9-Introduction

The vertebrates include fish (Ch. 9), reptiles and birds (Ch. 10), mammals (Ch. 11-12), and humans (Ch. 13). Unlike invertebrates, vertebrates have eyesight, smell, hearing, internal bone structure, large brain, complex nervous systems, and organs that allow them to grow large and live on land. The first vertebrate is in the Chengjiang Lagerstatten (517 Ma). This chapter describes the origin of the early jawless vertebrates as well as the evolution of jawed and bony fish from the Cambrian to Devonian periods.

The difference between invertebrates and vertebrates has been a long-standing problem in the study of the evolution of animals. One of the primary topics of the famous debates between French naturalists Georges Cuvier and Geoffroy St. Hilaire in the early 19^th century was the relationship between vertebrates and invertebrates. They were still confused about the origin of vertebrates in 1911, as Geoffrey Smith described, “From what existing phylum of Invertebrata, if from any, did the Vertebrate type take origin? That is the question which in the past and present has been debated with vigorous and sometimes ecstatic fervor.”[1]

In 1870, Ernst Haeckel proposed that vertebrates evolved from cephalochordates. Although unknown at the time, cephalochordates have a strong resemblance to the larval phase of the lamprey.

Likewise in 2012, Brian Eddy and Richard Handy stated, “What kind of animals were the ancestors of the chordates and what is their relationship to the vertebrate groups? This subject has long been debated and various theories have been advanced, but understanding has been limited by a scarcity of evidence.”[2] In 2018, Henry Gee, senior editor of Nature Journal and expert in vertebrate origins, made the following statement.

“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.” [3]

Geoffroy St Hilaire provided a basis for determination of common ancestry of two organisms with his concept of homology. Darwin extended this concept by identifying homologous structures as a basis for defining evolutionary relationships. Until March, 2021, scientists thought that the lampreys were basal vertebrates because the larval phase of the lamprey resembles a cephalochordate, which would indicate homology between a vertebrate and an invertebrate; however, the basis of the common evolutionary origin was recently disproven by a study of lampreys in the Devonian, which did not have a larval phase.[4] The larval phase of the lamprey evolved at least two hundred million years after the origin of the vertebrates. Frogs have a similar larval phase (tadpole) so the evolution of a larval phase is not unique to lampreys.

"Remarkably, we've got enough specimens to reconstruct a trajectory from hatchling to adult in several independent lineages of early lampreys," said Michael Coates, Ph.D, a biology professor at UChicago and co-author of the study along with Rob Gess, Ph.D at the Albany Museum and Kristen Tietjin at the University of Kansas. "They each show the same pattern: the larval form was like a miniature adult." [5]

"We've basically removed lampreys from the position of the ancestral condition of vertebrates," explains lead author Tetsuto Miyashita, Ph.D, a palaeontologist with the Canadian Museum of Nature. "So now we need an alternative." [6]

The Miyashita study was published in the journal Nature, and it came from the Coates lab at University of Chicago and the Canadian Museum of Nature, all of which are highly respected. In addition, it included fossils from different parts of the world. Removing the lamprey from the base of the vertebrates solves a few general problems with the base of vertebrate evolution such as differences in the pattern of Hox gene clusters in lampreys and probable stem vertebrates. Possibly the most important consideration is that we didn't start out as lampreys, one of the most disgusting creatures on the planet. Now, the early fish were probably cute. There are still many homologous characteristics of vertebrates and cephalochordates; however, the problem with this is that the modern type of cephalochordates, which has many homologous features with vertebrates, might not have existed at the base of the Cambrian Period. They might be degenerate vertebrates. The ancient type of cephalochordate might have been ancestral to vertebrates, but there are other candidates. For example, there are many homologous characteristics in vertebrates and annelid worms. The problem with this relationship is that there are also many nonhomologous characteristics (Section 9-2). Section 9-3 describes the features in vertebrates that are not homologous with any invertebrates.

The known history of vertebrates begins with 500 fossils of Haikouichthys in the Chengjiang Lagertstatten in 517 Ma. With the removal of lampreys at the base of the vertebrates, the first vertebrates might be classified as ostracoderms, which were early jawless fish.

"After their examination of the fossil record, the researchers now believe that extinct armored fishes known as ostracoderms might instead represent better candidates for the root of the vertebrate family tree, whereas modern lamprey larvae are a more recent evolutionary innovation." [7]

Fish with jaws began to appear almost 100 million years after Chengjiang in the Early Silurian. Fish rapidly diversified during the Devonian into cartilaginous fish (sharks) and bony fish. The bony fish diversified into the ray-finned fish (most of the fish in the sea), and lobe-finned fish (the ancestors of amphibians and land vertebrates). Amphibians moved to land in the Late Devonian and Early Carboniferous (Figure 9‑1).

The vertebrates have a known rate of DNA mutation, and this enables scientists to estimate their time of origin and/or divergence from other animals. This calculation is called the molecular clock technique. Section 9-6 describes the molecular clock estimates for the origin of the vertebrates and compares it to the fossil record. There is a link in this section to a debate between leading paleontologists who think that the origin of animals began with the Avalon Explosion in the Late Ediacaran and leading molecular clock researchers who think that the origin of the animal kingdom must be further in the past to account for the differences in DNA and morphology between vertebrates and invertebrates.

Section 9-7 (optional excursus) compares Moses’ account of the creation of the vertebrates (fish and birds) to paleontological data related to the origin of the vertebrates.

References

[1] Geoffrey Smith, Primitive Animals (Cambridge: Cambridge University Press, 1911), 75.

[2] Brian Eddy and Richard Handy, Ecological and Environmental Physiology of Fishes (Oxford: Oxford University Press, 2012), 18.

[3] Gee, Henry, Across the bridge: understanding the origin of the vertebrates. University of Chicago Press, 2018.

[4] Miyashita, Tetsuto, Robert W. Gess, Kristen Tietjen, and Michael I. Coates. "Non-ammocoete larvae of Palaeozoic stem lampreys." Nature 591, no. 7850 (2021): 408-412.

[5] Canadian Museum of Nature. "Long-accepted theory of vertebrate origin upended by fossilized lamprey larvae." ScienceDaily. www.sciencedaily.com/releases/2021/03/210310122538.htm (accessed October 19, 2021).

[6] Canadian Museum, Long-accepted.

[7] Canadian Museum, Long-accepted.