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Origin of brain lies in a worm, scientists say

Sur­pris­ing find­ings al­so sug­gest we’ve flipped over during the course of ev­o­lu­tion

April 23, 2007
Courtesy EMBL
and World Science staff

The rise of the cen­tral nerv­ous sys­tem—the brain and spi­nal cord—in an­i­mal ev­o­lu­tion has puz­zled sci­en­tists for cen­turies. 

In­sects, worms and the back­boned an­i­mals, ver­te­brates, are thought to have a shared an­ces­try. But their cen­tral nerv­ous sys­tems dif­fer, and were thought to have evolved on­ly af­ter their lin­eages had split dur­ing ev­o­lu­tion. 

The "liv­ing fos­sil" Pla­t­y­ne­reis du­mer­ilii. (Cour­te­sy Britt Han­sen, Pho­to­lab, EMBL Hei­del­berg)

All these spe­cies de­scend from a com­mon an­ces­tor called Ur­bila­te­ria—the first of the line­age of an­i­mals with sym­me­try be­tween the two sides of their bod­ies. 

In­sects, earth­worms and ver­te­brates are among the twigs of this ev­o­lu­tion­ary tree. Its stem rep­re­sents their last com­mon an­ces­tor, which is al­so the com­mon an­ces­tor of all bi­la­te­ri­ans—an­i­mals with two sym­met­ri­cal sides. Note that although most sci­ent­ists agree on the tree's broad outlines, de­bate con­ti­n­ues on the de­tails; dif­fer­ent trees could be con­struc­ted based on evi­dence avai­lable to­day.

If this slith­ery pa­tri­arch al­ready had a nerv­ous sys­tem, what it might have looked like and how it gave rise to the ar­ray of nerv­ous sys­tems seen to­day is what Det­lev Ar­endt and his group at the lab­o­r­a­to­ry stu­dy. To do so, they in­ves­t­i­gate the nerv­ous sys­tem of a ma­rine an­ne­lid worm, Pla­t­y­ne­reis du­mer­i­lii. 

“Pla­ty­ne­reis can be con­sid­ered a liv­ing fos­sil,” said Ar­endt. “It still lives in the same en­vi­ron­ment as the last com­mon an­ces­tors... and has pre­served many an­ces­tral fea­tures,” in­clud­ing a pro­to­type inver­te­brate cen­tral nerv­ous sys­tem. 

Ar­endt and his group in­ves­t­i­gated how the de­vel­op­ing cen­tral nerv­ous sys­tem in Pla­t­y­ne­reis em­bryos gets sub­di­vid­ed in­to re­gions that lat­er give rise to dis­tinct struc­tures. 

The re­gions are set apart by the the ac­tiv­i­ties of unique com­bi­na­tions of re­g­u­lato­ry or con­trol genes, each of which func­tions main­ly to con­trol sets of oth­er genes. These dis­tinct com­bi­na­tions en­dow each type of nerve cell with a spe­cif­ic mo­lec­u­lar fin­ger­print. 

Com­par­ing the mo­lec­u­lar fin­ger­print of Pla­t­y­ne­reis nerve cells with what’s known about ver­te­brates re­vealed strik­ing sim­i­lar­i­ties, the re­search­ers said. This mo­lec­u­lar anat­o­my “turned out to be vir­tu­al­ly the same in ver­te­brates and Pla­t­y­ne­reis,” said Alexan­dru Denes, who did the re­search in Ar­endt’s lab. Cor­re­spond­ing re­gions give rise to nerve cell or brain cell types with si­m­i­lar mo­lec­u­lar fin­ger­prints, he re­marked. These go on to form the same struc­tures in an­ne­lid worm and ver­te­brates, which in­clude hu­mans.

“Such a com­plex ar­range­ment could not have been in­vented twice through­out ev­o­lu­tion. It must be the same sys­tem,” added Gáspár Jékely, anoth­er mem­ber of the re­search team. “It looks like Pla­t­y­ne­reis and ver­te­brates have in­her­it­ed the or­gan­i­sa­tion of their [cen­tral nerv­ous sys­tem] from their re­mote com­mon an­ces­tors.” 

The find­ings pro­vide strong ev­i­dence for a the­o­ry that was first put for­ward by zo­ol­o­gist An­ton Dohrn in 1875, the sci­en­tists added. The the­o­ry holds that ver­te­brate and an­ne­lid nerv­ous sys­tems are of com­mon de­scent and ver­te­brates have flipped over from front to back dur­ing the course of ev­o­lu­tion. 

“This ex­plains per­fect­ly why we find the same [cen­tral nerv­ous sys­tems] on the back­side of ver­te­brates and the bel­lyside of Pla­t­y­ne­reis,” Ar­endt said. How the in­ver­sion oc­curred and how oth­er inver­te­brates have mod­i­fied the an­ces­tral sys­tem “are the next ex­cit­ing ques­tions,” he added.

This would­n’t be the on­ly time that re­search­ers have pro­posed a dra­mat­ic re­ver­sal of body plans dur­ing the course of ev­o­lu­tion. Hans Mein­hardt the Max Planck In­sti­tute for De­vel­op­men­tal Bi­ol­o­gy in Tübin­gen, Ger­ma­ny, has pro­posed that for­ward or head end of com­plex an­i­mals evolved from the rear end of an­cient an­i­mals re­lat­ed to cnidar­i­ans, such as jel­ly­fish. Their basic body plan pre­dates even that of Urbilat­eria.

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Surprising findings also suggest we’ve flipped upside down in the course of evolution. The rise of the central nervous system—the brain and spinal cord—in animal evolution has puzzled scientists for centuries. Insects, worms and the backboned animals, vertebrates, are thought to have evolved from the same ancestor. But their central nervous systems differ, and were thought to have evolved only after their lineages had split during evolution. Researchers from the European Molecular Biology Labora tory in Heidelberg, Germany now report that the vertebrate nervous system is probably much older than expected. Published in the April 20 issue of the research journal Cell, the study suggests that the last common ancestor of all three groups already had a centralised nervous system resembling that of vertebrates. Many animals have evolved complex nervous systems throughout the course of evolution. But the architecture of these systems differs substantially among species. Vertebrates have central nervous system in the form of a spinal cord down the back. Insects and so-called annelid or segmented worms, such as earthworms, have a rope-ladder-like chain of nerve cell clusters on their belly side. Other have nerve cells distributed diffusely over their body. Yet all these species descend from a common ancestor called Urbilateria—the first of the lineage of animals with symmetry between the two sides of their bodies. If this slithery patriarch already had a nervous system, what it might have looked like and how it gave rise to the array of nervous systems seen in animals today is what Detlev Arendt and his group at the labora tory study. To do so, they invest igate the nervous system of a marine annelid worm called Platynereis dumerilii. “Platynereis can be considered a living fossil,” said Arendt. “It still lives in the same environment as the last common ancestors used to and has preserved many ancestral features,” including a prototype invertebrate central nervous system. Arendt and his group invest igated how the developing central nervous system in Platynereis embryos gets subdivided into distinct regions that later give rise to different structures. The regions are distinguished by the the activities of unique combinations of “master” or regulatory genes that control sets of other genes. These distinct combinations endow each type of neuron with a specific molecular fingerprint. Comparing the molecular fingerpint of Platynereis nerve cells with what’s known about vertebrates revealed surprising similarities, the researchers said. This molecular anatomy “turned out to be virtually the same in vertebrates and Platynereis,” said Alexandru Denes, who did the research in Arendt’s lab. Corresponding regions give rise to nerve cell or brain cell types with similar molecular fingerprints, he remarked. These also go on to form the same structures in annelid worm and vertebrates, which include humans. “Such a complex arrangement could not have been invented twice throughout evolution. It must be the same system,” added Gáspár Jékely, another member of the research team. “It looks like Platynereis and vertebrates have inherited the organisation of their [central nervous system] from their remote common ancestors.” The findings provide strong evidence for a theory that was first put forward by zoologist Anton Dohrn in 1875, the scientists added. The theory states that vertebrate and annelid nervous systems are of common descent and vertebrates have flipped upside down over the course of evolution. “This explains perfectly why we find the same [central nervous systems] on the backside of vertebrates and the bellyside of Platynereis,” Arendt said. “How the inversion occurred and how other invertebrates have modified the ancestral CNS throughout evolution are the next exciting questions for evolution ary biologists.” This wouldn’t be the only time that researchers have proposed a dramatic reversal of body plans during the course of evolution. Hans Meinhardt the Max Planck Institute for Developmental Biology in Tübingen, Germany, has proposed that forward or head end of complex animals evolved from the rear end of ancient animals related to cnidarians, such as jellyfish, whose body plan predates even Urbilateria.