According to an international team of paleontologists, a Cambrian fossil arthropod found in China is the earliest known fossil to show a brain.
Embedded in mudstones deposited during the Cambrian period 520 million years ago in what today is the Yunnan Province in China, the approximately 3-inch-long fossil, which belongs to the species Fuxianhuia protensa, represents an extinct lineage of arthropods combining an advanced brain anatomy with a primitive body plan. The fossil provides a missing link that sheds light on the evolutionary history of arthropods, the taxonomic group that comprises crustaceans, arachnids and insects.
The researchers call their find a transformative discovery that could resolve a long-standing debate about how and when complex brains evolved.
?No one expected such an advanced brain would have evolved so early in the history of multicellular animals,? said Prof Nicholas Strausfeld, a neurobiologist with the University of Arizona and lead author of a paper describing the discovery in the journal Nature.
Paleontologists and evolutionary biologists have yet to agree on exactly how arthropods evolved, especially on what the common ancestor looked like that gave rise to insects. ?There has been a very long debate about the origin of insects. Until now, scientists have favored one of two scenarios.?
Some believe that insects evolved from the an ancestor that gave rise to the malacostracans, a group of crustaceans that include crabs and shrimp, while others point to a lineage of less commonly known crustaceans called branchiopods, which include, for example, brine shrimp. Because the brain anatomy of branchiopods is much simpler than that of malacostracans, they have been regarded as the more likely ancestors of the arthropod lineage that would give rise to insects. However, the discovery of a complex brain anatomy in an otherwise primitive organism such as Fuxianhuia makes this scenario unlikely.
?The shape matches that of a comparable sized modern malacostracan,? the authors write in the paper. ?The fossil supports the hypothesis that branchiopod brains evolved from a previously complex to a more simple architecture instead of the other way around. ?
This hypothesis arose from neurocladistics, a field pioneered by Prof Strausfeld that attempts to reconstruct the evolutionary relationships among organisms based on the anatomy of their nervous system. Conventional cladistics, on the other hand, usually look to an organism?s overall morphology or molecular data such as DNA sequences.
?I spent a frenetic five hours at the dissecting microscope, the last hours of my visit there, photographing, photographing, photographing,? he said. ?And I realized that this brain actually comprises three successive neuropils in the optic regions, which is a trait of malacostracans, not branchiopods.?
Neuropils are portions of the arthropod brain that serve particular functions, such as collecting and processing input from sensory organs. For example, scent receptors in the antennae are wired to the olfactory neuropils, while the eyes connect to neuropils in the optic lobes. When Prof Strausfeld traced the fossilized outlines of Fuxianhuia?s brain, he realized it had three optic neuropils on each side that once were probably connected by nerve fibers in crosswise pattern as occurs in insects and malacostracans. The brain was also composed of three fused segments, whereas in branchiopods only two segments are fused.
?In branchiopods, there are always only two visual neuropils and they are not linked by crossing fibers,? Prof Strausfeld said. ?In principle, Fuxianhuia?s is a very modern brain in an ancient animal. The fossil supports the idea that once a basic brain design had evolved, it changed little over time. Instead, peripheral components such as the eyes, the antennae and other appendages, sensory organs, etc., underwent great diversification and specialized in different tasks but all plugged into the same basic circuitry.?
?It is remarkable how constant the ground pattern of the nervous system has remained for probably more than 550 million years,? Prof Strausfeld said. ?The basic organization of the computational circuitry that deals, say, with smelling, appears to be the same as the one that deals with vision, or mechanical sensation.?
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Bibliographic information: Xiaoya Ma et al. 2012. Complex brain and optic lobes in an early Cambrian arthropod. Nature 490, 258?261; doi: 10.1038/nature11495
Source: http://www.sci-news.com/paleontology/article00654.html
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