Evolution
they are central to our understanding of the progression of evolution over long time scales. Gould used the fossil record to suggest almost all body plans originated in the Cambrian, with a few that persist into the modern day, but most being ‘ weird wonders ’ . He suggested these were experimental, evolutionary dead ends each in a phylum of its own and that only by sheer luck had the few successful body plans – e.g. Pikaia, an ancestor of all chordates and therefore ourselves 19 – survived. Could evolution on a grand scale, therefore, rely not just on the ‘ fitness ’ of species for their environments, but on random mass extinctions or chance circumstances? In other words, as Gould puts it, if you ‘ re-run the tape of life ’ , 20 would the result be entirely different, with modern day animals descended from the five-eyed Opabinia, and humans never having evolved? Evidence against Gould’s claim can be found in a reinterpretation of the fossil record: rather than a Cambrian ‘Explosion’, unique sequences of rock confirm what Darwin thought all along: though still rapid, the transition unfolded over millions of years in stages 21 – an Ediacaran-Cambrian transition (ECT). 22 Nor do major animal body plans have to originate primarily in the Cambrian, since a re- examination of Ediacaran trace fossils (such as the Australian Ikaria 23 ), suggests that broadly bilaterally symmetrical organisms, with a mouth, through-gut and anus did indeed exist, albeit rarely, in the pre-Cambrian. Furthermore, the concept of stem- and crown-groups may resolve the dilemma of ‘ weird wonders ’ existing early in evolutionary history before dying out. As new palaeontological evidence emerges, it becomes more apparent that they aren’t completely failed experiments: many Ediacaran animals, for example, are suggested to represent stemgroup Eumetazoans, based on new phylogenetic analyses. 24 What is termed the ‘stem - group’ is made up of small, extinct branches on the evolutionary tree leading up to a major, modern ‘crown - group’ – in this case, the Eumetazoa (true animals excluding sponges). Cambrian animals such as Opabinia and Hallucigenia (when reinterpreted as having legs and rather the aforementioned spines on its back 25 ) might slot into place on the arthropod stem-group. 26 This promises to explain the ‘ weird-wonders ’ not as representatives of a lost age of biological experimentation, but rather the natural consequence of the early stages of animal evolution. 27 However, though it is taken to be current scientific consensus, I am wary that it leaves many of the same unanswered questions as Gould’s ideas do: stem -groups by definition must go extinct, potentially due to competition from better adapted crown-groups, yet this doesn’t preclude the possibility of random chance being involved, and therefore an inherently non-deterministic view of evolution. The widespread presence of convergent evolution – such as ‘sabre - toothed’ mammals evolving separately multiple times 28 – are a line of argument in favour of a biological determinism,
19 Mussini et al. 2024. 20 Gould 2024. 21 Cribb 2019. 22 Seilacher et al. 2005. 23 Evans et al. 2020. 24 Dunn et al. 2021. 25 Ramsköld 1992. 26 Budd 1996. 27 Budd and Jensen 2000. 28 Conway Morris 1998.
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