On some historical and theoretical foundations of the concept of chordates

Abstract

The concept of chordates arose from the alliance between embryology and evolution in the second half of the nineteenth century, as a result of a theoretical elaboration on Kowalevsky's discoveries about some fundamental similarities between the ontogeny of the lancelet, a putative primitive fish, and that of ascidians, then classified as molluscs. Carrying out his embryological studies in the light of Darwin's theory and von Baer's account of the germ layers, Kowalevsky was influenced by the German tradition of idealistic morphology that was concerned with transformations driven by laws of form, rather than with a gradual evolution occurring by means of variation, selection and adaptation. In agreement with this tradition, Kowalevsky interpreted the vertebrate-like structures of the ascidian larva according to von Kölliker's model of heterogeneous generation. Then, he asserted the homology of the germ layers and their derivatives in different types of animals and suggested a common descent of annelids and vertebrates, in agreement with Saint-Hilaire's hypothesis of the unity of composition of body plans, but in contrast with Haeckel's idea of the Chordonia (chordates). In The Descent of Man Darwin quoted Kowalevsky's discoveries, but accepted Haeckel's interpretation of the ascidian embryology within the frame of a monophyletic tree of life that was produced by the fundamental biogenetic law. Joining embryology to evolution in the light of idealistic morphology, the biogenetic law turned out to be instrumental in bringing forth different evolutionary hypotheses: it was used by Haeckel and Darwin to link vertebrates to invertebrates by means of the concept of chordates, and by Kowalevsky to corroborate the annelid theory of the origin of vertebrates. Yet, there was still another interpretation of Kowalevsky's discoveries. As an adherent to empiricism and to Cuvier's theory of types, von Baer asserted that these discoveries did not prove convincingly a dorsal position of the nervous system in the ascidian tadpole larva; hence, they could not support a homology between different animal types suggesting a kinship between ascidians and vertebrates.

Exploring the past through the present

“…Also, I am far from criticizing Margherita Raineri's treatment of Kowalevsky's studies, but it seems to me that the “von Baer versus Kowalevsky” contraposition (see Raineri and her present remark) needs a commentary. Despite its length, the following quotation from the book (1955) by Leonid Blyakher (a corresponding member of the International Academy of the History of Science) may counterbalance Raineri's outlook on von Baer's 1873 paper:

“The cause of his writing this paper was sensational.

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Russian comparative embryology takes form: a conceptual metamorphosis toward “evo‐devo”

“…It has recently been claimed (Raineri ) that some of Kowalevsky's interpretations of his own experimental results may be incomplete or even incorrect and that von Baer had doubts about the “reliability” of Kowalevsky's conclusions on the phylogenetic relationship of ascidians and vertebrates. However, it appears to be undervalued or under‐recognized that: (1) Kowalevsky was not a pioneer of the annelid theory, as Raineri () states, but he was the first to demonstrate structural and developmental similarities between ascidian and lower‐vertebrate larvae; (2) the von Bear 1873 manuscript on ascidians was critical of dilettantes who believed that ascidians could be ancestors of humans, not against Kowalevsky's experimental data (commented in Lukina ), and (3) Kowalevsky substantially extended von Baer's empirical generalizations by demonstrating the existence of germ layers and gastrulation processes in both invertebrates and vertebrates (see Mikhailov and Gilbert ; Brauckmann and Gilbert ). In its abstract form, the work of Kowalevsky on ascidians was arguably the most influential at that time.…”

Revisiting the past in view of the future

“…To confuse matters even worse, the opinions of Karl Ernst von Baer, the mentor of Kowalevsky who never accepted the homologies between ascidians and vertebrates, are sometimes summarized in such a way that the reader can easily reach the opposite conclusion. For instance, in the January/February issue of Evolution & Development , Mikhailov () wrote:

“It has recently been claimed (Raineri ) that some of Kowalevsky's interpretations of his own experimental results may be incomplete or even incorrect and that von Baer had doubts about the “reliability” of Kowalevsky's conclusions on the phylogenetic relationship of ascidians and vertebrates. However, it appears to be undervalued or underrecognized that: (1) Kowalevsky was not a pioneer of the annelid theory, as Raineri () states, but he was the first to demonstrate structural and developmental similarities between ascidian and lower‐vertebrate larvae, (2) the von Baer () manuscript on ascidians was critical of dilettantes who believed that ascidians could be ancestors of humans, not against Kowalevsky's experimental data (commented in Lukina ), and (3) Kowalevsky substantially extended von Baer's empirical generalizations by demonstrating the existence of germ layers and gastrulation processes in both invertebrates and vertebrates (see Mikhailov and Gilbert ; Brauckmann and Gilbert ).

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“…To reply to the above‐mentioned points: (1) Of course, I remember that Kowalevsky was “the discoverer of the affinities between tunicates and vertebrates” in his “seminal papers on lancelet and ascidian development” (Raineri ). However, he did not elaborate on these homologies to suggest a common descent of ascidians and vertebrates, but he put forth the hypothesis that vertebrates could have taken their origin from the larva of ascidians he still considered to be molluscs (Kowalevsky ).…”

“…In my words, von Baer was so “enthusiastic” about these discoveries, that he gave twice his positive vote to Kowalevsky on occasion of the von Baer‐Prize. In particular, the discovery of a common origin of the nervous systems from the outermost layer “was of such primary importance … that von Baer had forcefully argued that the work of Kowalevsky was worthy of the first prize, although it was still unpublished” (Raineri ).…”