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.
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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.
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.
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. 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, 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.
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, 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. 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. 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, 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.
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. 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. 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, 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 neural-crest-like activity in tunicates, but none at all in the amphioxus or any other invertebrate.
Vertebrates have large brains. Nothing like the vertebrate brain is seen in either tunicates or the amphioxus, although there are traces of its ground plan in the amphioxus, tunicates, and even hemichordates, if one looks hard enough. Other animals have brains, notably arthropods and mollusks, and in some cases these are elaborate structures associated with complex and even intelligent behavior. One thinks of the octopus, a famously canny creature with a large and complex brain. But the brains of invertebrates are constructed differently from those of vertebrates, and are not enclosed within that other distinctively vertebrate feature — the skull.
In addition to all the features mentioned above, vertebrates have a wealth of internal features that, although less obvious, are unique to the group and serve only to widen the gap between vertebrates and other animals. These include a water management system centered on the kidneys, which has allowed vertebrates, among only a select few animal groups and uniquely among deuterostomes, to live their lives entirely away from water. The kidneys are connected to a unique system of sex organs, which are in turn connected, chemically, to a sophisticated network of internal, hormone-based signaling, complementary to that of the nervous system. Although many animals (and plants) have a degree of innate immunity to agents of disease, which can on occasion be highly discerning and sophisticated, only vertebrates have a system of acquired immunity in which the cells of the immune system can be trained to recognize and neutralize threats never before encountered. All this and lymphatic drainage, a closed blood circulation with vessels lined with a specialized tissue called endothelium, and powered by a chambered heart. Because of these internal refinements, vertebrate animals can live a life much more independently of the environments in which they are found, compared with many other animals.
At a deeper level, the genome of vertebrates seems to have been duplicated — not once, but twice — at some point in the earliest history of the group, although there is some debate about whether the second duplication happened before or after the emergence of the lineage leading to the most basal extant vertebrates, that is, the jawless hagfishes and lampreys. It has been thought that genome duplication allows for an increase in complexity. If two genes are produced where there was one before, each one can evolve in its own way, perhaps allowing for previously unattainable subtleties in gene regulation, morphological specification, and so on. However, what seems to happen is that many of the duplicates are lost, so the connection between gene duplication and complexity remains moot. The genomes of teleost fishes — the group of bony fishes that includes most familiar kinds, such as the cod with your fries to the guppies in your aquarium — have undergone a further duplication, and although these creatures exhibit a wide range of morphology (forms as varied as sea-horses and the ocean sunfish) they are all recognizably vertebrates.
The presence in vertebrates of the head, brain, hard tissues, notochord, distinctive nervous system, neural crest, kidneys, adaptive immune system, and so on, features seen nowhere else in the animal kingdom, serves to divide vertebrates from all other animals.
1.3 BREAKING BRANCHES
At first sight, many of the characteristic features of vertebrates appear to have evolved all at once. This explains why vertebrates appear so different from anything else in the animal world. However, it is legitimate to ask whether the apparently unique features of vertebrates evolved not simultaneously, but one at a time, and, if so, in which order; and whether some of them might be found, even if in some more modest form, among invertebrates.
These are reasonable questions, because we already know that some of the features we see in vertebrates, such as the neural crest, are to some extent presaged in tunicates; that the notochord and hollow dorsal nerve cord are also found in invertebrate chordates such as tunicates and the amphioxus; and that the pharyngeal gill slits are found in hemichordates and possibly some now- extinct echinoderms.
This allows us to reconstruct an order in which these features were acquired. Pharyngeal gill slits evolved first, in the common ancestors of all deuterostomes; with the notochord and hollow nerve cord evolving later on, in the common ancestor of all chordates. The rudiments, at least, of the neural crest appeared later still, in the common ancestry of tunicates and vertebrates. Therefore it should be possible to break down all the features we see in vertebrates and try to imagine how they might have evolved sequentially.(Continues…)
Excerpted from "Across the Bridge"
Copyright © 2018 Henry Gee.
Excerpted by permission of The University of Chicago Press.
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Table of ContentsPreface Chapter One: What Is A Vertebrate? 1.1 Vertebrates in Context
1.2 What Makes a Vertebrate?
1.3 Breaking Branches
1.4 Summary Chapter Two: Shaking the Tree 2.1 Embranchements and Transformation
2.2 Evolution and Ancestors
2.3 Summary Chapter Three: Embryology and Phylogeny 3.1 From Embryos to Desperation
3.2 Genes and Phylogeny
3.3 Summary Chapter Four: Hox and Homology 4.1 A Brief History of Homeosis
4.2 The Geoffroy Inversion
4.3 The Phylotypic Stage
4.4 The Meaning of Homology
4.5 Summary 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 12.1 Introduction
12.2 The Organizer
12.3 The Notochord
12.5 Segmentation and the Head Problem
12.6 The Nervous System
12.7 Neural Crest and Cranial Placodes
12.8 The Skeleton
12.9 Summary Chapter Thirteen: How Many Sides Has A Chicken? 13.1 Introduction
13.2 The Enteric Nervous System
13.3 The Head and the Heart
13.4 The Urogenital System
13.5 The Gut and Its Appendages
13.7 The Pituitary Gland
13.8 Summary Chapter Fourteen: Some Fossil Forms 14.1 Fossils in an Evolutionary Context
14.2 Meiofaunal Beginnings
14.9 The Earliest Fossil Vertebrates
14.11 Ostracoderms and Placoderms
14.12 Summary Chapter Fifteen: Breaking Branches, Building Bridges 15.1 Defining the Deuterostomes
15.7 The Common Ancestry of Tunicates and Vertebrates
15.12 The Evolution of the Face
15.13 Crossing the Bridge
15.14 Conclusions Notes