Well, it’s been almost four months since I last updated this blog. The nice thing about the hypothesis of front-loading is that with the passing of time, and the acquisition of new data, the hypothesis becomes increasingly plausible. Consider this:
A previous sequencing of the genome of the Amphimedon queenslandica — a sponge that lives in Australia’s Great Barrier Reef — showed that it contained the same genes that lead to the formation of synapses, the highly specialized characteristic component of the nervous system that sends chemical and electrical signals between cells. Synapses are like microprocessors, said Kosik explaining that they carry out many sophisticated functions: They send and receive signals, and they also change behaviors with interaction — a property called “plasticity.”
“Specifically, we were hoping to understand why the marine sponge, despite having almost all the genes necessary to build a neuronal synapse, does not have any neurons at all,” said the paper’s first author, UCSB postdoctoral researcher Cecilia Conaco, from the UCSB Department of Molecular, Cellular, and Developmental Biology (MCDB) and Neuroscience Research Institute (NRI).
This time the scientists, including Danielle Bassett, from the Department of Physics and the Sage Center for the Study of the Mind, and Hongjun Zhou and Mary Luz Arcila, from NRI and MCDB, examined the sponge’s RNA (ribonucleic acid), a macromolecule that controls gene expression. They followed the activity of the genes that encode for the proteins in a synapse throughout the different stages of the sponge’s development.
“We found a lot of them turning on and off, as if they were doing something,” said Kosik. However, compared to the same genes in other animals, which are expressed in unison, suggesting a coordinated effort to make a synapse, the ones in sponges were not coordinated.
“It was as if the synapse gene network was not wired together yet,” said Kosik. The critical step in the evolution of the nervous system as we know it, he said, was not the invention of a gene that created the synapse, but the regulation of preexisting genes that were somehow coordinated to express simultaneously, a mechanism that took hold in the rest of the animal kingdom.
See? Evolution didn’t need to invent a new gene. Organisms without nervous systems had enough of the parts to make the synapse (the key component of the nervous system) and it was just a matter of time before something would trigger them to be expressed at the same time.
If nervous tissue was front-loaded to exist, it would make sense to also front-load the appearance of muscle:
While the structure and function of muscles, especially of vertebrates, have been intensively studied, the evolutionary origin of smooth and striated muscles has so far been enigmatic……Phylogenetic comparisons showed that one of the crucial structural proteins of striated muscles of vertebrates, a “myosin” motor protein, originated by gene duplication. “As this specific myosin has so far only been found in muscle cells, we expected that its origin coincided with the evolution of muscle cells. We were very surprised to see that the ‘muscle myosin’ evolved probably in unicellular organisms, long before the first animals lived,” explains Ulrich Technau who led the study…… Due to the striking similarities between striated muscles of vertebrates and jellyfish, it was so far assumed both striated muscle types share a common origin. In fact, jellyfish striated muscles also express the ancient “muscle myosin,” but they lack several essential components that are characteristic for the structure and function of striated muscles of “higher animals.”This indicates that despite their striking similarities, striated muscles of jellyfish and “higher animals” have evolved independently.
Let’s see. The evolutionary potential of some ancestral, unicellular core is not only expanded by gene duplication (as we would expect from front-loading), but it is strong enough to trigger the independent appearance of striated muscle.