A study published this week in Science is looking to redraw the tree of life and place an unexpected organism at its root: comb jellies. These odd looking sea-creatures, called ctenophores by those in the know (and pronounced ten-uh-for), have been identified in the study as the first animal lineage to branch off from bilaterians, which makes them the first distinct phylogenetic lineage in the history of animal life on Earth.
It was previously thought that sponges, or porifera, were the oldest divergent lineage in the animal world, but this study found something remarkable about the connection between the genomes of comb jellies and sponges. Sponges were always thought to have broken off first because they were the simplest organism among the various candidates (which included other jellyfish and cnidarians along with ctenophores). Sponges lack the nerve cells and muscle cells that ctenophores have, so it would make sense that ctenophores developed them later.
But as it turns out this isn’t the case, as this group found that sponges have the same genetic pathways as ctenophores for developing muscle and nerve cells. As such, it seems that ctenophores most likely developed first with basic nerves and muscles, and then sponges lost those cell types further along the evolutionary timeline. Sponges also have mesodermal cells which bilaterians, the ultimate precursors to animal life, had as well. This further strengthened the sponges’ former position as the root of divergent animal life. However, this study found that the genetic pathways that lead to sponge mesodermal cells lack critical components from those in bilaterians, suggesting that sponges developed theirs independently.
Beyond showing us where life on Earth began, this study also demonstrates an important principle of evolution. Evolution doesn’t necessarily move from the simple to the complex. Here we can see how a simpler group of organisms, sponges, developed from the loss of components in a more complex group, ctenophores. Studies like this highlight how phylogenetic analysis can upset our established understanding of the sprawling progression of life and remind us of how intricate and surprising life’s history is. Also, it gives me a reason to type ctenophore, which I enjoy. Ctenophore.