A researcher's controversial claim to have found a fossilized brain from the dawn of complex life has company, with several more brains from the same era seemingly transformed into stone. These remnants of the brains of early animals could reolutionize our understanding of the evolution of nervous systems.
The failure of soft organs to fossilize represents a major frustration to our efforts to understand the history of life. Even bones and teeth need appropriate conditions to be preserved, and the discovery of fossilized skin or hearts is exceptionally rare.
So in 2012, when the University of Arizona's Dr Nicholas Strausfeld claimed to have found both the brain and the eyes of a 520-million-year-old Cambrian arthropod, there was plenty of disbelief.
"It was questioned by many paleontologists, who thought – and in fact some claimed in print – that maybe it was just an artifact or a one-off, implausible fossilization event," Strausfeld said in a statement. Similar doubts were expressed about previous examples.
Now, Strausfeld has hit back with two new papers that he hopes will settle the question of whether brains can fossilize, and possibly start a rush for further examples.
In Current Biology, Strausfeld compares two new Fuxianhuia protensa specimens with his original speciman. The extinct arthropod once resembled some modern crustaceans and the fossils reveal it had a similarly structured brain. The existence of multiple examples of the same species confirms we are seeing this arthropod's neural tissue, rather than a conveniently placed geological formation.
The presence of specimens at different stages of preservation allowed the authors to conclude in the paper that neural tissue "was initially preserved as carbonaceous film and subsequently pyritized. This mode of preservation is consistent with the taphonomic pathways of gross anatomy, indicating that no special mode is required for fossilization of labile neural tissue."
A: Under a light microscope, neural tissue appears black. B: Carbon (pink) and iron (green) do not overlap in the preserved neural tissue. Credit: Strausfeld et al./Current Biology
In Philosophical Transactions of the Royal Society B, Strausfeld provides an overview of what we have learned about the central nervous systems of ancient invertebrates.
Surviving arthropods “are hallmarked by stunning morphological and taxonomic diversity, but their central nervous systems (CNS) are relatively conserved,” the paper notes. The discovery of preserved arthropod brains offers us the first opportunity to explore at what point and to what extent the structures of these nervous systems diverged.
Intriguingly, Strausfeld adds, molecular estimations place the point at which the nervous systems of different arthropod groups diverged a little over 500 million years ago, very close to the estimated dates of all the fossilized brains so far discovered.
The rarity of fossilized brains raises questions about what allowed these examples to occur. As with all fossilization, swift burial was essential, lest scavengers get there first. A low-oxygen environment is also required to keep off bacteria.
Strausfeld experimented with burying sandworms and cockroaches in mudslides at 7°C (45°F) and found that this went some way towards creating the conditions where brains could be preserved. A further requirement is for the specimen to have a very dense nervous system. The fact that the modern species he studied have similarly dense brains fits with Strausfeld's hypothesis. "F. protensa's tissue density appears to have made all the difference," he said.
The Royal Society paper was published as part of a discussion on the origin and evolution of the nervous system, which was co-edited by Strausfeld, in recognition of how quickly knowledge in the area is advancing.
