Another way of putting this is as follows: if we rewound the tape of life just prior to the origin of metazoa, it would not be surprising to see neurons evolve all over again. The stage was set.
These essays were posted on the original blog sometime back in the early Fall 2008. It is therefore quite encouraging to note that recent research has supplied some significant supporting evidence.
First, consider this summary:
New work suggests that the so-called “lower” metazoans (including Placozoa, corals, and jellyfish) evolved in parallel to “higher” animals (all other metazoans, from flatworms to chordates). It also appears that Placozoans—large amoeba-shaped, multi-cellular animals—have passed over sponges and other organisms as an animal that most closely mirrors the root of this tree of life.
The root is actually that of the tree of multicellular, eukaryotic life. But the new tree has profound implications:
The phylogeny drawn from the new analysis places Placozoans as basal within the Diptoblasta, a group of animals that includes sponges, comb jellies, jellyfish, corals, and anemones. This means that sponges and comb jellies, both previously considered candidates for the most basal animal, fall within the clade as more derived than Placozoans and as sister taxa to each other. Study results also identify a very deep division between the Diptoblasta and the Bilateria/Triploblasta: when looking at all animals, scientists now see that Placozoans and their relatives are in a separate lineage from all other metazoans (starfish, bivalves, anthropoids, crustaceans and chordates). This means that the nervous system, once thought to have arisen once, must have evolved twice from the DNA that coded for these complex systems (keeping in mind that while Placozoans and sponges do not have nervous systems, many of the taxa related to them do.)
“Some people might initially be shocked to see that nerve cells in cnidarians and higher animals (Bilateria), the group of animals that includes humans, evolved independently,” says Schierwater. “But with this new phylogeny, we can take a closer look at the anatomy of these organisms—and we can see that their nervous systems are not all that similar at the morphological level after all.”
There is nothing shocking from the perspective of front-loading, for as I noted months ago, if we rewound the tape of life just prior to the origin of metazoa, it would not be surprising to see neurons evolve all over again.
But why do we see this remarkable convergence?
DeSalle agrees. “It is the underlying genetic tool kit that is similar amongst these basal animals. Placozoa have all of the tools in their genome to make a nervous system, but they just don’t do it.”
Exactly. If the toolkit for the development of a nervous system was present in unicellular eukaryotes without a nervous, we should not be surprised to see a nervous system evolve independently more than once. Yet this all leads to intriguing questions about the triggers behind front-loading. What has prevented Placozoa from evolving a nervous system? And what has enabled Metazoa to more fully extract the functional potential of the nervous system?