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.
Conflicting Phylogenies
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
