[MUSIC] Hello, and welcome to Early Vertebrate Evolution. My names is Scott Persons and I'm a paleontologist at the University of Alberta. I'll be your guide as we explore the biology of some of our most ancient relatives. We belong to a group of organisms called vertebrates, animals with an internal bony skeleton. You may be familiar with some of the famous fossil vertebrates you can learn about in our other University of Alberta online courses, like dinosaurs. But the animals we'll look at in this course are much older. We can trace our vertebrate ancestors back more than 500 million years. What did these earliest ancestors look like? Where did they live? How did they give rise to the animals that we're familiar with today, including ourselves? In this lesson, we'll start at the very beginning of the story of vertebrate evolution. By the end of this course, we'll have learned where our skeleton comes from, how we adapted to swim and bite. And how vertebrates came out of the sea and onto the land. Let's get started. First, let's review a few terms. In this course, we'll be assuming that you have a basic understanding of the mechanisms of evolution. And that you know how to interpret a phylogenetic tree. We'll also assume that you're familiar with the concept of geologic time and plate tectonics. And, that you've learned how fossils form and are preserved. Finally, you should be familiar with at least some basic vertebrate skeletal anatomy. If you're not sure about any of these topics. Or, if you'd like a refresher of some terms and concepts. You may want to take a look at the material in our course Dino 101. If you see any terms that you're not sure of, you can also check out our glossary. Humans, other mammals, reptiles, birds, amphibians, and fish. Are all part of a large clade of animals called the Vertebrata or Vertebrates. And you probably all ready know, that the clade is a group of organisms that includes the common ancestors of the group. And all of its descendants. Vertebrates are part of a larger clade called the Chordata. Which is a group of animals with a stiff cord in their back. Now, for a mini grammar lesson. The plural of fish can be fish, like the plural of sheep is sheep. Or fishes, like the plural of wish is wishes. Fishes is usually used when you're talking about more than one kind of fish. So, if you have a bucket of trout, you'd say you caught some fish. But, if you have a bucket with some trout, some perch, some bass, and some pike. You could say you caught some fishes. Okay, let's try a quick quiz. Which of these animals do you think is most closely related to humans, and is also a member of Chordata? Is it A, Clam, B, Jellyfish, C, Sea squirt, or D, Lobster? You might be surprised to learn that you share anything in common with sea squirt. Sea squirts, biologists call them tunicates. Are more closely related to humans than they are to these other ocean dwelling animals. Adult sea squirts may not particularly look fishy enough to be related to vertebrates. As adults, most sea squirts attach themselves to a spot on the sea floor. And filter food particles out of the water around them. However, juvenile or larval sea squirts can swim freely or float in the water, and have a very different appearance. Larval sea squirts have a structure called a notochord. Which is a stiff, but flexible rod made out of fibers of the material called collagen. A notochord is a useful thing to have. Because it provides an anchoring point for muscles to attach to, and pull against. The notochord is the beginning of the vertebral endoskeleton, or internal skeleton. This is different from the hard, exoskeletons or external skeletons found in invertebrates like crabs, spiders, and insects. Eventually, the notochord was structurally replaced by the vertebrae of your backbone. But even you had a notochord for a little while, when you were a developing embryo. The fact that we can see a remnant from the early days of our chordate ancestors, in our embryonic development. Is just one of the amazing ways that developmental biology contributes to our understanding of evolutionary processes, and our evolutionary history. We'll see many more examples of this throughout the course. Above the notochord, is another one of the important features found only in chordates. The hollow dorsal nerve cord. Some animals, like earthworms and insects, have a nerve cord that is solid and located ventrally in the animal. So, it runs along the belly. But chordates, have a nerve cord that runs along our back dorsally. Because of the way it forms, rolling up from the sides, it is hollow. It sits above the notochord, and has nerves that branch out to send commands to muscles along the body. In humans, the dorsal hollow nerve cord is enclosed by the vertebrae of our backbone. Which is properly called the vertebral column, but you may also know it as the spine. And so, it is called the spinal cord. Let's take a break from chordate features for a minute. And talk about what we mean when we say that a feature is modified to become something else. And this is an example of something called homology. Homology describes a shared ancestry between two or more different features, or traits in different organisms. Homologous traits or homologues, are traits that are descended from similar traits in a common ancestor. When the homologous feature is only shared by some of the members of a larger group. It's a shared derived character, called a synapomorphy. For instance, humans and birds are both vertebrates. We inherited our skeletons from a common vertebrate ancestor, that we share with all other vertebrates. So, our skeleton is homologous with that of birds. Humans and kangaroos are both members of a smaller group within vertebrates called mammals. And we share fur with kangaroos, which birds don't. Because we and kangaroos and all other mammals inherited fur from a common ancestor, that was more recent than the ancestor we share with birds. So fur is a synapomorphy of mammals. On the other hand convergent characters are characters that appear similar, but arose independently. And aren't derived from a common ancestor. Convergent evolution often results in analogous traits. Analogous traits, or analogues, are traits that may be similar due to a shared function. But they're not descended from similar traits in a common ancestor. Insects and birds both have wings. But they didn't inherit their wings from a winged common ancestor. So insect and bird wings are analogous traits. For another example, let's take a look at the wings of birds, bats, and Pterosaurs, which you may be familiar with. If we look at these animals on a simplified phylogenetic tree, we can see that the last ancestor these animals shared was a primitive terrestrial tetrapod with walking limbs. These animals didn't inherit their wings from a single common winged ancestor. The wings evolved independently in these distantly related lineages. The wings are therefore convergent, or analogous features. On the other hand, take a look at their arm bones. They all have the same limb bones. A humerus, radial, carpels, metacarpals, and phalanges. And these bones are also arranged in the same way. In all three limbs even though the shapes of some of the bones are different. If we look at the limbs of their terrestrial common ancestor, we can see that it had these same bones in the same arrangement. The bones in the arms of these animals are all similar in this way, because they inherited them from their common ancestor. So, the limb bones are homologous features. These limbs have been modified through the process of natural selection from their ancestral terrestrial limbs. To function for flight, forming analogous structures. It's important to remember when we talk about homologous and analogous traits, that the same feature can be homologous at one level and analogous at another. These features are analogous as wings. But homologous as limbs. In the same way, many of the traits we see in our earliest recorded ancestors are homologous, with many of the features we see in vertebrates today. But several had to be modified over time from their ancestral forms. Let's return to our chordate relatives. We've discussed the notochord, and dorsal hollow nerve cord that are features found in all chordates. Therefore, these are homologous characters within vertebrates. Even though in adult humans, the notochord is essentially no longer present. It has been functionally replaced by our vertebral column. If you were to cut a larval sea squirt in half, down the length of its body, and look at the interior part of its body, you would be able to see a series of narrow openings. These are pharyngeal slits, which become the gill openings in fishes. Pharyngeal refers to the region around the throat or pharynx. In fishes, like this shark, the gills are used for respiration, transferring oxygen into the bloodstream. Sea squirts use their pharyngeal slits for eating, they bring water into the pharynx and filter food out of it, and then expel the excess water. There's some evidence today that pharyngeal slits might actually pre-date chordates, and be present in the larger group that encompasses all the chordates, the hemichordates and the echinoderms. Now, hemichordates includes acorn worms, while echinoderms includes starfish and sea urchins. For now, we'll consider pharyngeal slits to be one of the characteristic features of chordates. Terrestrial chordates lost their gills. But again, if you look at a human embryo, you'll see the beginnings of the pharyngeal slits for a little while during development. The fourth unique feature of chordates is one of the weirdest, but also one of the most important. This is a structure called an endostyle. And the endostyle is a long groove on the bottom side of the pharynx found in sea squirts and some larval vertebrates like the lamprey. The endostyle is lined with tiny, hair-like structures called cilia. And these spread a coating of mucous that is secreted by cells which traps food particles. Recent evidence suggests that the endostyle is homologous with our thyroid gland. Since the thyroid gland regulates our metabolism, we owe a lot to the humble endostyle. Our final unique feature for chordates is one that might not seem like it's all that special, but it's found only in this group. In many animals, the digestive system ends at the anus. The anus is located at the very end of the body. So, if you think of something like an earthworm, the mouth is located at one end, and the anus is located at the very tip of the body at the other end. Now, chordates have a post-anal tail, meaning that the muscles and skeleton of our bodies continues beyond the end of the digestive tract. Humans don't have tails when we're born, but we do have tails as embryos. Sea squirts and humans look very different, and took very different evolutionary pathways that led to very different lives. But we still have some features in common during our early embryonic development. And those shared features show our shared evolutionary history as chordate animals. Now that we've looked at what chordate features we have in common with a sea squirt. Lets think about what makes vertebrates unique within the chordate group, vertebrate synapomorphies. Which of these features do you think are unique to vertebrates, but not chordates? A. Pharyngeal slits. B. A head and brain anterior to the notochord. C. A post-anal tail. And or D. Bones. More than one answer maybe correct, so check all that apply. Pharyngeal slits and a post anal tail are found in all chordate animals. And since vertebrates are a subset within chordates, we don't consider these features unique to vertebrates. Only vertebrates have a head with a brain located anterior to the notochord and have bones. So B and D are correct. Vertebrates built on the basic chordate body plan with new evolutionary innovations. Let's take a look at a hypothetical early vertebrate, something similar to <i>Haikouichthys</i>. Now <i>Haikouichthys</i> is a Cambrian vertebrate from China that lived about 525 million years ago. The earliest land vertebrates evolved almost 400 million years ago and the first dinosaurs didn't evolve until around 240 million years ago. The earliest modern humans didn't evolve until about 200,000 years ago. So when I refer to the early vertebrates as our ancient ancestors, I mean really ancient. <i>Haikouichthys</i> doesn't look very much like us, does it? But in several ways, it's closer to us than it is to a sea squirt. One of the most obvious differences is that vertebrates have a distinct head and brain at the anterior end of the body which extends out past where the notochord ends. In primitive chordates, both the nerve cord and the notochord extend all the way to the very front end of the animal, but in vertebrates, the nerve cord grows and expands at the front end, resulting in it reaching further forward than the notochord. Your brain is an expansion of this end of the dorsal hollow nerve cord found in all chordates. Part of this expanded nervous tissue is associated with sense organs such as your eyes, nose and sound receptors. In order to house this expanded bulge of neural tissue, the anterior end of the vertebrate bodies became enlarged into a distinctive cranium head. The brain and nerve cord are fragile and could be easily damaged, but they're protected by new kinds of tissue: bone and cartilage. Cartilage can be found in your nose, your ears, windpipes and joints. Sometimes it's fairly flexible and other times, it's quite stiff. Bone is a mineralized tissue that's made of the mineral calcium hydroxyapatite and collagen. Calcium hydroxyapatite confers compressive strength and collagen confers tensile strength meaning bone can withstand both pushing and pulling forces. Your bones form the rigid support for your muscles to pull against allowing muscles to move parts of your body. You might think that all of your bones formed in the same way, but there are actually different classes of bone that develop in different parts of your body. Most of the bones in your skeleton start out as cartilage in the embryo, and then the cartilage gets replaced by bone and this is a process called ossification which occurs during growth. These bones are called endochondral bones. Endochondral bones include the bones in your arms and legs, your pelvis, your vertebrae, and some parts of your skull. Another mode of bone formation is intramembranous ossification, where bone forms without the need for a cartilage scaffold. And some of these intramembranous bones are called dermal bones because they ossify some of the tissues in the skin layer called the dermis. Your flat skull bones, and many of the bones in your face, your clavicles, the spikes and plates of a stegosaurus, an armadillo's shell, and the shark's scales are all examples of intramembranous dermal bones. Many early vertebrates had extensive dermal armour covering their entire bodies. In the fossil record we can see the evolution of bone from non-ossified chordates, to ossified vertebrates and so we tend to think of bone as one of the defining characteristics of vertebrates. But it might be more accurate to think of the defining characteristic of vertebrates in terms of the developmental process that forms bone. As embryos, vertebrates have a unique set of cells called neural crest cells and these cells are a jack of all embryonic trades. When you were an embryo, neural crest cells first formed along the edges of your dorsal hollow nerve cord, they then migrated around your embryonic body and changed into new kinds of cells. The neural crest cells in the anterior part of your body form the cartilage and bone of your skull and the cells in your nerves. They are associated with your sensory organs like your eyes and ears. Dorsally, along your back, the neural crest cells change into the pigment cells in your skin and became incorporated into your nervous system. The neural crest cells also help form many of the hormone producing organs in your body. Neural crest cells are probably the single most important evolutionary innovation that differentiates vertebrates from other chordates.
