Across the Bridge

Understanding the Origin of the Vertebrates

Henry Gee

• Publisher: University of Chicago Press
• Year: 2018
• Print ISBN:
  9780226403052
  9780226402864
• eBook ISBN:
  9780226403199
• Topic: Ciencias biológicas
• Subtopic: Evolución

Our understanding of vertebrate origins and the backbone of human history evolves with each new fossil find and DNA map. Many species have now had their genomes sequenced, and molecular techniques allow genetic inspection of even non-model organisms. But as longtime Nature editor Henry Gee argues in Across the Bridge, despite these giant strides and our deepening understanding of how vertebrates fit into the tree of life, the morphological chasm between vertebrates and invertebrates remains vast and enigmatic.

As Gee shows, even as scientific advances have falsified a variety of theories linking these groups, the extant relatives of vertebrates are too few for effective genetic analysis. Moreover, the more we learn about the species that do remain—from sea-squirts to starfish—the clearer it becomes that they are too far evolved along their own courses to be of much use in reconstructing what the latest invertebrate ancestors of vertebrates looked like. Fossils present yet further problems of interpretation. Tracing both the fast-changing science that has helped illuminate the intricacies of vertebrate evolution as well as the limits of that science, Across the Bridge helps us to see how far the field has come in crossing the invertebrate-to-vertebrate divide—and how far we still have to go.

CHAPTER 1

What Is a Vertebrate?

1.1 VERTEBRATES IN CONTEXT

Most familiar animals are vertebrates—that is, animals with backbones. We are vertebrates, as are most of our domestic animals, such as cows, horses, poultry, sheep, and pigs. The numerous animals housed at various times chez Gee—dogs, cats, chickens, rabbits, guinea pigs, hamsters, snakes, axolotls, and fish, not forgetting the frogs that crowd our garden pond each spring, are vertebrates.

Most of the animals you will meet in a zoo, from lions to lorikeets, geckos to giraffes, are also vertebrates, so much so that non-vertebrates are usually confined to a single building labeled something like “creepy crawlies.” The invertebrates, though, comprise a wider and more diverse domain than that. With a proper zoological perspective, vertebrates represent one rather small branch of a riotously various and diverse array of animal life. To understand vertebrates and how they evolved, one has to have a good overview of the entirety of animal life.1

Perhaps the most important invertebrates, at least in terms of numbers of species, are the insects. Many of these will be familiar to the most wildlife-averse urbanite, even if they are only flies and cockroaches (see fig. 1.2). Bees, ants, wasps, butterflies, moths, beetles, dragonflies, and grasshoppers are all familiar insects. Most known animal species are, in fact, insects. And yet insects form just one branch on the much more extensive tree of arthropods, or jointed-legged animals. Besides insects, this includes spiders, scorpions, ticks, mites, crabs, lobsters, centipedes, millipedes, barnacles, and other, less familiar creatures such as pycnogonids (sea spiders) and xiphosurans (horseshoe crabs).

Other invertebrates include mollusks such as clams, squid, slugs, and snails; as well as a diverse range of worms, jellyfishes, starfishes, sponges, and so on, to name just the more familiar among a still wider array of animals. Many of these are small, rare, or obscure, and known mainly to professional zoologists, or those students who, like me, liked to explore the dusty end of the textbook in search of unpronounceable exotica.

1.1 A selection of vertebrates. A: rabbit Oryctolagus cuniculus (mammal) and chickens Gallus gallus (birds); B: dogs Canis familiaris (mammals); C: cat Felis domesticus (mammal); D: royal python Python regius (reptile); E: axolotl Ambystoma mexicanum (amphibian); F: plecostoma, possibly Pterygoplichthys sp. (fish). Photographs by the author, from the author’s menagerie.

1.2 A small selection of invertebrates. A: a house spider (arthropod); B: land snail (mollusk); C: a handful of sea gooseberries, Pleurobrachia (ctenophores); D: a sea cucumber (echinoderm): the rightmost object on the plate; E: jellyfish (cnidarian); F: octopus (mollusk); G: a colony of sea chervil, Alcyonidium (bryozoan); H: horseshoe crabs, Limulus (arthropod); J: bumble bee (arthropod); K: edible crab (arthropod). Photographs by the author.

Amateur microscopists will have seen the rotifers (wheel animalcules) and tardigrades (water bears) that swarm in water or crawl out of damp moss. Sharp-eyed beachcombers will have encountered sponges, tunicates, and bryozoa (moss animals). But it’s a fair bet that most people will never have seen, or even heard of, priapulids, pogonophorans, placozoans, or phoronids, and those are just the ones I could immediately think of beginning with the letter p.2 Yet each represents a “phylum,” that is, a distinct and distinctive kind of animal life.

1.2 WHAT MAKES A VERTEBRATE?

Despite this diversity, vertebrates seem to stand apart. They are so different from other animals that recognizing a vertebrate seems almost instinctive. Could it be because we ourselves are vertebrates, and so recognize our kin, even if only from a distance? This is undoubtedly a reason, yet even when one discounts our very understandable prejudice, vertebrates do seem qualitatively different from other animals.

The presence of a distinct head is a vertebrate feature, and the characteristic vertebrate arrangement of a “face” with two eyes, set side-by-side, and a mouth beneath, might explain the almost universal feeling of kinship with all vertebrates, whereas the arrangements seen in other animals—whether a panoply of eyes, tentacles, or spiny mouthparts, or a front end that is featureless or eyeless—seem alien to us and might be greeted with horror. The emoticon of a smiley face ☺ typifies the vertebrate arrangement and has universal appeal, whereas people have to learn to love many-eyed spiders and eyeless worms. This is, in fact, proven in the breach. Tiny flatworms called planarians,3 found in streams and ponds, are very different in their construction from vertebrates, and yet some have two large eyes at the front that make them seem curiously appealing, if not actually cuddly. You can see a couple of examples in fig. 1.3.

1.3 The eyes of Planaria. A: Cura cf. pinguis, from Australia. The head is on the left, and the whole animal is about 4 mm long (courtesy Miquel Villa-Farre). B: a close-up of the head of another species Dugesia sanchezi, just to show it’s not a fluke (courtesy Alejandro Sanchez Alvarado).

It’s worth listing some of the many ways in which vertebrates differ from other animals. I’ll go into these in much more detail later in the book, but for now it’s worth rehearsing them, to get to grips with that feeling we have that there is a substantial gap between vertebrates and other animals, a chasm we need to bridge if we are to understand vertebrate origins.

I’ve already alluded to the presence of a head, and, in particular, a face. A head is a concentration, at one end of an animal, of entry points for air, food, and sensory information. A head, in such a broadly defined sense, is only to be expected in bilaterally symmetrical animals with a preferred direction of travel. Other such animals include insects and other arthropods. These, too, have heads, but they are constructed differently from the heads of vertebrates. Insect eyes are made in a completely different way from vertebrate eyes, being constructed of many repeated units (think of pixels) rather than a single, camera-like unit with a flexible lens, as found in vertebrates. Insects’ ears are found on their legs, their noses on their feet; and they breathe not through their mouths, but through many tiny pores on their bodies. This suggests that the heads of insects and vertebrates evolved entirely independently, each from headless ancestors. This is supported by what we know of the evolutionary relationships of insects and vertebrates. Insects are more closely related to various more-or-less headless worms than to vertebrates. By the same token, the closest relatives of vertebrates among the invertebrates—the sea squirts, or tunicates, and the superficially fish-like amphioxus—do not appear to have distinct heads. However, I shall explain in this book, this does not mean that tunicates and the amphioxus do not have structures comparable with what we see in the vertebrate head—it is that they are not immediately obvious. Perhaps it is truer to say that these invertebrate relatives of vertebrates do not have the smiley faces we instinctively associate with the vertebrate state.

Vertebrates are built around an internal skeleton of cartilage, which in many cases is reinforced with harder tissues such as bone, dentine, and enamel. Although cartilage of various sorts is found throughout the animal kingdom,4 bone, dentine, and enamel are tissues unique to vertebrates. The principal mineral constituent of vertebrate hard tissues is hydroxyapatite, a form of calcium phosphate. The shells and other hard tissues of invertebrates are made of a different substance, calcium carbonate.5 The vertebrate skeleton comprises a brain case, housing the brain and sense organs such as the eyes, ears, and nose, to which might be attached skeletal supports for jaws and gill arches, and of course the backbone made of interlocking vertebrae, from which the group gets its name.

The skeleton also includes internal supports for fins and limbs, if present. During development, the backbone replaces a longitudinal stiffening rod called the notochord, which is found at some stage in the life cycle of vertebrates as well as tunicates and the amphioxus. Because of this, the vertebrates, the tunicates, and the amphioxus are united into a larger group, the chordates.

Along with the notochord, all chordates possess, at some part of their life cycle, a system of serially repeated pouches on each side of the throat region or pharynx, which in many cases pierce the body wall and open either directly to the outside, or into a protective cavity or atrium, which communicates with the outside through a smaller number of openings. In tunicates, the amphioxus, and the larvae of lampreys alone among vertebrates, these pharyngeal pores or slits form part of a unique filter-feeding system.6 Water is taken in through the mouth and propelled, by currents generated by cilia, outward through the pharyngeal slits. Mucus secreted by the endostyle—a region of glandular cells in a longitudinal gutter on the pharyngeal floor—is carried up the cartilage-supported bars between the slits, trapping any water-borne debris before it escapes. The food-laden mucus makes its way to the roof of the pharynx where it enters the oesophagus and the digestive system. Tunicates and the amphioxus feed like this throughout life. Filter-feeding lampreys lose this arrangement at metamorphosis. The endostyle is transformed into the thyroid gland, and in adult lampreys and all other vertebrates, the pharyngeal slits are transformed into supports for gills used to extract oxygen from water and, in fishes, to excrete excess salt. In most tetrapods (that is, land-living vertebrates) the pharyngeal slits never form at all and the elements that otherwise would have made up their bony or cartilaginous supports become incorporated into the inner ear, the jaw, or the hyoid skeleton that supports the tongue.

Pharyngeal slits are found in animals other than chordates, notably marine animals called hemichordates,7 even though these creatures do not appear to have endostyles, notochords, or other structures found in chordates. Hemichordates come in two forms: enteropneusts (acorn worms) and pterobranchs, neither of which will be familiar to anyone but professional zoologists. Enteropneusts are blind, brainless, flaccid, and sometimes foul-smelling worms that live in marine sediment; pterobranchs are small, often colonial organisms, feeding through an arrangement of tentacles called a lophophore.8

Some extinct echinoderms—a group of animals that today includes starfishes, sea urchins, and sea cucumbers—appeared to have had pharyngeal slits, although no extant echinoderm does so.9 Hemichordates and echinoderms together form a group called the Ambulacraria, and ambulacrarians and chordates together form a larger animal group called the deuterostomes.

The notochord of chordates provides support and purchase for muscles and other tissues such as nerves and blood vessels, arranged in a series of segments called somites. Although many other animals are segmented—arthropods, as well as segmented worms or annelids—these segments are constructed entirely differently. Tunicates appear to have lost their segmentation in evolution, whereas the segmentation in amphioxus differs from vertebrate segmentation in important ways.

As the notochord develops during the life of a chordate embryo, it secretes substances that induce the development, dorsal to it (that is, along the upper surface, or back), of a hollow, longitudinal nerve cord, the basis of the vertebrate central nervous system.10 The dorsal, hollow nerve cord is a unique feature of chordates. In all invertebrates that have a central nervous system, the nerve cord, if present, is ventral (that is, along the belly) and solid. Some invertebrates have two or more nerve cords. In some animals, paired, ventral cords are joined by cross-bridges at regular intervals like the rungs of a ladder.

The formation of the dorsal nerve cord is accompanied by the migration of cells from its lateral edges, along specified routes, to various parts of the body. These cells, collectively the neural crest,11 are responsible for many uniquely vertebrate features such as the bones of much of the head and face; parts of the organs of special sense, notably the ears; the formation of the skin, its pigmentation, and its appendages such as scales, hair, feathers, and teeth; and other parts of the anatomy such as the spinal ganglia, the adrenal glands, the nervous system that lines the intestines, and parts of some major blood vessels. In that much of the instantly recognizable vertebrate face is formed by the neural crest, one could argue that this is the single most important defining feature of vertebrates. There are, however, traces of modest neur...

Table of contents

• Cover
• Title Page
• Copyright Page
• Contents
• Preface
• chapter one:  What Is a Vertebrate?
• chapter two:  Shaking the Tree
• chapter three:  Embryology and Phylogeny
• chapter four:  Hox and Homology
• chapter five:  What Is a Deuterostome?
• chapter six:  Echinoderms
• chapter seven:  Hemichordates
• chapter eight:  Amphioxus
• chapter nine:  Tunicates
• chapter ten:  Vertebrates
• chapter eleven:  Some Non-deuterostomes
• chapter twelve:  Vertebrates from the Outside, In
• chapter thirteen:  How Many Sides Has a Chicken?
• chapter fourteen:  Some Fossil Forms
• chapter fifteen:  Breaking Branches, Building Bridges
• Notes
• References
• Index