous ground below. Instead, you should focus on biochemistry and gravity as the influencing environmental factors.

If your planet has a lot more natural calcium and less iron, then your organisms would be more inclined to sequester iron in thier bones and treat calcium as an on-demand micro nutrient instead. Also, a reduction in gravity could make the added weight of iron less of a disadvantage.

If both strong tides and iron skeletons are both essential part of your setting, then you should certainly keep them, just be mindful that one does not cause the other.

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  • $\begingroup$ Tougher? Or harder? Many common place materials are tougher than tungsten carbide and other cutting materials used in modern machining. Regular mild steel is tougher. You wouldn't want tungsten carbide bones. They would be quite fragile. Teeth might be okay, but that's for their hardness, not toughness. It is quite tough for how hard it is, but against other more common materials their absolute toughness is not exceptional. $\endgroup$
    – DKNguyen
    May 27, 2023 at 0:50
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    $\begingroup$ @DKNguyen They are both, science.org/doi/full/10.1126/sciadv.add4644, but the more important factor here is that nature already includes iron rich chiton like metamaterials that fit the OPs description. Even if only used as an example instead of an exact match, they justify the existence of iron enriched "bones" as a biological feature. $\endgroup$
    – Nosajimiki
    May 30, 2023 at 18:43
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Yes, but as an additive

Calcium is not bad to begin with. Various organism use calcium to enhance the material properties of otherwise pure organic materials. And there is not one single formula for bone, improvements have been made over time, to optimize for different scenario's.

Beavers utilize iron in their teeth. See among other resources: Making Teeth Tough: Beavers Show Way to Improve Our Enamel Ions of other metals have been used in biology for various purposes. The science is still out, this is a topic needing a lot of research.

What is probably not possible is the construction of a solid piece of metal. Or if it is, its practicality will be very limited. What is important for most organisms is the constant repair and maintenance of the material. The material needs to be alive, or it will fail quickly. Making a solid piece excludes this part to be alive. Which might be a viable compromise if individuals of the species have a high dead rate to begin with, like molluscs.

Also it is unlikely that enzymes exist that can produce a pure sample of the metal. The best that probably could be achieved is embedding ions in other material to enhance its strength. Or to make a brittle material like CaCO3 in pearls and corals.

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    $\begingroup$ Calcium is a highly reactive metal. It is also very soft; it can be cut with a knife. I am quite certain that no organism on Earth uses calcium to enhance the material properties of anything. You may be confusing calcium with hydroxiapatite or aragonite, which is a bit like confusing iron with rust, or like confusing chlorine with table salt. $\endgroup$
    – AlexP
    May 26, 2023 at 13:54
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    $\begingroup$ I'm not confusing it. I was less precise. I meant the calcium ion, no organism uses or can even produce elemental calcium. So from the context that should be evident. Also see my last paragraph were I make the exact point you are making. $\endgroup$ May 26, 2023 at 14:50
  • $\begingroup$ You may want to clarify your statement about materials that are not alive failing quickly inside of an organism. I have titanium pins and plates in my ankle right now. They are embedded in living material, but my doctor expects them to outlast the rest of my body. My wife has a knee replacement. It will eventually fail and probably before she dies, but its estimated useful period is measured in decades, which is not "quickly" on the time scales most mammals care about. $\endgroup$ May 27, 2023 at 0:18
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    $\begingroup$ @TimothyAWiseman. Good addition. Titanium quickly creates an oxide layer, if this can be reproduced by the organism the "pat", member could be durable. $\endgroup$ May 31, 2023 at 10:14
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It takes a LOT of energy to extract the pure metals you mention from natural compounds, as well as sorting through a lot of ore (which is particularly concentrated) let alone regular soil and rock. These organisms would require a lot of energy input to grow or be very slow growing. They would also need to sort through a lot of food (if the food even contains those elements to begin with. Otherwise they would need to process a lot of rock or soil.

If the bone breaks though, you can't heal the metal parts without breaking them down the area area and rebuilding it from the inside out starting from scratch. This would be most analogous to how enamel is constructed: with surrounding scaffold cells that then go away, rather than enamel growing itself. Which is why our enamel does not regenerate. Once it's gone, it's gone.

There are also corrosion issues with your chosen materials. Iron rusting is simple to understand since bodies tend to be full of oxygen and water. Aluminum and titanium rely on a passivation layer to not rust which means they need to be exposed to oxygen to develop this later and if this later is broken they need to be exposed to more oxygen so the layer re-develops before rust forms. If you do something like submerge the material in liquid so oxygen and then scratch it, it the passivation layer can't form and it will corrode.

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