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The two most fascinating questions about extraterrestrial life are where it is found and what it is like. In particular, from our Earth-based vantage point, we are keen to know where the closest life to us is, and how similar it might be to life on our home planet. This book deals with both of these key issues. It considers possible homes for life, with a focus on Earth-like exoplanets. And it examines the possibility that life elsewhere might be similar to life here, due to the existence of parallel environments, which may result in Darwinian selection producing parallel trees of life between one planet and another. Understanding Life in the Universe provides an engaging and myth-busting overview for any reader interested in the existence and nature of extraterrestrial life, and the realistic possibility of discovering credible evidence for it in the near future.
Rapid evolution can be observed happening in nature when selection is unusually strong. We are all familiar, these days, with the evolution of antibiotic resistance in bacteria and the evolution of pesticide resistance in insects. Less familiar, but also very rapid, is the evolution of resistance to heavy metals in populations of plants that have adapted to growing on the spoil-heaps surrounding zinc and lead mines. These cases of unusually strong selection and consequently rapid evolution are all associated with human modification of the environment. The classic case study of evolution happening – industrial melanism in moths – also fits into this category.
Evo-devo has come a long way since its origins a mere four decades ago. Many exciting things have been discovered, and there will be many more discoveries to come in the years ahead. I have tried, in this book, to give you a flavour of this new branch of science. Here, I summarize what I think are its most important conclusions so far and the most important challenges that lie ahead.
In the previous chapter we looked at several different kinds of developmental bias. One of our conclusions was that there are both specific biases, such as the numbers of centipede trunk segments and mammalian neck vertebrae, and general biases, such as the tendency for variant developmental trajectories – and in particular viable ones – to be clustered close to the ancestral trajectory. For example, in the case of snails we noted that the forms of developmental repatterning that were generally available for natural selection to act on were slight quantitative modifications of the pattern of development of the snail that was the ancestor of the clade concerned – an example being developmental trajectories leading to differences in adult shell size. Acknowledging this form of bias entails accepting that evolution of body form does not usually take place via radical-effect macromutations. This is interesting because we saw in Chapter 2 that from the late nineteenth century to the mid-twentieth century, prominent biologists who had a specific interest in the evolution of development, such as William Bateson, D’Arcy Thompson, and Richard Goldschmidt, took a macromutational approach.
Although today we call the scientific study of the relationship between evolution and development ‘evo-devo’, neither that term, nor its longer counterpart ‘evolutionary developmental biology’, existed before about 1980. Yet the study of the relationship between the two great creative processes of the living world has a much longer history – effectively starting in the nineteenth century, the first century in which there was a well-articulated theory of evolution (first Lamarck’s, then Darwin’s). We generally refer to evo-devo’s nineteenth-century antecedent as ‘comparative embryology’. Although in the subsequent period from about 1900 to 1980 there were further studies of the relationship between evolution and development, there is no collective term for this endeavour, because mainstream developmental biology and evolutionary biology were largely separate undertakings during that stretch of time. The few biologists who tried to deal with the two together over this 80-year period might be described as mavericks. Each of them produced interesting bodies of work, but these did not really link up to form a scientific discipline.
Body-plan features that have been discussed so far include symmetry, segmentation, skeletons, and limbs. When these are encountered in different phyla, are they homologous or convergent? There are examples of both of these, plus examples where the answer is not yet clear. Bilateral symmetry of the overall body plan seems to have originated just once. So the fact that vertebrates and arthropods are both bilaterally symmetrical is due to their having inherited that body layout from their last common ancestor; in other words, their bilaterality is homologous. However, although vertebrates and arthropods both have skeletons (whereas animals belonging to many other phyla do not) these represent convergent rather than homologous skeletons – this is clear from the fact that one is ‘endo’, the other ‘exo’. Turning to segments and limbs, the fact that both vertebrates and arthropods have these component parts is hard to interpret with certainty one way or the other. The reason for this is our lack of knowledge of that ancient animal that we call the urbilaterian, or ‘Urby’ for short. Direct evidence of this creature will probably never be forthcoming, since it was almost certainly small and soft-bodied, and has left us with no fossils from which to infer its living form. Instead, we can only make rather indirect inferences based on the point in the animal evolutionary tree at which we think bilaterality arose. However, indirect inference is better than nothing, so here goes.
Here, I list ten important issues where I think that there is a significant risk of misunderstandings among those who are new to the field. After each potential misunderstanding, there is a statement of the correct situation, as I perceive it. Many of these issues are related to the rationale underlying the emergence of evo-devo as a (relatively) new discipline.
Our starting point for discussion of evolutionary pattern is the word ‘clade’. This was introduced by the British biologist Julian Huxley (grandson of Darwin’s bulldog T. H. Huxley) in the 1940s. It means a taxonomic group of a particular kind: one that includes all the descendants of a particular ancestral species, and no others. This kind of group can also be called monophyletic. When the German taxonomist Willi Hennig founded the new approach to taxonomy that we now call cladistics, in the 1950s and 1960s, the idea of a clade was central. For those not familiar with cladistics, Hennig’s main concern was that the evolutionary trees that were used through much of the literature of evolutionary biology confounded two things: closeness of ancestry and similarity in body form.
The two great creative processes of biology are evolution and development. You and I, as adult human beings, are products of both. Evolution took about four billion years to make the first human from a unicellular organism that emerged from the primordial soup. Development, in the form of embryogenesis together with its post-embryonic counterpart, takes less than 20 years to produce an adult human from a different unicellular organism – a fertilized egg or zygote. By this measure, development operates more than 200 million times faster than evolution. However, despite their very different timescales, the two great creative processes of biology are intrinsically interwoven. Evo-devo is the scientific study of this interweaving. Its full name is evolutionary developmental biology, but because this is an unwieldy phrase it is almost universally referred to by its nickname.
It is constructive to approach this issue from a historical perspective. Some aspects of animal relatedness have been known for a long time – centuries – while some have only been established in the last few decades. And others remain to be worked out or confirmed. A useful starting point for this historical approach is the 1817 four-volume work Le Règne Animal (The Animal Kingdom) by the French comparative anatomist Georges Cuvier, who divided the kingdom into four embranchements (branches): vertebrates, molluscs, articulates (outwardly segmented animals), and radiates (radially symmetrical animals). We should note here that Cuvier was an anti-evolutionist; he was opposed to the evolutionary theories of his fellow Frenchmen Jean-Baptiste Lamarck and Étienne Geoffroy Saint-Hilaire, and he did not live to see the publication of Darwin’s Origin of Species. However, many non-evolutionists prior to Darwin (from Aristotle onwards) made good attempts at the classification of animals, even though the fruits of their labours would not be given an evolutionary interpretation until later. Here, I will discuss Cuvier’s suggested groups as being evolutionary ones, even though that is not how he saw them.
Although I received my doctoral training within the neo-Darwinian tradition, in a university department (at Nottingham) that was largely devoted to population genetics, there is a view of evolution adopted by some neo-Darwinians that I have always rebelled against. This is the view that evolutionary processes can be understood in terms of only two levels of biological organization – the gene and the population. At its worst, this view is associated with actually defining evolution in those terms alone. For example, in their 1971 book A Primer of Population Biology, the American biologists Edward O. Wilson and William H. Bossert defined evolution as ‘a change in the gene frequency of a population’. Evo-devo can be seen as a rebellion against this overly reductionist approach.