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Vertebrate characteristics such as hearing, sight, smell, large brain, and internal skeleton were not present in fossils prior to 517 Ma and the first appearance of the vertebrates; however, the fossil record after 517 indicates gradual increases in complexity of these features in the vertebrate clade after the first appearance of vertebrates. The vertebrate Haikouichthys (Figure 9‑7) appeared in the Chengjiang Lagerstatten (517 Ma). Myllokunmungia (Figure 9‑7) also appeared in Chengjiang and was a hagfish. Shu stated that Haikouichthys was a true vertebrate with a cranium and a notochord with vertebral elements. [1] Richard Cowen, author of History of Life, wrote the following: “Haikouichthys is not merely a chordate but a lamprey-like jawless fish, with gill bars supporting its gills. And Myllokunmingia (Figure 9-8) has pouch-like structures associated with its gills, which also makes it a jawless fish rather than a chordate.” [2] Haikouichthys ... had vertebrate characteristics such as eyes, nasal sacs, and cartilaginous vertebrae.[3]
Haikouichthys (517 Ma)
Haikouichthys (517 Ma)
Figure 9‑7. Haikouichthys (517 Ma), the first vertebrate in the fossil record, which was a lamprey. Credit: Nobu Tomura. Used here per CC BY-SA 3.0.
Mylokunmingia (517 Ma)
Mylokunmingia (517 Ma)
Figure 9‑8. Myllokunmingia (517 Ma), a possible vertebrate. Public domain.
Metaspriggina (505 Ma)
Metaspriggina (505 Ma)
Figure 9‑9. Metaspriggina (505 Ma), with a slightly evolved jaw, discovered near the Burgess Shale in Canada. Credit: Nobu Tomura. Used here per CC BY-SA 4.0.
An example of the early evolution of the vertebrates is Metaspriggina (Figure 9‑9) in the Burgess Shale (505 Ma). It had the beginnings of the formation of a jaw, which was not found in Haikouichthys.
In 2003, Shu et al. described the fossil Haikouichthys:
"Haikouichthys shows significant differences from other fossil agnathans: key features include a small lobate extension to the head, with eyes and possible nasal sacs (smell), as well as what may be otic capsules (hearing). A notochord with separate vertebral elements is also identifiable. Phylogenetic analysis indicates that this fish lies within the stem-group craniates." [4]
In 2014, Morris and Carron described Metaspriggina:
"This primitive fish displays unambiguous vertebrate features: a notochord, a pair of prominent camera-type eyes, paired nasal sacs, possible cranium and arcualia, W-shaped myomeres, and a post-anal tail." [5]
These Cambrian vertebrates had a head, internal skeleton, eyes (sight), ears (hearing), and nose (smell). The cartilage and bones in vertebrates are constructed from hydroxyapatite in a collagen matrix, which is strong and tough enough to form bones and teeth in vertebrates. Mollusks and arthropod external shells are chitin and calcium carbonate, which is hard but not strong enough to form vertebrate bones. The vertebrate internal skeleton and hydroxyapatite are vertebrate novelties; however, as with many of the vertebrate novelties and ancient gene or gene family contributes to the process. The Runt or Runx gene family contributes to the process of skeletonogenesis in vertebrates. [6] Although genes were coopted from invertebrates to make the internal skeleton, the vertebrate skeleton and the hydroxyapatite - collagen bone material is not homologous with cephalochordates or any invertebrate.
One of the unique features of vertebrates is the kidney and the heart kidney system. There is no fossil evidence of a kidney in Haikouichthys; however, Ditrich hypothesized that there was a kidney based on the hypothesized environment of the fossils in an estuary and the need to regulate salts in this environment, which appears to be circular reasoning.
"The habitat of the earliest vertebrates (craniates) is still being debated. Marine as well as freshwater habitats and anadromous behaviour have been proposed. In contrast, an estuarine origin of vertebrates is suggested here, based on ontogenetic, comparative anatomical and functional data. This approach should resolve inconsistencies between the probable existence of glomeruli in the vertebrate ancestors and the marine habitat of all related extant groups (e.g. urochordates and cephalochordates). The kidney, as the main osmoregulatory organ, must have been developed according to the environmental prerequisites even in stem vertebrates. In the absence of fossil evidence only deductions from contemporary animals are possible. These data indicate that ancestral stem vertebrates probably had well-developed glomeruli, and were capable of at least some ion-exchange between urine and the body. However, they were probably unable to cope with a strong osmotic gradient with respect to their environment." [7]
The kidney is essential to the regulation of salts. It was one of the key factors that enabled large vertebrates to colonize the land and live out of water [8]
Figure 9‑10. Two-chambered heart in fish . The fish heart is sometimes classified as four-chambered since it has four chambers. Credit: Ahnode. Used here per CC BY 3.0.
Fossils indicate a possible heart in Metaspriggina, but this is not certain. [9] The vertebrate two (Figure 9-10) or four chambered heart is unique and not homologous with invertebrates. Henry Gee described the nonhomologous features of the vertebrate heart. [10] The vertebrate neural crest forms the muscles of the heart.
Haikouichthys had a vertebrate muscular system. [11] In vertebrates, the series of muscles forms in a clock wavefront phenomenon. Waves travel down the body and end at a certain time. This begins the new wave, which forms the next v-shaped muscle structure. Annelid worms are segmented, but in a different way from vertebrates. There are several segmentation systems in vertebrates: pharyngeal arches, somites, and rhombobomeres form a range of segmented features in the endoderm, mesoderm, and ectoderm, respectively. The clock and wavefront system is not found in invertebrates.[12] Other vertebrate features that might have been in Haikouichthys are endothelium in blood vessels and the vertebrate adaptive immune system.
Fossils indicate that Haikouichthys and Metaspriggina had the senses of sight, smell, and hearing. Invertebrates have chemosensitive cells, but the vertebrate sense of smell has an entirely different level of complexity. The olfactory placode forms first, and this forms the olfactory system, which generates the sense of smell. The vertebrate sense of smell is based on olfactory receptor neurons (placodes) which extend tiny hairs into the mucus in the olfactory epithelium, which captures odorants. There are 1,000 different genes that code for different olfactory receptors. The combination of different receptors binding to odorants in different ways leads to the sense of smell. Receptors are joined together in glomerules at the olfactory bulb. The olfactory bulb communicates smell to the brain through the amygdala. Cephalochordates and annelids have no sense of smell although they have chemo sensitive locations on their skin that can detect food. Even though they don’t have the sense of smell, neurons in the vertebrate olfactory system are found in cephalochordates and annelid worms.[13]
Vertebrates have image forming eyes. Cephalochordates just detect the presence of light with four rows of ciliated photoreceptor neurons and a pigment.[14] The rows have different types of photoreceptor cells. The neurons from the photoreceptors extend out through a hole called the neuropore into the brain. The photoreceptor cells are homologous with the vertebrate retina.
There is no observed transition to the vertebrate image-forming eye in the fossil record. This does not mean that there was not a gradual transition or that image forming eyes did not evolve in other species. It just means that there is no observed gradual transition from the cephalochoardate photoreceptor cells to vertebrate eyes. Scientists had thought, based on the simplicity of the hagfish eye, that it was basal to the vertebrates and preceded the lamprey eye; however, recent research has shown that the hagfish eye initially had the complexity of the lamprey and then devolved to a simpler form due to lack of use. It was a case of evolution toward simplicity. The loss of the functionality of the eye in hagfish makes sense since they are bottom feeders, which does not require sight. Although there is some light at deeper depths in the sea, there was probably little advantage in having eyesight for the hagfish.
Figure 9‑11. Formation of the vertebrate eye in placode on ectoderm.[15] LP is Lens Placode. Credit: Cvekl and Paden (CC). Used here per CC BY-SA 3.0.
A natural path to formation of the overall structure of a lensed eye is conceivable and happened in mollusks over tens or hundreds of millions of years. In this hypothetical scenario, the formation of the vertebrate eye begins with the formation of the ocular vesicle (OV) below the ectoderm, and the formation of the lens placode (LP) on the ectoderm (Figure 9‑11). The optical vesical then extends out to the ectoderm and triggers the formation of the lens placode on the ectoderm. The lens placode then folds inward, placode cells form fibers form in the cavity, and eventually forms the lens. The many genes and processes involved in forming a functional lens is described in Cvekl and Paden.[15]
“Over 100 years of lens studies in vertebrates have produced a remarkably complex picture of the inductive process govern the formation and differentiation of lens cells. Findings from the last decade have provided novel insights into the cellular and molecular mechanisms of the entire cascade of lens embryonic induction and the dynamics of lens differentiation and morphogenesis.” [16]
Figure 9‑12. The vertebrate eye. Credit: RH Castilhos and J Marchin. Used here per CC BY-SA 3.0.
The shape shifting vertebrate lens allows it to focus on near and far objects and the adjustable pupil allows the eye to adjust to different levels of light (Figure 9-12). The choroid provides oxygen and nutrients to the retina. The sclera (the white of the eye) protects the eye. The ciliary body at attached muscles control the shape of the lens. The attaching skeletal muscles also change the direction of the eye. Although mollusk eyes resemble vertebrate eyes in outward appearance, they do not have the capability to move and focus.
While the some of the nerves in the retina are in cephalochordates, the vertebrate retina, which lines the back of the eye (shown in yellow in Figure 9‑12) has unique features and is not homologous with the cephalochordate photoreceptor cells. Light passes through transparent nerve layers to the rods and cones, which are at the back of the retina. The signals from the rods and cones then move forward through nerve fibers to the front of the retina, where they eventually reach the ganglion cells, which pass the information to the nerve fibers. The nerves in the retina process information and identify shapes and bright points of light prior to passing the information back to the brain through the optical nerves. Even though the vertebrate eye appears suddenly in the fossil record, the eye has evolved in other species since then.
Light passes through transparent nerve layers to the rods and cones, which are at the back of the retina. The signals from the rods and cones then move forward through nerve fibers to the front of the retina, where they eventually reach the ganglion cells, which pass the information to the nerve fibers. The nerves in the retina process information and identify shapes and bright points of light prior to passing the information back to the brain through the optical nerves. Even though the vertebrate eye appears suddenly in the fossil record, the eye has evolved in other species since then.
Only vertebrates have the sense of hearing although some invertebrates have sound sensitive organs. The vertebrate ear forms in three parts, the inner ear, outer ear, and middle ear. The inner ear and middle ear form in the pharyngeal arches based on directions from the neural crest in vertebrates. It is difficult to find information now that lampreys are ruled out as basal vertebrates, but it appears that the early vertebrates had at least the beginnings of the structure of the inner and middle ear.
A study that seemed to prove that the structure of the vertebrate brain was derived from Cambrian chordates might actually show opposite. The study was based on a comparison of the modern cephalochordate to development phases of the zebra fish.[17]
Figure 9-13. "Early brain development in zebrafish. A schematic side view of zebrafish embryonic head. Anterior (rostral) is to the left and dorsal at the top. Approximate anatomical positions of individual brain regions are reconstructed from photos of in situ hybridizations retrieved from ZFIN database (Bradford et al. 2011). Embryo during (A) segmentation stages (18 h postfertilization) and (B) pharyngula stages (24 h postfertilization). (C) Larva at the start of hatching (48 h postfertilization). Distinct parts of the CNS are marked by different colors: forebrain violet and orange, midbrain green, and hindbrain blue. D, diencephalon; DT, dorsal telencephalon; HB, hindbrain; MB, midbrain; OT, optic tectum; r1–r7, rhombomeres 1–7; SC, spinal cord; T, telencephalon; TG, tegmentum; VT, ventral telencephalon." Credit: Šestak, and Domazet-Lošo (2015) [18]. Used here per CC BY-NC
Sestak followed the progression of brain development patterns by looking at the zebrafish brain 18 hours, 24 hours, and 48 hours after fertilization (Figure 9-13). Sestak thought that the first period (A) corresponded with the period during which metazoans began to evolve, prior to the chordates. He thought that the second period corresponded with the cephalochordates, which he stated had the most influence on the vertebrate brain development pattern (B). Finally, at the last stage, there was just one change, the formation of the dorsal telencephalon, which is only observed in vertebrates. Thus, Sestak constructed Figure 9-14, which he thought indicated that the vertebrate brain structure developed in cephalochordates. However, this study was based on a cephalochordate that probably was a degraded vertebrate. If the vertebrate brain did not develop in a cephalochordate, then the implication of Figure 9-14 is that the vertebrate brain has little homology with brains of organisms that were before vertebrates in the Cambrian.
Figure 9-14. "Summary of the adaptive landscape of the zebrafish CNS. At left are shown the phylogenetic levels (phylostrata) from the ancestor of the cellular organisms (ps1) to the present day zebrafish (ps14). The gray-shaded area at top marks the vertebrate section of the phylogeny. A simplified version of every phylostratigraphic profile from figures 2 to 6 is shown by the vertical lines and the corresponding circles of various sizes. An adaptive signal with the strongest amplitude is represented by the largest circle, the second highest signal is marked by the medium size circle, and all other overrepresentation signals are marked by the circles of the smallest size. Only statistically significant signals are shown. The three phases in the evolution of zebrafish brain are labeled by rectangles of different colors (first phase—blue, second phase—orange, and third phase—green)" Credit: Šestak, and Domazet-Lošo (2015). Used here per CC BY-NC.
References
References
[1] Shu, D.G.; S. Morris, J. Han, Z.F. Zhang, K. Yasui, P. Janvier, L. Chen, X.L. Zhang, J.N. Liu, Y. Li, H.Q. Liu. 2003. Head and backbone of the Early Cambrian vertebrate Haikouichthys. 421(6922): 526-9.
[2] Richard Cowen, History of Life, 4th ed. (Malden, Massachusetts: Blackwell Science, 2005), 324 pp.
[3] Richard Cowen, History of Life (John Wiley and Sons, 2013), p. 86.
[4] Shu, D-G., S. Conway Morris, Jian Han, Z-F. Zhang, K. Yasui, Philippe Janvier, L. I. N. G. Chen et al. "Head and backbone of the Early Cambrian vertebrate Haikouichthys." Nature 421, no. 6922 (2003): 526-529.
[5] Morris, Simon Conway, and Jean-Bernard Caron. "A primitive fish from the Cambrian of North America." Nature 512, no. 7515 (2014): 419-422.
[6] Wagner, Darja Obradovic, and Per Aspenberg. “Where did bone come from? An overview of its evolution.” Acta orthopedic 82, no. 4 (2011): 393-398.
[7] Ditrich, Hans. "The origin of vertebrates: a hypothesis based on kidney development." Zoological Journal of the Linnean Society 150, no. 2 (2007): 435-441.
[8] Gee, Henry. Across the Bridge: Understanding the Origin of the Vertebrates. University of Chicago Press, 2018.
[9] Morris and Caron, Primitive Fish.
[10] Gee, Bridge
[11] Gee, Bridge
[12] Beaster-Jones, Laura, Stacy L. Kaltenbach, Demian Koop, Shaochun Yuan, Roger Chastain, and Linda Z. Holland. "Expression of somite segmentation genes in amphioxus: a clock without a wavefront?." Development genes and evolution 218, no. 11-12 (2008): 599-611.
[13] Holland, L.Z. “Invertebrate origins of vertebrate nervous systems.” In Evolutionary Neuroscience, pp. 51-73. Academic Press, 2020.
[14] Holland, Nervous System.
[15] Cvekl, Ales and Ruth Ashery Paden. The cellular and molecular mechanisms of vertebrate lens development. Development. 141 (2014): 4432-4447.
[16] Cvekl, Mechanisms
[17] Šestak, Martin Sebastijan, and Tomislav Domazet-Lošo. "Phylostratigraphic profiles in zebrafish uncover chordate origins of the vertebrate brain." Molecular biology and evolution 32, no. 2 (2015): 299-312.
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Human eye. Credit: Petr Novák, Wikipedia . Used here per CC BY-SA 2.5
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