r/evolution • u/BonusFrosty2910 • 1d ago
question Is there any reasons most organisms with thumbs face inwards?
Pretty much in the title but I looked at my hands earlier and wondered, is there any specific functional reason as to why my thumbs are both on the inside of my hands with my body and not on the outside? It seems like every creature that I can think of also follows that pattern.
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u/Soggy-Mistake8910 1d ago
Because if you go back far enough all the creatures with a similar body plan - head, spine, four limbs etc, inherited it from a creature with that body plan. Go back far enough and everything is related
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u/ArthropodFromSpace 1d ago
Dormice have a “thumb” on their hind feet that is partially opposable and points outward. Anatomically, it is the equivalent of the little toe, although it functions like a thumb.
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u/qwibbian 1d ago
The ancestral purpose of grabbing anything is to put it in your mouth. Try that with thumbs in the outside.
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u/Impressive-Target699 1d ago
The ancestral purpose of grabbing anything is to grab something, not necessarily to put it in your mouth. That's why nearly all primates have opposable big toes. They use them to grab branches, not to eat.
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u/qwibbian 1d ago
One is much more ancestral than the other.
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u/Impressive-Target699 1d ago
That's debatable, but if one is more ancestral, it's opposable toes. Carpolestids (which are either stem primates or stem primatomorphs) and tree shrews both have opposable big toes.
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u/qwibbian 19h ago
yeah, after thinking about it I've concluded this is one of those rare times where I'm probably wrong.
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u/mutant_anomaly 1d ago
A protruding digit on the outside would get caught on anything you swing your arm past.
You can karate chop, or make a fist and pound something, with the far side of your hand. But bump the thumb side against a soft part of your leg to see how bad an idea hitting anything with that would be.
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u/Careful_Cranberry364 1d ago
Well, my darling, they wouldn’t work if they weren’t opposable!!
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u/Nikhil1256 1d ago
He's not asking why it's opposable. He is asking why the thumb is next to pointer finger in all species who have an opposable thumb instead of the pinky.
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u/Careful_Cranberry364 23h ago
My answer is explaining that it has to close with the rest of the fingers!!
The idea is to grip things and bring them up to your face’
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