{"id":512,"date":"2025-06-19T22:19:18","date_gmt":"2025-06-19T22:19:18","guid":{"rendered":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/?post_type=chapter&p=512"},"modified":"2026-04-07T18:55:45","modified_gmt":"2026-04-07T18:55:45","slug":"vertebrate-origins","status":"publish","type":"chapter","link":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/chapter\/vertebrate-origins\/","title":{"raw":"Vertebrate Origins","rendered":"Vertebrate Origins"},"content":{"raw":"
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Focus Questions\u2014to Guide Your Reading of This Chapter<\/p>\r\n\r\n<\/header>\r\n

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  1. What are some of the defining characteristics of vertebrates? Gnathostomes? Eugnathostomes?<\/li>\r\n \t
  2. What familiar vertebrates fall into each taxonomic group of vertebrates?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

    3.1 Introduction\u2014What Is a Vertebrate, Anyway?<\/h1>\r\n

    In C<\/span>hapter<\/span> 2<\/span><\/span><\/a>, we introduced you to deuterostomes and, more specifically, the chordates. We also introduced you to the vertebrates in general, including what early vertebrates may have looked like. Let\u2019s dive deeper into some features that vertebrates share, and we\u2019ll add some new ones to your list.<\/p>\r\n

    Neural crest cells<\/strong><\/span> have been regarded as one of the most important unique vertebrate features. The cells develop along either side of the neural plate<\/strong><\/span>, which makes up the embryonic nerve cord (neural tube) and embryonic brain (Figure 3.1). You will read more about these and what they become in Chapter <\/span>4<\/span><\/span><\/a>, but they have two important properties. The first is that they are migratory. Across vertebrates, we see these cells develop at the neural plate from anterior to posterior, and then they distribute themselves around the body. Once they arrive at their destination, we see their second important property: They are multipotent. That means that they can become many different things. They can make the structural tissues in your head (bone, cartilage, dentine in your teeth), nervous system cells, or pigment cells, depending on which genes are expressed and when. Altogether, neural crest cells are responsible for many vertebrate features.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"450\"]\"\" -
    Figure 3.1\u2014Neural crest cell formation during the process of neurulation. Note that the neural tube develops into the dorsal nerve cord.[\/caption]\r\n

    Let\u2019s now imagine a vertebrate\u2014it might be you, your cat, or your goldfish. Regardless, whichever vertebrate you\u2019ve imagined has a head. If we had to define what a head is, we would say something like \u201cit\u2019s the part of the body where the mouth, brain, skull, and sensory organs are located.\u201d Note that we didn\u2019t say there was a jaw or teeth\u2014not all vertebrates have these (more on that later). It\u2019s also worth noting that many invertebrates have heads (albeit without skulls) too\u2014insects are great examples.<\/p>\r\n

    Considering the vertebrate head leads us to more vertebrate features. The first was mentioned in the previous chapter, cranial ectodermal placodes<\/strong><\/span>, also called neurogenic placodes<\/strong><\/span> (Figure 3.2). These embryonic features are a thickening of tissue at various places in the head region. Based on interactions between cells and tissues, placodes can differentiate into a number of different structures depending on their location. These include the lens of the vertebrate eye, portions of various cranial nerves (nerves that come directly off the brain), structures related to olfaction (sense of smell), and more. You will read more about these placodes in Chapter <\/span>4<\/span><\/span><\/a> and about some of their derivatives in the chapters about the nervous and sensory system (Chapters 18<\/span><\/span><\/a>, 19<\/span><\/span><\/a>, and 20<\/span><\/span><\/a>).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"600\"]\"\" -
    Figure 3.2\u2014The olfactory (yellow), lens (blue), trigeminal (green), otic (purple), and epibranchial (orange) cranial ectodermal (neurogenic) placodes locate to characteristic positions, here shown in a side view of a three-day-old chick embryo. You will learn much more about these names and structures in later chapters.[\/caption]\r\n

    Speaking of nervous systems, vertebrates are also known for their fairly complex brains. If the vertebrate you thought of earlier was a goldfish, you may not categorize its brain as \u201ccomplex.\u201d However, it\u2019s all relative\u2014if we compare vertebrate brains to those of other chordates, for example, even that goldfish brain is pretty complex. Vertebrate brains are divided into three regions: a forebrain<\/strong><\/span>, a midbrain<\/strong><\/span>, and a hindbrain<\/strong><\/span> (Figure 3.3). These are very distinct regions that have fairly distinct functions, even in goldfish. Cephalochordates, on the other hand, do not have any obvious divisions at all. They have a slightly larger mass of neural tissue in their head region called the cerebral vesicle. There is some difference in gene expression as you move anterior to posterior in the cerebral vesicle, but that\u2019s it. Urochordates have no brain at all. Neural crest cells are not responsible for making any vertebrate brain tissue itself. However, they do help make some of the cranial nerves coming off the brain. You\u2019ll learn more about these in Chapter 19<\/span><\/span><\/a>.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"338\"]\"\" -
    Figure 3.3\u2014Divisions of the embryonic vertebrate brain during different periods of development.[\/caption]\r\n

    Vertebrates protect their brains with a skull<\/strong><\/span>, also called the cranium<\/strong><\/span>. This is typically made up of either cartilage or bone, which means that the skull is a feature that is derived from those very important neural crest cells. Skulls are fascinating parts of the body that can tell you about sensory systems, the brain itself, posture, feeding mode, and much more. In fact, we\u2019ve devoted an entire chapter just to the skull (Chapter 8<\/span><\/span><\/a>).<\/p>\r\n

    If we could summarize everything we\u2019ve just discussed, we could say that vertebrates are defined by their heads. Indeed, as scientists began to understand the importance of neural crest cells and the neurogenic placodes, Carl Gans and Glenn Northcutt proposed the \u201cNew Head Hypothesis\u201d in 1983. The hypothesis contains several different strands that concern the origin of unique vertebrate tissues, homology, and much more. The general idea is this:<\/p>\r\n\r\n