Learn about Giving What We Can’s 10% Pledge and Trial Pledge at https://givingwhatwecan.org/scishow
Frogs aren’t known for their fabulous manes, but the claws of frogs are helping us learn why humans have hair — thanks to a homeobox gene and a weird evolutionary shift.
Hosted by: Hank Green (he/him)
———-
Support us for $8/month on Patreon and keep SciShow going!
https://www.patreon.com/scishow
Or support us directly: https://complexly.com/support
Join our SciShow email list to get the latest news and highlights:
https://mailchi.mp/scishow/email
———-
Huge thanks go to the following Patreon supporters for helping us keep SciShow free for everyone forever: Toyas Dhake, Reed Spilmann, Gizmo, Garrett Galloway, Friso, DrakoEsper , Kenny Wilson, Lyndsay Brown, Jeremy Mattern, Jaap Westera, Rizwan Kassim, Harrison Mills, Jeffrey Mckishen, Matt Curls, Eric Jensen, Chris Mackey, Adam Brainard, Piya Shedden, Alex Hackman, Kevin Knupp, Chris Peters, Kevin Bealer, Jason A Saslow
———-
Looking for SciShow elsewhere on the internet?
SciShow Tangents Podcast: https://scishow-tangents.simplecast.com/
TikTok: https://www.tiktok.com/@scishow
Twitter: http://www.twitter.com/scishow
Instagram: http://instagram.com/thescishow
Facebook: http://www.facebook.com/scishow
#SciShow #science #education #learning #complexly
———-
Sources: https://docs.google.com/document/u/1/d/e/2PACX-1vQHO0rk8yh2N3xVoHD_2N3QYYEDHCkFAyZID71_mCvrfWGqQ5hDk8AUXGlMQjb0N2Qy-85-tU-whyeH/pub
If you want to know how we evolved hair and fingernails, ask a frog. They are impartial judges, since they do not have any of those things. I joke, but this is actually something researchers are doing to understand how four-footed animals evolved to live on land.
00:15:Because despite their lack of fabulous manes, amphibians do have evidence of that transition hiding out in their DNA. And this can help us understand everything from life on land to why humans have hair today. Claws, hair, feathers, scales, and even horns can be grouped together
01:37:under the umbrella of cornified skin appendages . I know… that’s gross. They’re all made of similar proteins and they usually develop in similar ways. That said, they’re not for everybody. Take frogs for example. Frogs: not typically associated with luscious locks or impressive horns.
01:55:And you probably don’t normally think of them as having claws either. But some species do have claws, like the, western clawed frog, a favorite experimental model for developmental biologists everywhere. They don’t have, like, a ton of claws. There're three on each hind foot! But they are there.
01:11:And claws don’t come up often in the world of amphibians. Plus, these claws grow differently than claws found on other four-footed creatures. So these frogs’ toe-gear was thought to be something they evolved totally separately.
01:24:That meant non-frog claws must have made their first appearance after the amniotes, which today includes the mammals, the reptiles, and the birds, split off from the amphibian ancestors. Or so we thought. But in a 2024 paper in Nature Communications, researchers challenged that idea,
02:42:and the consequences are surprisingly long-reaching. They were studying keratins, the proteins that make up cornified skin appendages. God, I hate that. And they reached the conclusion that frog claws share an origin with those of amniotes.
02:57:In humans and the other amniotes, the expression of keratin is overseen by specific genes. Namely, the Hoxc13 gene. Hoxc13 belongs to a group called Hox genes, short for homeobox genes. These are the master blueprints of a developing organism, directing what happens where
02:15:as a blob of cells turns into an embryo with limbs and organs and all that good stuff. They can do this because Hox genes code for specialized proteins called transcription factors. And transcription factors essentially turn specific genes on or off,
02:29:so that the right functions are showing up in the right place at the right time and you don’t grow hair in your kidney. They’re a little like the middle management of the genome. Their one job is to boss around all the genes under them.
03:40:And Hoxc13 has been put in charge of whether or not we grow nails, hair, and the like. Now, there’s a funny quirk of Hox genes, which is that each group of them is laid out on the chromosome in the same order that they work in the body, from head to toe.
03:55:That means where a gene falls in its cluster gives some clue as to where it does its work. You can expect a Hox gene with a name ending in 1 to be involved near the head, going all the way up to 13 which happens at the tail end. And we literally mean the tail.
03:10:Hoxc13 is active in the tail fin of ray-finned fish, for example. But as ancient fish continued to evolve, Hoxc13 came to be turned on not just at the tip of the tail, but also the tips of the fins of lobe-finned fish. Those fins would give rise to the limbs of all four-footed animals.
04:31:Which means Hoxc13 is the director of the film we are about to see. This SciShow video is supported by Giving What We Can! An organization that works with experts to help you find and donate to the charities that can do the most good per dollar donated.
04:47:You don’t have to be a scientist to help make progress on some of the world’s biggest problems, because you can also contribute by donating a portion of your income. When you donate with Giving What We Can, your money goes to evidence-based charities who have been vetted by experts for impact.
04:01:Giving What We Can has a global community of almost 10,000 people who have pledged to give at least 10% of their income to effective charities over their lifetimes, with thousands more pledging 1% or more for a shorter period of time.
04:13:To join a community making an impact on global problems, no matter your career, explore taking a pledge to give at givingwhatwecan.org/scishow In that 2024 paper, the researchers showed that in clawed frogs, keratin genes are regulated by Hoxc13 – same as they are in amniotes.
05:32:This led them to look for Hoxc13-regulated keratin in other amphibians, and they also found some in everyone’s favorite forever-baby, axolotls! And I know, It’s also pronounced “asholot”, but look… I am who I am. While axolotls don’t have actual claws,
05:47:they do have brownish layers at the tips of their toes. And sure enough, it was discovered these were made of Hoxc13-regulated keratin too. This research revealed that there was a shared evolution of claws long before amphibians and amniotes made their evolutionary split.
05:04:And that might reveal how important keratin was in transitioning to life on land. Instead of being a later innovation, it was there the whole time. It seems likely that keratin structures first evolved as a means of protecting
05:18:those vulnerable tippy toes while also helping with movement, and eventually prey capture. From there, keratin took off to take on a bunch of other important roles, like covering mammals in fuzz. That said, we’re still doing a lot of head scratching around the relationship between
06:33:these early origins of claws, and our magnificent haircuts today. Given that Hox genes have such a thing for order, it’s not clear how Hoxc13 would have gone from just making claws, like at the tips of limbs, to regulating hair growth throughout a mammal’s body.
06:50:Because remember, they don’t wanna be everywhere. That’s other Hox genes’ turf. It’s possible Hoxc13 genes were put to work in new places thanks to a type of genetic sequence called enhancers. Briefly, enhancers can help turn on genes, including those for transcription factors,
06:07:like your manager’s manager. And researchers have identified enhancers that help boost expression of Hoxc13. But we don’t know the whole picture yet. There were likely some intermediate evolutionary stages as keratins took on new functions, but exactly what that looked like, we just don’t know.
06:24:Plus, those same enhancers weren’t found outside of mammals, so this doesn’t explain how other amniotes got their specialized skin coverings. It’s especially puzzling in terms of bird feathers, which might have evolved separately from other cornified skin appendages.
07:39:Oh my God, I want to stop saying it so bad… cornified?! Their keratin proteins are different from those found in hair. So we also still don’t know the full role of Hoxc13 in bird evolution. What’s clear, though, is that keratins, especially claws,
07:55:were there for us all the way through our rocky, precarious transition to life on land. It’s nice to know they’ve had our backs – or, our toes, at least – all this time.
Watch Now
