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Cambrian Fossil Pushes Back Evolution of Complex Brains

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Geplaatst op 14 oktober 2012 - 09:11

Complex brains evolved much earlier than previously thought, as evidenced by a 520-million-year-old fossilized arthropod with remarkably well-preserved brain structures.

Geplaatste afbeelding
This picture shows a nearly intact fossil of Fuxianhuia protensa. The inset shows the fossilized brain in the head of another specimen. The brain structures are visible as dark outlines.

The remarkably well-preserved fossil of an extinct arthropod shows that anatomically complex brains evolved earlier than previously thought and have changed little over the course of evolution. The fossil is the earliest known 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. 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 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 [of the fossilized brain] matches that of a comparable sized modern malacostracan.”
The researchers argue the fossil supports the hypothesis that branchiopod brains evolved from a previously complex to a more simple architecture instead of the other way around.

Geplaatste afbeelding
A modern brain in an ancient body: A reconstruction of the brain of the 520 million year-old fossil Fuxianhuia protensa (left), which has a very simple body shape, yet shows unexpected similarity to the complex brain of a modern crustacean, such as the land hermit crab (Coenobita clypeatus) pictured on the right.

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University of Arizona

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