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I was thinking of a situation, in which there is a mass extinction that wipes out all large land animals, and after this mass extinction some of the descendants of modern geckos evolve to fill the niches left by the large mammals. Some of these descendants of geckos also evolve to walk on two legs similar to how some dinosaurs walked on two legs during the Mesozoic

I understand that human thumbs were originally used to help with climbing trees before getting used to hold things when the ancestors of modern humans started walking on two legs. Geckos use van der waals forces to help with climbing trees as they have hairs on their feet that have hairs on their hairs, and so on down to the molecular level.

Just as human thumbs originally evolved for climbing and then got used for holding things in our hands could the van der waals forces that geckos use for climbing be used for holding objects in the hands of descendants of geckos?

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There are two ways to go about handling this problem - one that keeps van der Waals forces and the other that loses it.

As you've pointed out van der Waals (more like van der Walls) forces help geckos stick to walls. What this force is is basically an attraction formed due to the polarity between two surfaces (i.e the gecko and the surface of the wall). Now this is isn't as strong as Dipole dipole forces but much stronger than London Dispersion forces.

So assuming your future-geckos hold things with their hands like we do, this means that we want to improve their dexterity. To improve dexterity, I would recommend having very strong van der Waals forces on some part of their hands versus others, acting a bit like the grip on the bottom of shoes. Just having van der Waals forces could create problems - if you just had van der Waals forces on all parts of your hand simple things like throwing something may not just be difficult, but almost painful if the polarity between your hand and the object is great enough.

How strong this force is depends on the complex arrangement of hairs on the feet of geckos. So this would mean that eventually the future-gecko's hands would have either an overall less-dense arrangement of hairs or have a very dense arrangement of hairs at specific parts of the being's hands to maximize the effectiveness of different kinds of grips.

Keep in mind that there's the chance that future-geckos lose the ability to grip onto walls entirely because they don't do that anymore. We don't have van der Waals forces helping us out - we do fine with our mostly-bare skin on our palms. Being reptiles, there's a chance that the flexible/elastic nature of reptilian skin would be a good enough grip so that the future-gecko's hands also become hairless.

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You can get around the problem by getting your geckos to evolve human-like hands with more dexterity, and lose the Van der Wall forces.

Since they're filling the niches formerly occupied by humans and other large animals, that would mean they're now more land based. This can create an evolutionary pressure towards hands with dexterity over hands that let you climb trees. Geckos already have five 'fingers', so it's not far-fetched to imagine that they'd evolve them into hands resembling ours complete with an opposable thumbs.

In fact, this would actually be more meaningful than keeping the Van der Wall forces. Again, since they're going to be occupying land, there's a good chance they'd evolve into bigger body sizes. This would mean we can even have them as big as monitor lizards or bigger; I'm not sure about the math, but it is entirely possible that there's a maximum weight after Van der Wall forces cease to be effective. So you could argue that with the incentives the newly vacant land promises, there's an intense evolutionary pressure towards ditching Van der Wall forces and evolving 'better' hands.

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