Evolutionary Development: Chicken Teeth - Crash Course Biology #17

Key Takeaways

when little kids say they want to grow up to be a scientist here's what they actually mean they want to blow things up in a laboratory setting they want to get bitten by a radioactive monkey which will turn them into a terrifying humanoid battle monkey or they want to make a fly with eyeballs on its butt or like a chicken with fangs most of the time scientists don't get to do that stuff like you may blow something up but it's either going to be in like a really controlled setting or it will be an accident in which case is bad like the lab where I first worked the first lab I ever worked in had a blood stain on the ceiling but if you're scientist specializing in The Amazing new discipline of evolutionary development biology you may just get to make a fly with eyeballs on its butt or even a chicken with Thief but no battle [Music] monkeys so evolutionary developmental biology or evoo for all of us cool kids is a new science that looks deep into our genes to figure out how exactly they give instructions to make different parts of our bodies and as the name suggests it's giving us some hot leads into the nature of and mechanisms behind Evolution one big thing it's showing us is that animals all animals are way more similar than we ever imagined you know how you always hear about how humans and chimps are 98.6% genetically similar it kind of makes sense right cuz chimps and humans you could you could see that we kind of kind of look alike like if you walk into a coffee shop and there's a chip sitting in a chair and it's like maybe wearing a fedora or something you might briefly mistake that chimp for a human you might not even notice it's sitting there it could happen but what about a mouse you are not going to mistake a mouse for a person how genetically similar do you think we are with mice how about 85% similar shut up no I won't shut up humans and mice are 85% genetically identical so why then are mice like little in skittery covered in white fur and have beady little eyes while I can like walk upright in a non- skittery way and have beautiful deep serious eyes I'll give you the long answer in a minute but for now the short answer is it's all because of the incredibly weird and amazingly powerful genes called developmental regulatory genes mostly when we're thinking of genes we think of the things that code for some useful enzyme or protein like the ones that determine what our ankles are going to look like but those ankle genad don't just come on and off at random they have to be turned on and off that's what these developmental regulatory genes do they activate the genes that put the body parts together they don't tell them how to do it mind you they just tell them when or if it's time to get to work and since they're the ones pretty much calling the place regulatory genes start working rather early in embryonic development for instance a kind of regulatory Gene called Gap genes are responsible for telling the blastula that little Hollow ball of cells that forms during the early stages of development like make a mouth here and let's put an anus over on this other end probably the most amazing kind of regulatory genes are the homeobox genes or Hawk genes which kick into gear after the embryo is more developed haw literally control the identity of body parts setting up how an animal's body is organized like here's where you put the leg and here's where you put the tail and like I said these Hawks genes don't give instructions for how to create legs and Tails there are a bunch of other genes that are in charge the actual craftsmanship of the body parts you could think of the Haw jeans as like the head architects in the construction of a building they've got the master plan but they don't do any of the construction themselves that's way beneath them because under this top tier of regulatory genes there are scads of genes that act as like subcontractors like if a Haw Gene tells its direct subordinates make an eye here the subordinates then turn around activate other regulatory genes to give more specific instructions like this is where we got to put the collagen for the outer shell of the eyeball and make some nerve tissue for a retina right here again these second tier genes in third tier and fourth tier and on Down the Line don't actually do any of the work they just send instructions down the chain of command adding more specific information to the instructions as they go it's a really rigid hierarchy no Gene your body aside from that very first one does anything until it's told when and how much to do it so because I know that you're such a sort of intelligent and curious student I know what you're wondering right now what activates that first regulatory Gene and how in the name of Bill McInnis did they tell each other to do stuff well since evoo is a relatively new discipline we don't really know all the stuff that I wish we knew that's for you to figure out when you become a biologist scientists are starting to think that a lot of the human genome that has until recently been considered junk DNA because it apparently doesn't code for anything might actually be regulatory genes for instance just in the past few years we've learned that humans have about 230 separate hawx genes in our genome and they appear on every one of our chromosomes even the sex chromosomes how regulatory genes are inherited is also still being studied from what scientists have been able to deduce so far most regulatory genes are inherited very much the same way as all of your other genes but for some really early stage regulatory genes the proteins that they're coded to produce called Gene products have already been made and are sitting in the egg before it's fertilized waiting to tell the embryonic cells what to do to get the ball rolling another thing that your mom did for you that you probably never thanked her for so here's the really cool thing even though most regulatory genes are inherited each individual within a species tends to have the exact same DNA sequence in those genes there aren't even different leals and when you think about it they kind of have to be the same since all individuals of a species should be built from the same basic blueprint like you don't want people walking around with thumbs sticking out of their heads now this gets me back to me and my Beed friend the mouse Hax jees and other regulatory genes that are at the very highest tier the ones that say like head here and eye here not only tend to be the same within a species they're also very similar across different animal groups like between all mammals or even all vertebrates the differences between my regulatory genes and a mouse's regulatory genes are way down the chain of commands where the instructions are the most specific but the big picture stuff like you're a vertebrate and you have four Limbs and you have hair and breast tissue and ear bones and all that stuff that all mammals have all those General instructions are the same and that's why 85% of human's genetic makeup is the same as mice mices Mouse mice mises okay you've been very patient my student so I've got a surprise for you we're going to make some butt eyeballs in 1995 in a very cool and also totally messed up experiment a team of researchers in Switzerland took a hawk Gene from a mouse embryo one that said I goes here and inserted it into the DNA of a developing fruit fly embryo but they activated the mouse eyeball Gene in a region of the fly that would become the fly's back leg and so what do you think happened I'm not going to tell you yet cuz I want you to guess wrong the fruit fly did not grow a mouse eyeball next to its back leg it grew a fruit fly eye next to its back leg remember the gene didn't say how to make an eye it just gave the instruction to make an eye if it had said how to make the eye you'd get a mouse eye on a fruit fly but instead it told the fruit fly cells make an eye here and those fruit fly cells had their own instructions regulated by another whole set of regulatory genes once they got the order to make the eye they made it the only way they knew how that is pretty freaking messed up but also freaking awesome now in addition to getting me in touch with my Ander Mally unstable child scientist this kind of experiment is where evoo has begun to really revolutionize our understanding of evolution because we've known that Evolution can take place over a really long time but we haven't really been able to figure out how it sometimes happens really fast traditionally one of the main ways that scientists have explained evolution is through genetic mutations but an organism would have to do a lot of mutating to evolve from say a dinosaur into a bird it used to be thought that a 50% change in form would require 50% mutation in genes which would take a long time way longer than the pace at which we see things actually evolving but it turns out that a small change in a regulatory Gene up at the top of the chain of command can have huge effects on how an organism is actually assembled to understand how this works let's look at why birds don't have teeth so Birds evolved from therapod dinosaurs which are just these freaking sweet dinosaurs like Velociraptors which look a lot like birds but you know way more awesome and with big razor sharp teeth but you may have noticed that birds don't have razor sharp teeth they have beaks under the old way of thinking about Evolution the loss of the teeth would have had to happen very slowly as the genes make enamel and dent and gradually mutated to make less and less and less of each of those things until they made none at all and for a long time that's just how we thought dinosaurs evolved into Birds but there was one problem it would have taken way longer for all of those mutations to occur than it actually took for dinosaurs to evolve into Birds based on the fossil record fortunately evoo is offering us an explanation a single mutation in the regulatory genes could have shut off the enamel and Dentin production and another mutation in another regulatory Gene could have upped the Keratin production from the level of Mak some scales to the level of make a beak so birds actually do still have genes for teeth from their dinosaur and ancestors they're just not express because The Regulators don't turn them on but how do we know that well in 2006 a biologist at the University of Wisconsin named John Fallon who studies birth effects was looking at some mutant chicken embryos and noticed that they had formed little teeth like little baby reptile teeth it turns out that the mutations affected the chicken's Gene regulation allowing the teeth a feature lost to birds around 60 million years ago to just pop back up again the same sort of crazy throwback features have been observed in snakes born with legs like their ancestors once had or blind Cave Fish suddenly born with eyes if you turn those genes back on those ancient repressed features come back it's crazy I know it's so cool I don't I just I this is it's all fairly new science so this is still like in my head is like really fantastic fantasting that's a word I made up thank you for watching this episode of Crash Course biology I hope that I blew your mind or that you learn something or that you do well on your test or whyever you came to watch this episode if you go over there you can click to catch up on things that you may have missed or just rewatch the whole episode cuz you got to emphasize it in your mind otherwise you don't remember these things thanks to everybody who helped put this episode together if you have any questions you can catch us down in the YouTube comments below or on Facebook or Twitter and we will endeavor to answer them goodbye