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October 10, 2012

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Fossil said to be earliest to show complex brain

Oct. 11, 2012
Courtesy of the University of Arizona
and World Science staff

A re­markably well-pre­served fos­sil of an ex­tinct, worm-like an­i­mal re­veals that an­a­tom­ic­ally com­plex brains evolved ear­li­er than pre­vi­ously thought, re­search­ers say in a new stu­dy.

Found in rock formed from mud de­posits dur­ing the so-called Cam­bri­an pe­ri­od 520 mil­lion years ago, sci­en­tists said, the roughly three-inch-long fos­sil rep­re­sents a prim­i­tive arthro­pod. Arthro­pods are the part of the ev­o­lu­tion­ary family tree that has pro­duced in­sects, spi­ders and crus­taceans, such as crabs and lob­sters.

A newfound fossil of Fuxianhuia protensa. The inset shows the fossilized brain in the head of another specimen. The brain structures are visible as dark outlines. (Credit: Specimen photo: Xiaoya Ma; inset: Nicholas Strausfeld)

The fos­sil be­longs to the spe­cies Fux­i­an­huia pro­tensa, which com­bines an ad­vanced brain anat­o­my with a prim­i­tive body, and can now be seen as a “miss­ing link” that sheds light on the ev­o­lu­tion of arthro­pods, the re­search­ers said.

The find­ing is de­scribed in a pa­per pub­lished in the Oct. 11 is­sue of the re­search jour­nal Na­ture. 

The au­thors, Uni­vers­ity of Ar­i­zo­na neuro­bi­ol­o­gist Nich­o­las Straus­feld and col­leagues, say the find is “trans­for­ma­tive” and could re­solve a long-stand­ing de­bate about how and when com­plex brains evolved.

The fos­sil was un­earthed in Chi­na’s Yun­nan Prov­ince.

“No one ex­pected such an ad­vanced brain would have evolved so early in the his­to­ry of mul­ti­cel­lu­lar an­i­mals,” said Straus­feld. 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, he added.

“There has been a very long de­bate about the or­i­gin of in­sects,” Straus­feld said, adding that two sce­nar­i­os have dom­i­nated that dis­cus­sion. Some say in­sects evolved from an an­ces­tor of the mala­cos­tra­cans, a group of crus­taceans that in­clude crabs and shrimp. Oth­ers point to a line­age of less widely known, and simpler-brained crus­taceans called bran­chiopods, which in­clude, for ex­am­ple, brine shrimp.

A re­con­struc­tion of the brain of the 520 million-year-old fos­sil Fux­i­an­huia pro­tensa (left), com­pared to that of a mod­ern crus­ta­cean, such as the land her­mit crab (Coeno­bita clypea­tus) pic­tured on the right. (Cred­it: Nich­o­las Straus­feld)

The com­plex brain structure in a deeply an­cest­ral crea­ture such as Fux­i­an­huia sup­ports the first idea, mem­bers of Straus­feld’s group said. That’s be­cause it sug­gests such a brain is a nor­mal ar­thro­pod trait. If so, the bran­chiopods pre­sum­ably owe their plainer brains to some lat­er sim­plifica­t­ion, and they are a side branch in the evo­lu­tion­ary tree.

The shape of the fos­silized brain “matches that of a com­pa­ra­ble sized mod­ern mala­cos­tra­can,” the au­thors wrote. “There have been all sorts of im­plica­t­ions why bran­chiopods should­n’t be the an­ces­tors of in­sects,” Straus­feld added. “Many of us thought the proof in the pud­ding would be a fos­sil that would show a ma­la­cos­tracan-like brain in a crea­ture that lived long be­fore the or­i­gin of the bran­chiopods; and bin­go! This is what this is.”

But Gra­ham E. Budd of Upp­sa­la Uni­vers­ity in Swe­den, writ­ing a com­men­tary ac­com­pa­nying the pa­per in Na­ture, ar­gued that the case is not yet closed. Pos­si­ble al­ter­na­tive sce­nar­i­os, he wrote, in­clude that F. pro­tensa has been mis­clas­si­fied, or that the si­m­i­lar­ity of its brain to that of mod­ern arthro­pods is more of a co­in­ci­dence than ev­i­dence of a di­rect ev­o­lu­tion­ary link.

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A remarkably well-preserved fossil of an extinct, worm-like animal shows anatomically complex brains evolved earlier than previously thought, researchers announce in a new study. Embedded in stones formed from mud deposits during the so-called Cambrian period 520 million years ago, scientists said, the roughly three-inch-long fossil represents a primitive lineage of arthropods. Arthropods are the part of the evolutionary family tree that has produced insects, spiders and crustaceans, such as crabs and lobsters. The fossil belongs to the species Fuxianhuia protensa, which combining an advanced brain anatomy with a primitive body plan, and can now be seen as a “missing link” that sheds light on the evolution of arthropods, the researchers said. The finding is described in a paper published in the Oct. 11 issue of the research journal Nature. The authors, University of Arizona neurobiologist Nicholas Strausfeld and colleagues, say the find is a “transformative” discovery that could resolve a long-standing debate about how and when complex brains evolved. The fossil was unearthed in China’s Yunnan Province. “No one expected such an advanced brain would have evolved so early in the history of multicellular animals,” said Strausfeld. 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, he added. “There has been a very long debate about the origin of insects,” Strausfeld said, adding that two main scenarios have been under consideration. Some say insects evolved from an ancestor of the malacostracans, a group of crustaceans that include crabs and shrimp. Others point to a lineage of less widely known, and simpler-brained crustaceans called branchiopods, which include, for example, brine shrimp. But the discovery of a complex brain anatomy in an otherwise primitive organism such as Fuxianhuia makes this scenario unlikely, members of Strausfeld’s group said. That’s because the finding of a complex brain so early in arthropod history suggests that is a normal arthropod trait—so the branchiopods got their plainer brains due to some later simplification that makes them a side branch in the tree. The shape of the fossilized brain “matches that of a comparable sized modern malacostracan,” the authors wrote. “There have been all sorts of implications why branchiopods shouldn’t be the ancestors of insects,” Strausfeld added. “Many of us thought the proof in the pudding would be a fossil that would show a malacostracan-like brain in a creature that lived long before the origin of the branchiopods; and bingo! This is what this is.” But Graham E. Budd of Uppsala University in Sweden, writing a commentary accompanying the paper in Nature, argued that the case is not yet closed. Possible alternative scenarios, he wrote, include that F. protensa has been misclassified, or that the similarity of its complex brain to that of modern arthropods is more of a coincidence than evidence of a direct evolutionary link.