In Chapter 1, when introducing nervous systems, we noted Haeckel's well-known biogenetic law: 'ontogeny recapitulates phylogeny'. Legend has it that the origins of this 'law' lie in an instance of very un-Teutonic serendipity. It is said that the labels fell off von Baer's specimen jars and he suddenly found it impossible to tell which early embryo was which. K.E. von Baer (1792-1876) was one of the founding fathers of modern embryology. Figure 18.1 illustrates his difficulty. Although this story of the mixed-up embryos is nowadays believed to be apocryphal it still serves to make the point: the early embryos of a wide variety of vertebrates look very alike.
Von Baer was, in fact, quite cautious in his interpretation of this early similarity. He merely noted, in his famous 1828 text on animal embryology, that 'the more general characters of the large group of animals to which an embryo belongs appear earlier than the more special characters', 'from the most general forms, the less general are developed until the most special appear' and 'instead of passing through the adult stages of other animals the embryo departs more and more from them'. Few would dissent from these propositions today.
Later in the nineteenth century Fritz Muller (1864) and Ernst Haeckel (1866) popularised von
Baer's ideas in a more extreme form: the so-called 'biogenetic law'. Miiller writes that 'Descendants ... reach a new goal by deviating sooner or later whilst still on the way towards the form of their parents, or by passing along this course without deviation, but then instead of standing still advancing still further.' Haeckel, in his 1866 Generelle Morphologie, says that the organism 'repeats during the rapid and short course of its individual development (ontogeny) the most important of the form changes which its ancestors traversed during the long and slow course of their palaeontological evolution (phylogeny)'.
It is easy to see that all these formulations encapsulate the same perception: progression from general to particular. It is also easy to see that damage or genetic mutation at an early stage in development is likely to be far more serious than damage at a later stage. The MUller-Haeckel 'law' was taken to an extreme in the last decades of the nineteenth century, and the twentieth century saw something of a reaction against it. Much more emphasis has been placed on the adaptation of the embryo to its surrounding conditions. We no longer see ontogeny as a straight-line recapitulation of evolutionary history. Indeed, the expression of ancestral characters is often shifted with respect to each other during development, a process known as heterochrony. We met an instance of this in Section 17.7 when discussing the pineal's role in the delayed onset of reproductive maturity in humans.
It is salutary to find that the developmental genetics of the 1990s are beginning to come full circle back to confirm von Baer's formulations at the beginning of the nineteenth century, though at a far greater depth. Profound similarities in the molecular genetics of early embryos as widely separated as nematodes, flies and mammals are beginning to provide insight into the early evolutionary history of the Metazoa. Indeed, von Baer's interpretation is being generalised. It is not merely that individuals in the same phylum, for instance the Chordata of Figure 18.1, share common beginnings but that at the level of the gene and the protein all multicellular animals retain signs of a common origin. The early onset genes are to biology as the microwave background is to astronomy: whispers from the earliest of times. And, some of the quietest of these whispers tell of striking molecular homologies in the early development of animal brains.
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