Some animal neuropeptides have been around longer than nervous systems.
Our human brains can seem like a crowning achievement of evolution, but the roots of that achievement run deep: The modern brain arose from hundreds of millions of years of incremental advances in complexity. Evolutionary biologists have traced that progress back through the branch of the animal family tree that includes all creatures with central nervous systems, the bilaterians, but it is clear that fundamental elements of the nervous system existed much earlier.
How much earlier has now been made dramatically clear by a recent discovery by a team of researchers at the University of Exeter in the United Kingdom. They found that the chemical precursors of two important neurotransmitters, or signaling molecules used in nervous systems, appear in all the major animal groups that preceded creatures with central nervous systems.
Researchers found that, “choanoflagellates made protein precursors of two mature neuropeptides, phoenixin and nesfatin.”
Image: Thibaut Brunet
Their presence in choanoflagellates was a surprise because neuropeptides typically appear in the context of sender and receiver neurons. “In a unicellular organism, it’s more difficult to make sense of,” Yañez-Guerra said. “This shows that these neuronal molecules started evolving even before the need for this extensive communication between cell and cell. That’s why it was kind of shocking.”
The question that naturally arises is: What were those neuropeptide precursors doing in choanoflagellates, since it couldn’t have been neural signaling? There isn’t a definitive answer yet. Choanoflagellates do appear to produce the mature phoenixin neuropeptide, but not the mature nesfatin neuropeptide. It’s possible that choanoflagellates used their phoenixin neuropeptides to communicate with each other, for instance to coordinate the formation of choanoflagellate colonies.
A variety of molecules very similar to neuropeptides are made by nearly all the early animal groups, including the ctenophores (comb jellies) and the cnidaria (jellyfish, corals and sea anemones).
The neuropeptides aren’t the only thing that’s unique about ctenophore nervous systems: The structures of their neural networks are so unusual that researchers suspect they evolved independently of those seen in humans and other animals. Why ctenophores do things differently is a mystery, but it’s clear that nervous systems went through a period of tremendous experimentation and innovation early in their evolution — and that at least some of that experimentation began before animals even existed.
The first couple of sentences in this article make claims that exhibit such a wondrous lack of scientific clarity that they’re more like a political propaganda piece than an excerpt from a science article. “Incremental advances in complexity” that are far less demanding than anything leading to the modern human brain have been shown to lie outside of nature’s ability (see, for example, Douglas Axe’s book, Undeniable).