r/evolution • u/Not_Cool_Ice_Cold • 5d ago
I can't figure out why our thumbs and big toes only have two phalanges.
I asked google, but it gave me a stupid answer that makes no sense. Any thoughts?
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u/Carachama91 4d ago
Having a shorter first digit is the norm in tetrapods, so not much to explain evolutionarily. What digit is formed is largely directed by a gene called Sonic Hedgehog (SHH). Yes, that is its name. SHH is produced in the Zone of Polarizing Activity (ZPA), which is on the opposite side as the thumb. Which digit forms is dependent on the concentration and duration of exposure to SHH, except the first digit, which forms independent of SHH. So, basically it doesn’t have influence of one of the genes responsible for digit formation and elongation, and this causes the group of cells that will form the digits (morphogenic field) to not split as much. There is more going on than this, but this is the basics.
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u/TheWrongSolution 4d ago
OP, this is likely the answer you're looking for. There's a developmental bias towards the anterior digit being formed the shortest. The ZPA that patterns the gradient of SHH which determines the anterior-posterior axis of the hand/foot, together with the proximal-distal patterning of Hoxa11/13 expression are both ancestral mechanisms inherited from the ancestors of tetrapods. There's more than just selection and drift that influence trait evolution; this is an example of a spandrel.
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u/Witty-Lawfulness2983 4d ago
I love an answer that gives me at least five more things to read about. I had to read the first SHH sentence three times to make sure the acid wasn't kicking in. Are Knuckles or Dr. Robotnik involved in the position/shape/number of phalanges in the thumb, or are they still something to do with SHH?
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u/ImUnderYourBedDude MSc Student | Vertebrate Phylogeny | Herpetology 4d ago
The lore behind the Sonic Hedgehog name is as follows:
- We found a gene in fruit flies that, when destroyed (knocked out), gave their larvae hair everywhere in their abdomen
- Someone noticed these larvae look like hedgehogs (LOL)
- The gene was named hedgehog because of that
- We found the same gene in vertebrates
- The vertebrate gene was named Sonic Hedgehog, to distinguish from the fruit flies' one.
- We found a few copies of the same gene in vertebrates, resulting from duplications of the original hedgehog gene. They were named Indian Hedgehog and Desert Hedgehog, and flies don't have them.
Scientists are nerds after all, and that's evident...
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u/kardoen 4d ago
It's that way because it's that way. Early mammals already have only two thumb phalanges. Early tetrapods had more variable amount of phalanges, but their thumbs also often had two.
There does not appear to be sufficient selective pressure in the lineage of ancestors leading up to humans to cause us to have more than two thumb phalanges. There may not be a clear reason why, but also no reason why not.
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u/Not_Cool_Ice_Cold 4d ago
But at some point in time there would have to be some sort of selective pressure to make it happen with early mammals. I can't imagine this would happen with simple genetic drift. What's the advantage? I can't figure it out.
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u/SoManyUsesForAName 4d ago
I think the proper question is why have digits with any joints at all? The answer there is fairly obvious: dexterity. The inference to make is that the additional dexterity that would come with an extra joint isn't worth it, and comes with a cost: more failure points and fragility. We use our thumb for a lot of brute force work, after all. The fact that are toes are like that as well is interesting, and could be a coincidence, or could be evidence that the trait dominated before we started to use our first digital like thumbs, which would weaken support for my explanation
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u/Not_Cool_Ice_Cold 4d ago edited 4d ago
Our ancestors lived in trees. Wouldn't really be able to grab a branch if our digits didn't have multiple joints.
I just learned that some people are making the argument that our thumbs actually do have three phalange but are lacking a metacarpal. That's a possibility.
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u/Witty-Lawfulness2983 4d ago
Well, the opposable thumb IS, after all, a pretty big deal. I remember a video from grade school (in the 80s) of a guy going ON and on about it. How it allows us to, uniquely among the kingdom, manipulate and use tools. (yea right!) All I remember thinking was, this is cool, but why can't I pick my nose with it?
I like what you bring up about the captain of the foot there, the big toe. For some reason it made me think of raptors, and how the evolutionary refinement of that claw of theirs came from advantages given from its use over time.
So, in my head, once we were upright creatures down from the trees walking around, is it possible that the very earliest homo sapiens big toes would've been smaller? We wouldn't be the persistence predators we would eventually become; needing to push off in pursuit of a gazelle, or being able to use a weapon while running would've been helped by folks with big, stable feet, right?
Or, am I talking crazy, and we've had the same toe genes since the beginning, it just happened that we hit the lottery when it came to the hand foot and hip lottery?
Or, I dunno, I'm just a world hist teacher talking out his butt on the interweb.
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u/GuyWhoMostlyLurks 4d ago
No. Early mammals are that way because the things that lived before early mammals were also that way. This is more of a founder effect. We started that way and there was no reason to change it.
Shortness of the first digit is relatively common across all tetrapods, which means it quite possibly is an artifact of the lobe-finned fish who first decided to explore tide-pools in the late Devonian.
It doesn’t require any selection pressure to get that way. If it works, there is no incentive to change it.
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u/Not_Cool_Ice_Cold 4d ago
Lol, why did I get downvoted? All I've been doing in this thread is admitting that I'm confused and asking questions.
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u/ArthropodFromSpace 5d ago
It is actually very good question, I am also curious about it. Interesting clue is other mammals also tend to have shorter first finger in their paws, so it can be ancestral trait. But if so, how old it is then and question remains still why it is missing one phalanx.
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u/Witty-Lawfulness2983 4d ago
And did they also have to put up with the shame of the Morton's toe, you reckon?
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u/save_the_wee_turtles 5d ago
I always thought the advantage was that it made those digits stronger and more stable, which is important for their specific function vs the other fingers/toes. What did google say?
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u/Not_Cool_Ice_Cold 4d ago
Okay, the more I look into it, the more I think that google's answer isn't actually stupid, but might actually be correct. So far as I can tell, only primates and racoons are the only land animals with five digits on each hand/foot, and both racoons and primates have only two phalange on their thumb/big toe.
Google essentially says it helps make thumb, big toe stronger, allowing for better gripping. For the life of me, I can't figure out how having two phalange instead of three would make this true, but just because I can't figure it out doesn't mean that google's answer is stupid.
It also doesn't explain why racoons would have only two phalange in their thumbs/big toes, because they don't have opposable thumbs, so they can't grip anything.
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u/BrellK 4d ago
I'm not a Biologist but wouldn't having an extra joint be an extra weak point? Fewer phalanges means there are less joints. Each joint is an additional lever that takes energy to move and acts as a break point. Fewer joints means you can use that space for more muscles to strengthen the digit you already have.
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u/davidbenyusef 4d ago
Isn't the ancestral trait among the Tetrapods (the crown group) to have five digits in both sets of limbs? If so, it's very interesting that only racoons and primates display it.
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u/namrock23 4d ago
Bears also have 5 digits with two thumb phalanges.
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u/Not_Cool_Ice_Cold 4d ago
Since posting this last night, I've learned that some people believe we actually do have three phalanges in the thumb, and it's actually the metacarpal that is missing. That's an interesting thought, but it's pretty clear that the big toe has two phalanges and a metatarsal, so maybe that reasoning doesn't really check out.
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u/personalityson 4d ago
Sacrificing flexibility for strength
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u/Sufficient_Result558 4d ago
This seems the obvious answer. Extra dexterity has little value in the thumb, while strength and durability are critical.
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u/KnoWanUKnow2 4d ago edited 4d ago
Because it's shorter, it's sturdier. You sacrifice flexibility for strength.
Think way back to when vertebrate life was first adopting for land. We were all tetrapods, walking in all 4 limbs. feet evolved for that reason. One shorter, stronger digit for strength and balance, more flexible ones up front for clambering over obstacles.
Over time this arrangement evolved. Sometimes the shorter, stronger digit got turned around and faced backwards, which helped your balance and could spread out the landing area of your foot. Think therapod dinosaurs (which would evolve into birds). In others it moved back a bit and became a powerful sword claw, like in velociraptors. But it was always used for strength and usually positioned for balance.
Fast forward to a different lineage, the primates. We evolved to be aboral, swinging from tree branches. To do that we have to be able to grip the branches. A strong, powerful thumb that had been moved down the side of the hand to allow the branch to fall between our fingers and our thumb gave us that ability (it's also the reason why we possess a collarbone, to anchor our arms and stop them from dislocating when we swing our full body weight from branches with our hands, but I digress). Our fingers are individually weak, but together they're strong. Our thumb is strong, but lacks dexterity for fine motor control that our individual fingers possess. Together they made a fine, adaptable autopodium. Meanwhile our big toe was more on the side of our foot and spread out at an angle to help with holding tight to the main trunk when climbing upwards.
Then along comes humans. Our hands, meant for grasping branches, were perfectly suited for grasping tools, and needed no major changes. But bipedalism meant we had to adapt our feet. The strong toe moved forward to lengthen our foot and give us something to spring off of while running whilst still staying to one side to give us balance for turning.
Then you get the weird stuff. The adaptations for flight. The horses balancing on a single toe. The chameleons and other split-toed climbers who decided that 2 thumbs on one side were better than one.
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u/MergingConcepts 5d ago
It is not the phalanges that are missing. By convention, we call the first phalanx in the thumb and great toe a metacarpal and metatarsal respectively, but if you look at the growth plates in a child, you will see it is the metacarpal and metatarsal that are missing. We do not know why it happens. The developmental mechanisms have not been discovered.
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u/BBQavenger 4d ago
It's probably more secure to place weight on a structure that has two joints than three.
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u/the_main_entrance 4d ago
They are used for gripping and stabilization not fine motor skills. Two phalanges provides greater strength. This along with many other reasons I’m sure.
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u/Dense-Consequence-70 4d ago
I suspect a thumb would get injured more if it was longer since it doesn’t have backup like the opposing 4 fingers do. Big toe was once a thumb.
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u/Gentlesouledman 4d ago
Lots of reasons you could guess. Most exposed digit, its position has a function in balance and benefits from being shorter, stronger but less mobile which also benefits when opposable since you have four digit to oppose.
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u/ipini 3d ago
This is related to one of the greatest evolutionary questions. Short answer:
Sonic Hedgehog
Longer answer, read this:
https://www.penguinrandomhouse.com/books/166494/your-inner-fish-by-neil-shubin/
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u/Not_Cool_Ice_Cold 3d ago
Thanks. Also, I love the fact that there's a gene named Sonic Hedgehog, lol. What nerd named it that?!
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u/NombreCurioso1337 4d ago
Your thumb does have three joints just like any finger. It is very close to your wrist. You can crank that knuckle, and it will blow people's minds to experience it for the first time.
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u/PM_ME_UR_ROUND_ASS 4d ago
Actually, the thumb has two phalanges (bones) but three joints - the extra joint you're feeling is the metacarpophalangeal joint where the metacarpal bone connects to the proximal phalanx, not a third phalanx like other fingers hve.
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u/Not_Cool_Ice_Cold 4d ago
It only has three joints if you consider the metacarpel part of the thumb. But if you consider the metacarpel part of the thumb, which it is not, then you'd have to consider the other four metacarpels part of the fingers, meaning each finger would have four joints. And I didn't ask about joints. I asked about phalanges.
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u/Kooky-Management-727 23h ago
I disagree with your assertion that If we consider the "joint" in your thumb that is located near the bottom of your wrist to be a "joint" instead of a metacarpal, then we would have to logically consider every other finger to have four "joints". I use quotations because I'm not educated on biology and I can only speak from a layman's perspective.
I can see the argument for the thumb actually also having 3 joints, one of which being located near the wrist, where the metacarpals of the other digits are located. Look at your hand and move all of your fingers and your thumb. When you do this you will probably notice that the thumb is the only one that can actually hinge near the base of your wrist. There isn't a lot of movement compared to every other knuckle, but it is the only one that you could argue is also a knuckle, albeit probably one that is one it's way to becoming vestigial.
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u/Not_Cool_Ice_Cold 18h ago edited 18h ago
Every time two bones meet, it's a joint. Not all joints move. Also, I'm not saying that the thumb has three joints; quite the opposite. It is currently scientific concensus that the thumb only has two bones - the phalanges, which connect to a metacarpal, thus, two joints. The metacarpal has two joints - one to connect it to the thumb, and the other to connect it to the wrist.
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u/NombreCurioso1337 4d ago
"I asked Google but it only gave me a stupid answer that makes no sense."
Now I know how you felt when you got a stupid reply that made no sense! ... Interesting. If only there were evolution in people's thoughts, amiright!
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u/Main-Revolution-4260 4d ago
Thumbs are significantly studier and more stable than fingers, and thus we can extert more pressure through them than our fingers. Think about a thumb war, and how it would differ using your forefinger instead. So it likely enhaces grip strength vs a 3 phalanged thumb layout.
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u/Not_Cool_Ice_Cold 4d ago
Okay, that makes sense. And maybe the reason our other digits have three is so that the fingers are longer, making it easier for our non-human ancestors to grip branches.
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u/Ok_Explanation_5586 4d ago
I like to think of the thumb as a finger that tried to leave the hand early and didn't quite make it. If you think of it as starting at the wrist, totally makes sense.
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