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Fingers originated in fish ancestors: study

Sept. 21, 2008
Courtesy Nature
and World Science staff

Finger-like di­vi­sions in the fins of an­cient fish are the pre­de­ces­sors of our fin­gers and toes, ac­cord­ing to a study pub­lished on­line in the re­search jour­nal Na­ture this week. 

The find­ing comes from a re-examina­t­ion of a 385-mil­lion-year-old fish fos­sil, Pan­der­ich­thys, be­lieved to be ev­o­lu­tion­arily re­lat­ed to land ver­te­brates.

A computer-aided re­con­struc­tion of the fin bones of the fos­sil fish Pan­de­r­ich­thys show­ing what sci­ent­ists say are fin­ger pre­cur­sors, to­ward the bot­tom in brown. (Credit C. Boisvert and P. Ahlberg)
Bi­ol­o­gists since the 1800s have spec­u­lat­ed that the fin­gers and toes, or dig­its, of land ver­te­brates were de­rived from bony pro­tru­sions in­side fish fins called ra­di­als. 

But this idea fell out of fa­vor in the 1990s, largely based on early stud­ies of Pan­der­ich­thys, which ap­peared to lack di­git-like fin ra­di­als. 

In the Na­ture pa­per, Cath­er­ine Bois­vert of Upp­sa­la Uni­ver­s­ity in Swe­den and col­leagues pre­sented a study of the fos­sil us­ing com­pu­ter­ized to­mog­ra­phy, or CT, scan­ning, a form of X-ray im­ag­ing.

The old in­ter­preta­t­ion was in error—Pan­der­ich­thys did in­deed have fin­ger-like di­vi­sions in its fins, ac­cord­ing to the group. 

Boisvert and col­leagues claim pre­vi­ous re­search went off-track be­cause a thin sed­i­men­ta­ry film cov­er­ing the fins in the rel­e­vant place had con­cealed the un­der­ly­ing skel­e­ton.

The team wrote that the find­ing, to­geth­er with new da­ta from oth­er fish spe­cies, “makes a strong case for fin­gers not be­ing a nov­el­ty of te­tra­pods,” or four-limbed, ver­te­brate an­i­mals.

Rath­er, fin­gers and toes are “de­rived from pre-ex­ist­ing dis­tal ra­di­als,” the fin sup­port struc­tures, in lobe-finned fish, they added. Lobe-finned fish are fish with round­ed, fleshy, limb-like fins. 

One group of lobe-finned fish are thought to be an­ces­tors of am­phib­ians and oth­er land-dwelling ver­te­brates. Lobe-finned fish first ap­peared in the Or­do­vi­ci­an per­i­od, about 500 mil­lion to 425 mil­lion years ago, and are ex­tinct ex­cept for the coe­la­canth and lung­fish.

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Finger-like divisions in the fins of ancient fish are the predecessors of our fingers and toes, according to a study published online in the research journal Nature this week. The finding comes from a re-examination of a 385-million-year-old fish fossil, Panderichthys, believed to be evolutionarily related to land vertebrates. Biologists since the 1800s have speculated that the fingers and toes, or digits, of land vertebrates were derived from bony protrusions in fish fins called radials. But this idea fell out of favor in the 1990s, largely based on early studies of Panderichthys, which appeared to lack digit-like fin radials. In the Nature paper, Catherine Boisvert of Uppsala University in Sweden and colleagues presented a study of the fossil using computerized tomography scans, a form of X-ray imaging. The old interpretation was in error—Panderichthys did indeed have finger-like divisions in its fins, according to the group. Boisvert and colleagues claim previous research went off-track because a thin sedimentary film covering the fins in the relevant place had concealed the underlying skeleton. The team wrote that the finding, together with new data from other fish species, “makes a strong case for fingers not being a novelty of tetrapods,” or four-limbed, vertebrate animals. Rather, fingers and toes are “derived from pre-existing distal radials,” the fin support structures, in lobe-finned fish, they added. Lobe-finned fish are fish with rounded, fleshy, limb-like fins. One group of lobe-finned fish are thought to be ancestors of amphibians and other land-dwelling vertebrates. They first appeared in the Ordovician Period, about 500 million to 425 million years ago, and are extinct except for the coelacanth and lungfish.