14 DAY TRIAL // Sponges are the oldest multicellular animals with living representatives.
They are made just of two cell layers that form a tube, living fixed on the substrate and the only movement they make is closing or opening their pores through which food, waste and gases enter or go. Being so simple, they have no neurons or synapses.
But a team at the University of California, Santa Barbara has found in the genes of the sponges crucial clues about how the nervous system emerged.
"It turns out that sponges, which lack nervous systems, have most of the genetic components of synapses," said Todd Oakley, co-author and assistant professor in the Department of Ecology, Evolution and Marine Biology at UC Santa Barbara.
"Even more surprising is that the sponge proteins have 'signatures' indicating they probably interact with each other in a similar way to the proteins in synapses of humans and mice. This pushes back the origins of these genetic components of the nervous system to at or before the first animals -- much earlier than scientists had previously suspected." said Oakley.
"When analyzing something as complex as the nervous system, it is difficult to know where to begin," explained Ken Kosik, senior author and co-director of UCSB's Neuroscience Research Institute, who holds the Harriman Chair in Neuroscience Research.
The first neurons and synapses are found in "cnidarians", creatures like hydra, sea anemones, jellyfish and corals emerged more than 600 million years ago.
"We look at the evolutionary period between sponges and cnidarians as the period when the nervous system came into existence, about 600 million years ago," said Kosik.
The team listed all the genes involved in a human synapse, as synapses are the key of the nervous system functioning, involved in cell communication, learning and memory. After that, the scientists looked for the synapse genes in the sponge.
"That was when the surprise hit. We found a lot of genes to make a nervous system present in the sponge." said Kosik.
Those genes appeared to work together in the sponge. They were very similar to the human ones in how they interacted and in their chemical structure.
"We found this mysterious unknown structure in the sponge, and it is clear that evolution was able to take this entire structure, and, with small modifications, direct its use toward a new function. Evolution can take these 'off the shelf' components and put them together in new and interesting ways." said Kosik.