Transcription
Hello there, guys. Hope your day concerning the consumption of a fat, artificially evolved dinosaur went [Music] well.
Today, we're taking a break from cryptids and focusing a bit on an untraditional topic. As a person interested in, well, all aspects of biology and not just extinct things or mythical things, I'd like to make some videos talking about some of the fascinating evolutionary phenomena observable in nature and in the billions of organisms that fill the world around us. From the origins of spider silk to nylon-eating bacteria to baleen whales, I've always been fascinated with what makes organisms tick and function. I would constantly ask, "Why does this exist? How does this work? And how did this evolve from reduced forms?"
Well, over the years, I've tried to answer those questions through science by reading and researching these topics for myself. Consequently, I found that evolution is an amazing thing that gives all organisms a wide diversity and uniqueness. So why not make videos talking about the knowledge I've learned about our origins and our biology over the years with you guys? In addition to the several others I am making, this is just one part of my evolution series.
Today, I'd like to start out the series by examining an interesting evolutionary development that has shaped the history and evolution of organisms for millions of years, and that I have found is relatively obscure. This phenomenon is called neoteny, or paedomorphosis.
Now, before I explain what that is, it's important to get an understanding of ontogeny. Ontogeny is the name of a biological process that occurs in basically all individual animals and goes a bit like this: animals change morphologically and physiologically as they grow older and closer into maturity. Unlike what was once believed in prior centuries—yes, people actually believed sperm was little adult men—individual animals start out as immature larvae forms and they're not born resembling just miniature versions of what they would look like when they're adults. Instead, their body structure alters gradually as they grow into adulthood. They are born with juvenile traits in their body that are over time lost and replaced with adult ones as they develop. For example, we transition from a big-headed, short-limbed baby to a longer-legged juvenile, to a teenager, to a fully adult human with a fused skull, stronger jaws, and much longer limbs. We exhibit many growth stages in our life cycle.
This is easily observable in other animals as well. Insects start life as larvae, frogs start life out as tadpoles, and crocodiles start out life as adorable, big-headed, big-eyed babies. Man, that's cute. Between their original baby bodies and their adult ones, transitions or growth stages can be found in their life cycles, each illustrating either slow or fast developments. All these changes are often triggered by hormones released and manipulated by the endocrine system in these animals' bodies.
As we have learned more and more about ontogeny, we have learned that sometimes the juvenile forms contrast greatly with the adult ones. Big horn sheep are born with basically non-existent horns, but as they grow closer to sexual maturity, these horns grow much larger in order for them to defend themselves or fight other members of their own species for dominance. Lion cubs and deer fawns are born covered in spots, which help them blend into their surroundings, but as they grow older, these are lost in both animals. Male deer develop horns, and male lions develop massive manes.
Numerous dinosaur fossils originally described as separate and unrelated species are turning out to be immature and juvenile forms of already known adults. The dome-headed *Dracorex*, *Stygimoloch*, and *Pachycephalosaurus* all once considered to be separate species, are likely now considered the same species, just at different time periods in the animal's ontogeny or growth cycle. A similar thing was discovered in ceratopsids, in particular *Triceratops*. We now know that juvenile *Triceratops* start out life with tiny, backward-curved horns. In their teenage years, these horns continue to curl backwards and grow larger. In their sub-adult years, these horns gradually point forwards until they curl the completely other way. Similarly, *Nanotyrannus* is now considered by some a juvenile form of *Tyrannosaurus rex* and not a separate species. Thanks to numerous studies by T. Carr, we now know that tyrannosaurs start out life with thin, elongated, almost crocodilian-like skulls and extremely long legs for sprinting down prey. And as they grow older and their diet changes, they grow bulkier and have stronger jaws. They apparently gradually lost teeth as they grow older, starting out life with 17 or more and losing them as they mature. So, anybody that's banking on *Nanotyrannus* being a separate species, sorry.
Now, ontogeny doesn't always turn out the way it's supposed to. Sometimes mutations in an organism's DNA will result in it retaining juvenile traits into adulthood, disrupting an animal's growth cycle and removing a stage or few. This is called neoteny. In neotenic physiological developments that normally help restructure an animal as it grows up are delayed or slow, and as a result, traits exhibited in juveniles of the species are kept even when the animal is sexually mature. This obviously doesn't mean an animal doesn't age. This simply means traits such as the shape of the skull or limb proportions of babies are exhibited in adults instead of being changed or replaced by new ones. Think of it like a cat caterpillar remaining a caterpillar instead of metamorphosizing into a butterfly, or a frog that retains its gills into adulthood instead of losing them. Very strange.
As bizarre as this may sound, numerous animals—a vast majority of them, actually—are neotenic, and the retention of immature traits into sexual maturity has actually benefited many organisms, sometimes heavily affecting the evolution of an animal population. Axolotls are a species of Mexican salamander that are neotenic. Like all salamanders, it starts out life in the water with a tadpole-like body and external gills. In its teenage years, they develop limbs and begin to lose their tails. Normally, most other salamanders lose these larval traits, such as external gills and fish-like flukes, entirely and walk onto land as fully mature and terrestrial adults. However, the axolotl is neotenic. It remains in its teenage form for the rest of its life, keeps its external gills, and remains in the water, unlike its terrestrial cousins. This retention of the aquatic larval form allows axolotls to capitalize on the many resources in their underwater environment that their land-dwelling relatives can no longer reach. This allowed the axolotl's neotenic ancestors to survive longer than the normal salamanders, and thus a population of salamanders that didn't grow up evolved.
Another example of neoteny is in domesticated dogs. Selective breeding by humans has greatly impacted the morphology of dogs. We kind of screwed a lot of them over. The wolf ancestors of our modern dogs would normally grow from a big-eyed baby skull puppy to a long-snouted, powerful adult with strong jaws and skulls suited for grabbing prey. However, when humans came and domesticated dogs, we also began a process of artificial evolution that favored big-eyed, baby skull dogs that retain these puffy traits into maturity. This is easily recognizable when viewing the skulls of Chihuahuas and comparing them to an undomesticated baby wolf skull. One can see that Chihuahuas and many other dog breeds are mutated wolves that never developed the strong jaws and elongated snouts of the adults, but instead kept the puppy faces and weak jaws they possess in their juvenile forms. The puppy dog faces that appeal to humans were favored by selective breeders, and soon neotenic wolves became more and more prevalent in domesticated dogs.
All modern birds, likely, are also nothing more than mutant, neotenic skull dinosaurs. Yes, birds are dinosaurs. Get over it. This study researched the ontogeny of the skulls of archosaurs, the group that encompasses all dinosaurs, including birds and crocodilians. The study found that the changes in physiology that occur in the skulls of crocodilians and non-avian theropod dinosaurs when they grow to maturity was pretty noticeable. As juveniles possess short, blunt skulls with large eye sockets and big heads comparable to the rest of their bodies. As they grew older, their skulls elongated and thinned out into a long snout. The eye sockets got smaller, and the cranium reduced in size. However, the more avian or bird-like theropod skulls, like *Archaeopteryx* and *Confuciusornis*, remained relatively the same when they grew past their juvenile growth stage, keeping the large, short heads with large eye sockets and big craniums into their adulthood. Just look, there is very little difference from teenage to adult. This is the case with modern birds too. All modern birds have the skulls of baby dinosaurs, even in their sexually mature state. The study found that birds are nothing more than mutant dinosaurs that stopped growing past their juvenile growth stage. Unlike most other theropods, this retention of large eyes and larger brain case exhibited in juveniles likely is tied to the further evolution of flight in birds, as larger eyes and larger brains are crucial to interpret vision during flight. These mutant dinosaurs sacrificed the alligator-like snout exhibited in their ancestors and cousins for the ability to fly and develop larger brains and more sensitive eyes. Interestingly, this appears to be the only part of modern birds to be neotenic. They only appear to have baby skulls. Everything else is full adult dinosaur.
So, everything from birds to dogs to cute salamanders are bizarre examples where disrupting the normal growth cycle and retaining juvenile traits can actually be an evolutionary advantage. Well, there are two more extremely fascinating examples of neoteny that not only are relevant to other animals but are relevant to us. Yes, even the old *Homo sapiens*. Humans are neotenic. Now, I know what you're going to say, "What? That's not true. You're lying." And I'll answer with, "Yes, we are mutant apes that retain the juvenile traits of our ancestors." Just like the birds and the Chihuahua, it is our skulls that are neotenic. We can see our retention of juvenile traits in the ontogeny of the skulls of our close relatives, the great apes, such as chimpanzees, orangutans, gorillas, and bonobos.
When all great apes are first born, they possess a large, circular cranium and small, weak lower jaws, much like our own, to the point the baby skulls of chimpanzees and humans are almost indistinguishable. Now, as the apes grow older, their mandibles grow larger, protruding and shifting forward. In addition, the top of the skull elongates, and the cranium thins. At the top of the skull, a sagittal crest forms. This development is because when juvenile apes are first born, they possess weak jaws that are unable to produce much strength or power for crushing things like food. However, as these apes grow older, muscle development in the jaw begins, as the temporal muscle gets larger, and the sagittal crest on the top of the skull where the muscle attaches to becomes more and more prevalent. The sagittal crest elongation causes the cranium to become narrower. This jaw development sadly reduces the size of the cranium and subsequently the size of the brain. The thinner the cranium, the thicker the muscles can be, giving stronger jaw power. This is why gorillas, the great apes with some of the strongest jaws, subsequently have the smallest cranium.
Now, humans and our extinct ancestors and relatives are born just like these guys, with skulls possessing weak, undeveloped jaws with large craniums, lacking a sagittal crest entirely. But unlike other apes, the mutation in our DNA resulted in our skulls delaying the other growth stages so that the sagittal crest almost never forms, and our temporal muscles remain weak. This is why we look so much like the juvenile forms of chimpanzees and bonobos, because we are stuck with the toddler skulls of great apes. This alteration in the growth stages allowed us to keep our larger cranium space for a larger brain. Subsequently, we are stuck with the weak, undeveloped baby jaws, unable to create much power at all. We quite literally sacrifice brawn for brains, as our jaws are useless compared to how they are supposed to be in unmutated apes.
One of these mutations responsible for our neotenic skulls has been identified. MH16 is a gene found in all primates that codes for protein development in the temporal muscle. Without the proteins created by this gene, the muscles in the jaw will be smaller and less developed than they are in normal genes. DNA comparison studies found that this gene in all humans, and only all humans, is mutated, but in all primates is normal. The mutant gene is likely responsible for allowing us to keep our larger brain size and weaker jaw muscles, and is responsible for keeping us with the skulls of juvenile apes and not developing adult skull structures such as the sagittal crest. So, here we are, weak-jawed mutant apes. Without neotenic mutations in our DNA, we likely wouldn't have been able to evolve intelligence. Without them, and instead would have been stuck with dumb, old, massive jaws.
Surprisingly, that's not the only thing neotenic about us. All backbone vertebrates are likely neotenic. I know it might be hard to imagine, but once upon a time, us vertebrates, that is, us minorities with a backbone and internal skeleton, didn't [Music] exist. Okay, I'll stop that. A long time ago, I mean, a really long time ago on Earth, our ancestors, the forefathers to all chordates, were nothing more than primitive, sponge-like creatures that likely did nothing but filter feed at the bottom of the ocean for their entire lives. In these early forms, jaws, fins, teeth, legs, etc., didn't exist yet, making them look very alien. Even so, these early animals possessed many of the traits we possess today. Tunicates, or sea squirts, are one of the vertebrates' closest living invertebrate relatives, and like we resembled our earliest ancestors, they, like us, have a stomach, intestine, heart, anus, mouth, sex organs, and even gills in their adult forms. They attach themselves to the sea floor permanently and peacefully, filtering water through a siphon, collecting whatever food particles it can find. But like us, they possess ontogeny, with their juvenile larval form greatly contrasting their adult ones.
The larval forms of tunicates are interesting, as when they hatch from eggs, they possess a trait that is lost when they grow into adulthood: a notochord, a primitive version of our backbone, which allows them to swim with a tail. Subsequently, these larvae most resemble little tadpoles as they travel through the ocean in search of a good place to sacrifice their newfound mobility and grow into sessile adults. Scientists can infer from the growth stages of tunicates that all vertebrates are actually neotenic themselves. What likely happened so long ago is that a tunicate-like chordate refused to grow into adulthood and retained its notochord, as well as its mobility. This animal could find new niches and food opportunities over sessile ones, and thus a population of mobile, backbone-tailed animals rose from normally sessile ones. Over time, this tail and backbone became more and more efficient with fins, muscles, and well, the rest is history. That's right, we are neotenic versions of neotenic versions.
Neoteny is an extremely important biological phenomenon that is responsible for many evolutionary developments visible in organisms today, ourselves included. So, the next time somebody tells you to grow up, tell them you can't because you are genetically imprisoned to be a baby for your entire life, as a result of numerous benefits that staying young forever entails, like a backbone over being a weird anus stuck at the bottom of the ocean, or a bigger brain over strong jaws, or the ability to fly over giant dinosaur heads. Sometimes growing up doesn't always benefit the biological population. So, stay neotenic, my friends.
Thanks for watching. Hope you enjoyed this video and learned something new about neoteny and ontogeny. Loch Ness Monster is coming soon, part two, and a Paleo Profile is definitely coming sooner as well, as you know about those other, you know, several hundred series I'm starting. So, yeah, see you guys. Thanks for watching.