Sergei Yastrebov. II. Evolution of the First Chordates and Paleontology

The Batrachos Lounge. The second article by Sergei Alexandrovich Yastrebov, lecturer at Moscow State University and author of the blog Caenogenesis. The previous article is here, the next and last of the three is here. S. A. Yastrebov Evolution of the First Chordates and Paleontology What Are Chordates? At the very beginning of the nineteenth century, French zoologist Georges Cuvier divided all animals into four major groups: vertebrates, mollusks, articulates, and radiates. Very soon these groups came to be called phyla. The phylum Vertebrata comprised animals possessing a brain and spinal cord, a skull, and a vertebral column. The phylum was divided into five classes: fishes, amphibians, reptiles, birds, and mammals. These animals were familiar to everyone, and the boundaries of the phylum seemed entirely clear. Questions such as "should this or that animal be considered a vertebrate?" simply did not arise. [IMG_1] Fig. 1. An ascidian and its larva (from https://www.darwin.museum.ru). The "slit-like pharyngeal openings" are gill slits. In the adult ascidian their number increases considerably. The nervous tube, however, is reduced to a single small ganglion, and the notochord disappears entirely. The situation changed dramatically in 1866, when Alexander Kovalevsky published his work "The History of Development of Simple Ascidians." Adult ascidians are sessile, sac-like marine animals that feed by filtering fine organic particles from the water (Fig. 1). They possess neither a vertebral column nor a spinal or cerebral brain. Prior to Kovalevsky's work they had been placed, for reasons that are now unclear, among the mollusks. Ascidian larvae, however, are free-swimming and tailed; in the tail a curious "axial rod" had been described. Kovalevsky demonstrated that this "axial rod" is the notochord — the axial skeleton — constructed in the same manner as in the lower vertebrates. Furthermore, the ascidian larva possesses a tubular nervous system, again as in vertebrates. It became clear that the ascidian is our close relative. The 1860s were in general a period of major breakthrough in zoology. At about the same time, Branchiostoma — a translucent, marine, fish-like creature that had previously been assigned to the fishes — was studied in detail (Fig. 2). Instead of a vertebral column it possesses a notochord, which did not greatly trouble the investigators: a notochord is present also in certain undisputed vertebrates, such as the sturgeon or the lamprey. The difficulty, however, was that Branchiostoma lacks a skull, a cerebral brain, a heart, and kidneys — organs present in every vertebrate without exception. It became clear that Branchiostoma is something different. [IMG_2] Fig. 2. The lancelet Branchiostoma lanceolatum (after Gui). As a result, in 1874 the celebrated German naturalist Ernst Haeckel proposed the establishment of a new animal phylum — Chordata. It comprises animals possessing a notochord, a dorsal nerve tube, and pharyngeal (gill) slits — not necessarily at the adult stage, but at least at the embryonic or larval stage. The majority of chordates are vertebrates, but not all. Chordates that are not vertebrates are traditionally referred to as "lower chordates." There are two modern groups of lower chordates: tunicates and lancelets. Tunicates include ascidians and their relatives. As for the lancelet, we know it from the description in school zoology textbooks. Fewer than thirty species of lancelets are currently known, and all of them closely resemble one another. If Chordata is the phylum, then tunicates, lancelets, and vertebrates are its three subphyla. Other classifications of this group exist, but for the sake of simplicity we shall use this one for now. In 1955 Canadian zoologist Norman Berrill proposed that all other chordates descended from the larvae of ascidian-like creatures that ceased undergoing metamorphosis and acquired the ability to reproduce — a condition known as neoteny. This hypothesis, however, never gained wide acceptance. The tadpole-like ascidian larva lives for only one or two days and does not feed. Moreover, tunicates at all stages of their life cycle are virtually devoid of another important chordate character — segmentation. Yet segmentation is beautifully expressed in both the lancelet and the vertebrates, and there are serious grounds for considering that the common ancestor of the chordates was also segmented¹. Who, then, did this ancestor more closely resemble: the lancelet or a vertebrate? [IMG_3] Fig. 3. The anterior end of the lancelet (from https://www.orgs-evolution-knowledge.net). This is not a diagram but an exceptionally fine drawing, most probably made directly from microscope observations. Certain details not discussed in the text of the article are indicated. To clarify this question, let us examine in greater detail how the lancelet differs from the vertebrates. 1. Vertebrates invariably possess a brain composed of specific, always identical divisions familiar from human anatomy. Every vertebrate has a medulla oblongata, a midbrain, a diencephalon, and a telencephalon (forebrain). The lancelet has no brain at all. It is true that in the anterior part of its nerve tube there is a small expansion that modern neuroanatomists compare with the diencephalon of vertebrates, and in the region behind it neurons have been found that secrete the same substances as certain neurons of our midbrain (dopamine and serotonin). Nevertheless, this resemblance is sufficiently remote. 2. Vertebrates always possess a cartilaginous or bony skeletal covering of the head — the skull (cranium). The lancelet lacks a cranium. In many zoology textbooks the subphylum to which the lancelet belongs is accordingly named Acrania (headless). 3. In the lancelet, the notochord extends anteriorly beyond the nerve tube all the way to the front tip of the body (Fig. 3). This apparently serves to reinforce the anterior end, which the lancelet uses to burrow into mud or sand. Nothing of the sort occurs in vertebrates or in tunicates. Even in those vertebrates in which the notochord is very well developed, it terminates at the level of the boundary between the midbrain and diencephalon and never extends further forward. From this character derives yet another traditional name of the lancelet's group — Cephalochordata (head-chord animals). 4. Lancelets possess far more gill slits than any vertebrate. In an adult lancelet there are more than one hundred. In those vertebrates that retain gill slits in the adult state, the number is usually five pairs, much more rarely six or seven. The maximum number of gill slits in a vertebrate is seventeen pairs (in certain hagfishes). 5. The gill slits of the lancelet do not open directly to the exterior but into a special enclosed cavity formed by folds of the body wall. This cavity is called the peribranchial or atrial cavity. Water, from which the lancelet filters food particles, enters through the mouth, passes through the pharynx, exits through the gill slits into the atrial cavity, and only from there reaches the exterior through a small aperture. Vertebrates never possess an atrial cavity. It is, however, present in the adult ascidian. [IMG_4] Fig. 4. Cross-section through the notochord of a vertebrate embryo with surrounding tissues (after Yeo). a – cartilage, b – loose connective tissue, c – notochord tissue, d – notochordal sheath. This illustration shows the principal feature of notochord tissue: the greater part of the volume of each of its cells is occupied by an enormous vacuole. The vertebral column is not included among the characters listed above. This requires explanation. Above all, let us clearly distinguish between the concepts of "vertebral column" and "notochord." The vertebral column is a system of vertebrae — that is, cartilaginous or bony segmental elements arranged along the nerve tube. "Segmental" here means repeated and mutually similar, as we know from the human skeleton. Unlike vertebrae, the notochord is entirely non-segmented and is composed of a special tissue resembling neither bone nor cartilage (Fig. 4). The notochord may be incorporated into the vertebral column, surrounded on either side by paired vertebral elements (Fig. 5), or it may be displaced by them to the point of complete disappearance. But in the general case, the presence of vertebrae by no means implies the absence of a notochord. The jawless vertebrate lamprey, for instance, has a notochord that is well developed while its vertebrae are very rudimentary. Another representative of the jawless vertebrates — the hagfish — entirely lacks even traces of vertebrae! In the words of American paleontologist Alfred Romer, "calling a hagfish a vertebrate is possible only as a courtesy." In respect of this character, then, there is no sharp distinction between the lancelet and vertebrates. [IMG_5] Fig. 5. Photograph of a cross-section through the notochord of a vertebrate with developing vertebrae (from https://www.zoosyst-berlin.de). Above the notochord is the nerve tube; on either side are the myomeres; below is the dorsal aorta. The cartilaginous tissue of the vertebrae can be seen enclosing the notochord. Moreover, we see that the very name "vertebrates" is in general imprecise. And indeed, many comparative anatomists prefer to call this group of animals not Vertebrata but Craniata (skull-bearing animals). The cranium is a far more reliable character. In classical zoology the terms "vertebrates" and "craniates" were treated as synonyms, but relatively recently French paleontologist Philippe Janvier proposed distinguishing them. In his view, the group Craniata should subsume the group Vertebrata, since not all craniates possess a vertebral column: under this system the hagfish is a craniate but not a vertebrate. Thus one must be careful with the terms "vertebrates" and "craniates," as their meanings may differ. Furthermore, certain organs whose presence clearly distinguishes vertebrates from lower chordates are poorly preserved in the fossil record. These are first and foremost the heart and the kidneys. They must not be forgotten, but paleontology can as yet say little about them. We can now formulate several important questions concerning early chordates. Did they possess a brain? Was there an atrial cavity? Did the notochord extend anteriorly beyond the front end of the nerve tube? And how many gill slits were there? A Digression into Embryology: The History of the Neural Crest In the embryonic development of almost all metazoan animals there is a stage called the gastrula. A typical gastrula has the form of a two-layered sac with an internal cavity (Fig. 6). The cell layers forming the wall of the gastrula are called germ layers. The outer germ layer is the ectoderm; the inner germ layer is the endoderm. In bilaterally symmetrical animals a third germ layer — the mesoderm — soon (and sometimes immediately) forms as well. [IMG_6] Fig. 6. The gastrula of a lancelet (from https://www.southtexascollege.edu). The final illustration shows the formation of the mesoderm and nerve tube. From the ectoderm arise the skin (more precisely, its outer layer, the epidermis) and the nervous system. From the endoderm arises the digestive system (more precisely, its mucous membrane). In addition, there are organs that develop in the embryo as outgrowths of the gut tube. In our own case these include the liver, the pancreas, and even the lungs. All these organs are likewise composed of endoderm. From the mesoderm there typically arise the circulatory, excretory, reproductive, and musculoskeletal systems. Soon after the gastrula stage, the ectoderm on the dorsal side of the chordate embryo folds inward and closes to form a tube (Fig. 7). This gives rise to the nerve tube, which serves as one of the principal distinguishing characters of our phylum. It becomes the spinal cord and brain. [IMG_7] Fig. 7. Formation of the nerve tube and neural crest in a typical vertebrate (after Baker). Cross-section through the dorsal part of the embryo. Compare with Fig. 6: in the lancelet the nerve tube forms in a somewhat different manner. At the moment of nerve tube formation, the ectoderm divides into two parts: the neuroectoderm (which forms the nerve tube) and the surface ectoderm (which forms the epidermis). At the very boundary between the neuroectoderm and the surface ectoderm lies a group of cells called the neural fold or neural crest (Fig. 7). The majority of neural crest cells do not contribute to either the nerve tube or the epidermis. Instead, these cells are capable of dispersing throughout the entire organism, migrating in the manner of amoebae. From neural crest cells arise the ganglia situated outside the brain (spinal and sympathetic ganglia); the adrenal medulla; the pigment cells of the skin (melanocytes); the cells secreting dentin (odontoblasts); and much else besides. The most remarkable discovery in this field was made at the very beginning of the twentieth century by the Swiss woman embryologist Julia Platt. She found that a substantial portion of the skull also develops from neural crest cells. This finding provoked great scepticism in the scientific community, since according to the classical germ-layer theory the skull, like the entire musculoskeletal system, was "supposed" to develop from the mesoderm. More than forty years passed before Platt's discovery received clear experimental confirmation. Indeed, the greater part of the skull is formed from neural crest cells (Fig. 8). The exception is only the posterior portion of the braincase: this develops from ordinary mesoderm, as does the vertebral column. [IMG_8] Fig. 8. Contribution of the neural crest to the skull of the chick (after Dupin et al.). Parts of the skull derived from different portions of the mesoderm are shown in blue and green; those derived from the neural crest are shown in red. The most interesting feature of the neural crest is that the migration of its cells beyond the nerve tube occurs only in vertebrates (in the traditional sense of the word) and not in lower chordates. The neural crest appears such a vitally important and unique character of our subphylum that in 2001 embryologist Nicholas Holland and paleontologist Jun-Yuan Chen proposed naming this group of animals not Vertebrata and not Craniata, but Cristozoa (crested animals). And in some articles this name is indeed used. Whether the neural crest should be considered part of the ectoderm is a question of little meaning. The very concepts of germ layers were introduced for convenience, and the boundaries between them are conventional. It is simplest to regard the neural crest as a separate, fourth germ layer. This view is very widespread in modern literature. We have thus obtained yet another interesting question concerning the first chordates. Did they possess a neural crest? And if not, when did it arise? What Paleontology Tells Us The earliest unambiguously identifiable remains of craniates appear at the beginning of the second period of the Paleozoic Era — the Ordovician². The most ancient craniates are considered to be Arandaspis and Astraspis. These are already craniates in the full sense of the word, possessing a braincase, a cerebral brain, complex sense organs, and a hard skeleton. The first lancelet (named Palaeobranchiostoma) was described from the beginning of the fifth period of the Paleozoic — the Permian. This is already a more or less typical lancelet, similar to modern forms. For understanding the pathways of evolution, it would be most instructive to examine lower chordates that cannot yet be assigned to either craniates or lancelets. Animals of this kind must undoubtedly be sought in the first period of the Paleozoic — the Cambrian. Yet for a long time precisely such forms remained unknown. [IMG_9] Fig. 9. Reconstruction of Pikaia (from https://sandwalk.blogspot.com). In 1979 the eminent British paleontologist Simon Conway Morris described a very primitive chordate from the Middle Cambrian, named Pikaia. This animal is approximately the size of a lancelet — that is, several centimetres in length. As befits a chordate, Pikaia possesses segmented musculature, with up to one hundred segments in the body (a lancelet usually has somewhat more than sixty). Pikaia also has unique features of its own. First, there is a pair of long tentacles on the head; second, there are several pairs of branched appendages that resemble the parapodia of annelid worms but are most probably external gills. It is interesting to note that Pikaia was discovered as early as 1911, but was initially identified as an annelid worm. In foreign publications it is often written that Pikaia resembles a slug, and there is some truth to this, though mollusks are never as highly segmented. In 1991, in the Early Cambrian deposits of Yunnan Province in southwestern China, a creature was discovered and named Yunnanozoon. Initially, Yunnanozoon was characterized as a "segmented, worm-like animal of unknown affinities." Yet even then it was apparent that the body form of Yunnanozoon was less worm-like than fish-like. Its size is also roughly that of a lancelet. [IMG_10] Fig. 10. Photograph of an impression of Yunnanozoon (from https://burgess-shale.rom.on.ca). The first discovery of Yunnanozoon proved far from unique, and the organism was soon studied in greater detail. It became clear that it possesses true gill pouches separated by gill bars. Furthermore, Yunnanozoon most probably has two more organs highly characteristic of chordates. The first is the notochord itself. The second is the endostyle — a ciliated groove running along the floor of the pharynx in tunicates, in the lancelet, and in lamprey larvae. The great majority of biologists therefore now regard Yunnanozoon as a chordate. In 1995 Polish paleontologist Jerzy Dzik proposed assigning Yunnanozoon to a distinct class, the Yunnanozoa. Initially this class was monotypic, comprising only a single genus. Subsequently a second representative of this class was described from South China — Haikouella, named in honour of the capital of Hainan Island. [IMG_11] Fig. 11. Photograph of an impression of Haikouella (after Chen). The gill pouches are beautifully visible. The black bar indicates scale; its length is 5 millimetres. Chinese paleontologist Jun-Yuan Chen provides a characterization of the Yunnanozoa that may be briefly summarized as follows: (1) Number of gill slits: seven pairs in Yunnanozoon, six pairs in Haikouella. This number is encountered in vertebrates, whereas in the lancelet it is considerably greater. (2) An atrial cavity is present and is organized similarly to the atrial cavity of the lancelet, at least in Haikouella. (3) A cerebral brain is present and includes divisions closely resembling the medulla oblongata and diencephalon of vertebrates. (4) The notochord does not extend anteriorly beyond the front end of the nerve tube. In all probability it does not even reach the front end. (5) Sense organs: in the anterior part of the head there are definite eyes, paired and rather large. Nothing of this kind is found in either tunicates or the lancelet. The visual organs of modern lower chordates are very simply constructed, are never paired, and are as a rule located directly within the brain. (6) On either side of the notochord in Haikouella there are transverse rods resembling the arches of a weakly developed vertebral column — such as that found, for instance, in the lamprey. Haikouella and Yunnanozoon closely resemble one another (Fig. 12). The most conspicuous difference between them is in the number of gill pouches (character (1)). As we can see, with respect to characters (1), (3), (4), (5), and (6), they are clearly more closely allied to the vertebrates, and only with respect to character (2) to the lancelet. [IMG_12] Fig. 12. Comparison of Yunnanozoon and Haikouella (after Chen and Huang). A – Yunnanozoon, B – Haikouella. Ba – branchial arches, Pt – pharyngeal teeth, Go – gonads. Haikouella differs from Yunnanozoon in two significant respects: six gill pouches instead of seven, and transverse rods along the notochord resembling vertebral arches. It is not easy to answer the question of whether the Yunnanozoa possess a cranium. The cranium is a rather complex and heterogeneous structure. In all modern vertebrates it consists of two parts: the braincase (neurocranium) and the branchial skeleton. The branchial skeleton in the form of a cartilaginous lattice is certainly present in the yunnanozoans, but the braincase is absent. And if we hold that a cranium must necessarily include a braincase (as is usually assumed), then the yunnanozoans lack a cranium. Comparisons of modern animals had frequently led investigators to conclude that the cranium is evolutionarily older than the vertebral column, but paleontological data now call this into question. And do the Yunnanozoa possess a neural crest? Jun-Yuan Chen is convinced that they do, on the grounds that they possess cartilaginous branchial arches. If so, the yunnanozoans should be assigned to the very group that different authors call Cristozoa or Vertebrata — that is, in plain language, to the vertebrates. Very recently Chen proposed the following system of the chordates (Fig. 13): [IMG_13] Fig. 13. The new evolutionary tree of the chordates (after Chen). Lancelets (Cephalochordata) are more distant from the craniates than are tunicates (Tunicata) — a result consistent with the majority of current molecular-biological data. Further explanations are given in the text. In Chen's system, the Cristozoa (animals with a neural crest) are divided into Procraniata ("pre-craniates") and Craniata (craniates). The only currently known representatives of the Procraniata are the yunnanozoans. Hagfishes, lampreys, fishes, and terrestrial vertebrates belong to the Craniata. The group named Vertebrata is absent altogether! Vertebral elements are regarded as a shared character of the Cristozoa; their absence in hagfishes is considered secondary. Contrary to Janvier's view cited above, Chen is convinced that the vertebral column is an evolutionarily more ancient character than the cranium. Another remarkable chordate described from the Early Cambrian of Hainan is Haikouichthys. It broadly resembles the yunnanozoans but is even more fish-like. It has seven pairs of gill slits. In Haikouichthys, segmental muscular blocks (myomeres) and the connective-tissue septa between them (myosepta) are beautifully expressed. Moreover, the lines of the myosepta are zigzag-shaped, precisely as in vertebrates — for example, in the lamprey (Fig. 14). The paleontologist who discovered Haikouichthys, Degan Shu, accordingly considered it a relative of the lamprey. The presence of a braincase in Haikouichthys remains, however, doubtful. [IMG_14] Fig. 14. Haikouichthys (after Zhang and Hou). Above — tracing of the impression; below — reconstruction of the animal. The boundaries between myomeres are W-shaped, as in the lamprey, rather than V-shaped, as in the lancelet (cf. Fig. 3). Scale division 5 millimetres. At this point we must pause and honestly acknowledge that the situation with respect to Early Cambrian chordates is currently rather confused. There are at least two completely different views on their phylogenetic relationships and systematic position; which of them is correct remains unclear. Professor Jun-Yuan Chen holds that all three animals described above — Yunnanozoon, Haikouella, and Haikouichthys — are closely related to one another and are already vertebrates ("cristozoans"), only slightly more primitive than modern jawless vertebrates. Professor Degan Shu holds that Haikouichthys is indeed a genuine vertebrate, but that Yunnanozoon and Haikouella are not chordates at all. He assigns these two genera to the vetulicolians — a special, completely extinct animal phylum. In Shu's view, the yunnanozoans are close relatives of the chordates but not yet chordates themselves. Shu draws attention to the fact that for the yunnanozoans there is no reliable evidence of the presence of a notochord. According to his assertion, in one of the two described species of Haikouella it is even rather clear that there is no notochord. If this is correct, then the yunnanozoans are not chordates after all, and Chen has mistaken something else for vertebral arches in Haikouella. On the other hand, Chen reasonably proposes that we should not regard it as coincidental that the yunnanozoans closely resemble Haikouichthys — and judging from all photographs and reconstructions, this is indeed the case. Animals as similar to one another as these can scarcely belong to different phyla. If this is correct, then Haikouichthys will in all probability eventually be officially included in the class Yunnanozoa, which will be referred to the vertebrates (or to the Cristozoa, if Chen's terminology prevails). Which of these two eminent Chinese paleontologists is right will be shown by new discoveries. Paleontology is at present one of the most rapidly advancing biological sciences, and it is quite probable that the wait for new findings will not be long. [IMG_15] [IMG_16] Fig. 14. Contemporary Chinese paleontologists specializing in Cambrian fauna: the first photograph shows Professor Jun-Yuan Chen; the second shows Professor Degan Shu (on the right, receiving an academic award). In any case, the Early Cambrian chordates (or their relatives) discovered thus far have already helped us learn several important things that are independent of the resolution of the dispute between Professors Chen and Shu. It appears that there are two such things. First, our probable Early Cambrian ancestors were considerably more similar to the vertebrates than to the lancelet. This remains true even if they were not yet chordates. Second, in the brain of these animals (to the extent that we now know it), much already resembles the brain of vertebrates, yet the division most "important" to us is absent: the telencephalon (forebrain), to which the cerebral hemispheres belong. The emergence of the forebrain was apparently a separate evolutionary step. Perhaps even a decisive one for the vertebrates. And of how it came about, we currently know almost nothing. This is a challenge not only for paleontology but for developmental biology as well. Brief Bibliography Mallatt Jon, Chen Jun-Yuan. Fossil sister group of craniates: Predicted and found. // Journal of Morphology. Volume 258, Issue 1, pages 1–31, October 2003. Chen Jun-Yuan. Early crest animals and the insight they provide into the evolutionary origin of craniates. // Genesis. Volume 46, Issue 11, pages 623–639, November 2008. Chen Jun-Yuan. The origins and key innovations of vertebrates and arthropods. // Palaeoworld. Volume 20, Issue 4, December 2011, Pages 257–278. Shu Degan, Conway Morris Simon, Zhang Z. F., Liu J.N., Han Jian, Chen Ling, Zhang X.L., Yasui K. and Li Yong. A New Species of Yunnanozoan with Implications for Deuterostome Evolution. // Science 28 February 2003: Vol. 299 no. 5611 pp. 1380–1384. Shu Degan. A paleontological perspective of vertebrate origin. // Chinese Science Bulletin 2003, Vol. 48, No. 8, p. 725–735. ¹ This question is discussed in detail in the article "The Origin of Chordates: A Modern View of the Problem," published in the journal Potentsial No. 3 for 2012 (on this site, here). Back to text. ² A brief but authoritative account of how the chronology of Earth history is organized can be found here. Back to text.