Of Embryos and Transmutation III – Darwinian recapitulations

By Robbert Striekwold

The name that is most often associated with recapitulation is that of Ernst Haeckel, who believed that the study of embryos provided the best means of reconstructing evolutionary history. In this he took his cue from Darwin, who wrote in 1860 that embryology was the “strongest single class of facts in favour of change of form”.[1] Darwin’s enthusiasm for the study of embryos was inspired in part by the principle of recapitulation, which flourished during the early 19^th century. Not to everyone’s satisfaction, however.

It was the great German embryologist Karl Ernst von Baer (1792-1876) who set out to become the arch-nemesis of recapitulation. The theories of recapitulation current in the early 19^th century had the form of the so-called Meckel-Serres law, which held that embryos of animals would pass through stages resembling adult animals of lower species. Von Baer replaced this with his own universal law of development, which entailed that embryonic development is essentially a process of differentiation from the general to the special.

When the American naturalist Louis Agassiz (1807-1873) introduced an intriguing example of recapitulation, von Baer’s response was devastating. Agassiz had studied salmon embryos, and found that whereas the adults had symmetrical tails, embryos started out with asymmetrical tails. This was significant because the adults of higher fish (such as salmon) had symmetrical tails, while those of lower fish (such as sharks) were asymmetrical – a clear example of recapitulation (see figure 1) according to Agassiz. Nonsense, according to von Baer. Other organs in the salmon embryo, after all, did not follow this pattern, and other species of higher fish did not show the asymmetrical tail. Therefore, it was hardly plausible to see a general pattern here.

Transforming von Baer

It was the British naturalist Charles Darwin (1809-1882) who had the happy inspiration of transforming von Baer’s law back into a theory of recapitulation. Contrary to von Baer, he was very impressed by Agassiz’s fish tails. Darwin was well aware of the rather sad state of the fossil record, which did not show the gradual evolutionary series that his theory predicted. In the Origin of Species he wrote on the promise of recapitulation to discover a species’ history in its embryonic development, which would make it possible to fill in the pages that were missing from the geological record.

Darwin was most impressed by von Baer, however, and their ideas of embryonic development were very similar in form. However, in the similarity of embryos in their early stages, Darwin saw clear evidence of common ancestry, rather than of a general law of embryology (figure 2). His theory provided a simple explanation of this observation. Embryos would not need to adapt to changing environments nearly as much as adults, so natural selection would add new features mainly to the end stage of development, leaving the earlier stages more or less the same. Recapitulation had thus changed: the embryo would not go through stages of currently existing, lower organisms (as in the Meckel-Serres law), but through stages resembling ancestral organisms.

Darwin used embryology at various points to solve problems of classification and evolutionary history. In the late 1840s he found that the somewhat enigmatic barnacles had larvae that were clearly crustaceans, and thus had to be seen as a specialized type of crustaceans rather than the molluscs they were generally taken to be. More significantly, embryology provided Darwin with a possible transition between invertebrates and vertebrates, a divide that was widely taken to be unbridgeable. The larvae of the supposedly invertebrate ascidians looked very much like adults of the primitive vertebrate species amphioxus. Thus, the ascidian larva provided the ideal model for the common ancestor of both ascidians and vertebrates.

Late 19^th century recapitulation

Darwin was encouraged in these musings by the German morphologist Ernst Haeckel (1834-1919), who pushed him to become more explicit about recapitulation in later editions of the Origin. Indeed, Haeckel took the embryological principles that Darwin had hinted at and used them to found an embryological research programme with the sole purpose of reconstructing evolutionary history. He generalized Darwin’s principle of recapitulation into a “biogenetic law”, which held that the “rapid and brief ontogeny [embryonic development] is a condensed synopsis of the long and slow history of the stem (phylogeny)”.[5] Haeckel famously used this principle to identify the embryonic ‘gastrula’ stage, common to all living animals, as their actual common ancestor (figure 3).

In addition to this Darwinian embryological programme, the late 19^th century also saw the rise and flourishing of neo-Lamarckian theories of development. The Lamarckian explanation of recapitulation was very elegant: heredity was seen as analogous to memory, so embryonic development basically consisted of an embryo reliving its evolutionary past. This immediately solved an important problem of recapitulation theories – if new stages are added to the end of embryonic development all the time, shouldn’t it eventually become impossibly long? The Lamarckian answer was simple; with practice, everything gets easier, including development, so embryos can get through their old stages faster and faster as evolution progresses.

The 20^th century

But then, after a century of highly productive interaction between embryology and evolution, the two seemingly divorced. Seen as suspect by protagonists of both the new Mendelian genetics and the evolutionary synthesis, embryologists found themselves somewhat isolated from the rest of biology. It was not until the second half of the 20^th century that the ties between embryonic and species change were reaffirmed. But that is a story for next time.

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Robbert J Striekwold is a M.Sc. student in the History and Philosophy of Science programme at Utrecht University, specializing in the history and philosophy of evolutionary theory. He is writing his thesis on conceptual issues in modern evolutionary developmental biology.