By Sam Firminger
The well-known chicken or the egg conundrum first posed by classical philosophers such as the likes of Aristotle asks a simple biological question. Reduced down to its very essence, it’s a concept often applied in the field of phylogenetics. Here, a range of tools available to biologists are used to study the evolutionary history and relationships of the entire biological scale, from genes to species and phyla. In the past, these tools utilised mainly morphological data but with recent progressions in gene sequencing and evolutionary modelling, genetic data is swiftly changing the landscape of phylogenetics. With this advent of new technologies, the phylogeny of the early complex multicellular organisms, the basal Metazoans (more widely known as the kingdom of Animalia or Animals) has been turned on its head with contentious results being presented to the scientific community.
The Basal Metazoans
The Cambrian Period (541 to 485.5 Mya) is well known for the ‘Cambrian explosion’, a remarkable event that encompassed the explosive radiation of organisms into most of the animal phyla we know and love today. Immediately predating this period however, is the Ediacaran, also commonly referred to as the Vendian (635-542 Mya). Fossil records have shown that this is actually when soft-bodied organisms first appeared on Earth. It is here that the Porifera (Sponges), Ctenophores (Comb jellies), Cnidarians (Jellyfish and corals) and Placozoans first emerged in the murky depths of the ancient oceans. Ctenophores could easily be mistaken for jellyfish with their layered, jelly-like bodies but instead move by the movement of ‘combs’, or cilia, running along the body. They also lack the stinging cells which Jellyfish are famous for. Species found at depth can often be seen to have wonderfully striking multi-coloured LED-like ripples along their bodies, caused by scattering of light through the moving combs. Similarly, most (but not all) Ctenophora species are capable of using proteins that cause bioluminescence: you may well have seen these curious creatures in a documentary or two.
The traditional phylogeny of these taxa places the Porifera as the most basal group, with the rest splitting off from this lineage and evolving later. This is probably the view a layman would also adopt, simply by looking at the organisms. Sponges are sessile and look relatively simple, even plant-like, due to the lack of any features that you might typically associate associates with animals, such as limbs, eyes or muscles. However, multiple academic groups in recent years have challenged this view using transcriptome data (sequenced data from all types of RNA found in a cell). They suggest that it was actually the Ctenophores which evolved first, placing them as a sister group and the most distant to all known animals. This controversial view of the evolutionary history of these animals immediately sent ripples through the scientific community, as it went against all previous textbooks and published papers, including a 2014 paper published in the highly revered Nature journal.
If one is to run with this hypothesis then there are some things that need explaining. Ctenophores have relatively complex epithelial nerve nets, along with muscles and a gut. These characteristics are absent in Porifera, suggesting that if the Ctenophores did in fact evolve first, there would have had to have been a secondary loss of these features to a simpler body plan found in the sponges, followed by another novel evolution of a nervous system into those found in Cnidarians (Jellyfish). An alternative explanation is that the Ctenophores evolved their nervous system independently of the other phyla. However, this has been proven to be unlikely due to both the Ctenophores and the Cnidarians having specific common features, including neuronal fate patterning genes and the presence of vital components for synaptic function. This provocative claim of the sponges no longer being viewed as the ancestral phyla led to widespread questioning of whether it was time for the history books to be rewritten, completely changing our understanding of evolution as we know it. Are they right?
The Recent Research
A team at the University of Bristol led by Dr Davide Pisani with colleagues from around the world published a paper in December 2015 using genomic data to tackle these controversial claims. The team used data sets from previous experiments suggesting Ctenophore-early hypotheses and showed that the choice of evolutionary models, which are run on data, is crucial for obtaining correct results, and in previous papers these were inappropriately chosen. They discuss how the previously used models had failed to take into account important biological factors which affect the rate of change of genes such as the hydrophobicity of bases. Subsequent analyses by Dr Pisani and his team using more appropriate models along with powerful statistical methods lead to the conclusion that it was in fact the sponges which came first, not the Ctenophores, which supports and reinforces the classically held hypothesis: a sigh of relief for many scientists.
Dr Davide Pisani told the University of Bristol press team: “Knowing whether sponges or comb jellies came first is fundamental to our understanding of evolution. Take the nervous system for example; this is the fundamental organ system that mediates our own perception of self. It is what makes us human, so is pretty important! Depending on whether sponges or comb jellies came first underpins entirely different evolutionary histories for this organ system. If comb jellies came first, then the last common ancestor of all the animals might have had a nervous system, and as all comb jellies are predators this ancestor might have even been a predator.”
These results highlight the issue of revolutionary claims that all too often surface in the scientific community. Upon closer inspection with rigorous testing and analyses, they are not always what they seem. It begs the use of proper methodology with thorough self-analysis before publishing a paper. It seems as if the history books are safe, for now.
Dr Pisani’s paper can be found in full at: http://www.pnas.org/content/early/2015/11/24/1518127112.full.pdf