Recently, several researchers claimed to have observed the brain, and other parts of the nervous system, in fossils of an ancient arthropod from China called Fuxianhuia. Based on this, the scientists were able to compare these structures to modern arthropods and to make several inferences about how the arthropod brain is likely to have evolved. A Chinese–German collaboration led by Jianni Liu from the Early Life Institute of the Northwest University, Xi’an challenges these interpretations by offering an alternative explanation for the brain-like structures in fossils of Fuxianhuia: namely that they are in fact microbial biofilms which develop from bacteria in the gut of the animal as it decays. In a wider context, Liu et al. note that studies of how modern animals decay suggest that the brain is actually one of the first organs to be lost, and caution that researchers claiming to have found the remains of delicate organ systems need to consider (and exclude) the possibility that the structures they are seeing are merely the products of bacterial degradation. The study of fossil brains (palaeoneurology) is an exciting field, but is only possible if the brain has been correctly identified!
The arthropod Fuxianuia protensa from the ca. 500 million year old Early Cambrian Chengjiang deposit in China is a well-known species which has received much study, and may also be a key fossil for understanding various aspects of arthropod evolution. Well-preserved specimens not only show the external anatomy, but also internal features; some of which were interpreted as parts of the brain and nervous system, or else as parts of the circulatory system. These observations seemed, on the face of it, a little surprising as they relate to delicate internal organs which are intuitively unlikely to have survived the fossilization process. This prompted an international research term from the Early Life Institute in Xi’an, the Free University Berlin and the Museum für Naturkunde Berlin to reinvestigate this phenomenon by studying several hundred specimens of Fuxianuia protensa in different states of preservation. The results were published in the top international journal Proceedings of the Royal Society B.
The researchers found that the structures previously interpreted as brains with a quite specific and distinctive anatomy were actually quite often present, albeit with a range of different shapes and sizes. This led the team to doubt whether the original interpretations and comparisons with modern arthropods were correct. By looking at how modern arthropods decay after death, an alternative explanation presented itself. In modern arthropods, structures like the brain and heart decay very quickly. Instead, the bacteria contained in the gut can form so-called ‘biofilms’ which eventually leak out of the gut and can form radiating patterns which, in the head region, might be mistaken for a brain and radiating series of nerves. Liu et al. propose that these biofilms offer a better explanation for the structures seen in Fuxianuia; which in turn suggests that the brains of these fossils (unfortunately) remain unknown. Liu et al. conclude that because brains are delicate and do not preserve easily, it is important for researchers who claim to have found such structures to exclude the possibility that they are not just looking at artefacts of preservation; namely bacterial biofilms which may be preserved in ways which makes it tempting to interpret them as part of the original animal’s anatomy.
Published in: Liu, J., Steiner, M., Dunlop, J. A. & Shu, D. 2018. Microbial decay analysis challenges interpretation of putative organ systems in Cambrian fuxianhuiids. Proceedings of the Royal Society B.
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