Wednesday, May 24, 2017

Sponges Reclaim Their "Rightful" Place on the Tree of Life

So in 7th grade I remember being taught that sponges were "the first group of animals to evolve" - or more aptly put now - the earliest branching animal lineage. This thus made the sponges the sister group to all other animals, and I went on believing this lie for many years. But then my whole life was uprooted when, in CCS bio, John enlightened us to the current interpretation of the tree of life where ctenophores (comb jellies) kicked sponges out of their original position on the tree of life. I was shook. But as I was looking further into this topic after class, I found a recently published paper which discussed a new phylogenetic analysis that took place and restored sponges to their original position on the tree of life. Yay!

The new analysis used a much larger and more robust data set for their phylogenetic analysis than any other previous study tackling the issue - using a set of 1,719 "high quality" genes and 79 species. The researchers found that the most parsimonious interpretation of their results yields a tree of life where sponges branch off first in the metazoans and are monophyletic. These results also remove the implication that neurons and muscle cells had to evolve twice in the animal lineage - once in the ctenophores, then being lost in sponges, and evolving once again in bilaterals - as would have been the case if ctenophores branched first.

Previous phylogenetic analysis studies likely developed a different view of the tree of life due to an artifact or error known as long branch attraction which occurs when organisms with a high rate of molecular evolution, such as ctenophores, are grouped together simply because of this high rate of molecular evolution. This groups lineages that are not necessarily related together and thus produces errors.

If you want to look into this study in greater depth, here is a link to the article:

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