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Darwin’s dilemma resolved? Evolution’s “big bang” explained by 5x faster rates

Sept. 11, 2013
Courtesy of Cell Press
and World Science staff

A “Big Bang” of ev­o­lu­tion­ary di­vers­ity early in the his­to­ry of an­i­mal life is ex­plain­a­ble by a rea­son­a­ble up­tick in ev­o­lu­tion­ary rates, sci­en­tists claim.

The find­ings re­fer to an in­cred­i­ble burst of nov­el­ty in an­i­mal body forms and habits dur­ing the Cam­bri­an ex­plo­sion, be­tween 540 and 520 mil­lion years ago.

A liv­ing ar­thro­pod (the cen­ti­pede Cor­mo­cephalus) crawls over its 515-million-year-old rel­a­tive that lived dur­ing the Cam­bri­an ex­plo­sion (the tri­lo­bite Es­tain­gia). A study of ar­thro­pods re­veals that mor­phol­o­gy and genes evolved five times faster dur­ing evo­lu­tion's "big bang" com­pared to all sub­se­quent pe­ri­ods: Fast, but still com­pat­i­ble with Dar­win's the­o­ry. Both the cen­ti­pede and tri­lo­bite are found on what is now Kan­ga­roo Is­land, Aus­tral­ia. (Cred­it: Mi­chael Lee)


Us­ing new es­ti­mates of ev­o­lu­tion­ary rates in the most di­verse an­i­mals at the time, re­search­ers con­clud­ed the event is com­pat­ible with ev­o­lu­tion as Dar­win en­vi­sioned it. The find­ings ap­pear in the jour­nal Cur­rent Bi­ol­o­gy Sept. 12.

“This sim­ul­ta­ne­ous burst of life, with few or no pre­cur­sors, had seemed at odds with Dar­win’s idea of grad­u­al ev­o­lu­tion through nat­u­ral se­lec­tion,” said re­search­er Mike Lee of the Uni­vers­ity of Ad­e­laide in Aus­tral­ia. 

But ev­o­lu­tion of­ten speeds up when or­gan­isms start to “fill up a newly opened range of en­vi­ron­ments and nich­es,” he added. “Our re­sults show that mod­er­ately ac­cel­er­ated ev­o­lu­tion, sus­tained over a few tens of mil­lions of years, could have pro­duced this pat­tern. A five-fold in­crease in rates of ev­o­lu­tion would com­press about 100 mil­lion years’ worth of change in­to about 20 mil­lion years—a rel­a­tively brief pe­ri­od in ge­o­log­i­cal terms.”

The re­search­ers con­sid­ered arthro­pods—the group in­clud­ing in­sects, spi­ders, and crus­tacean­s—be­cause they were the most di­verse and suc­cess­ful an­i­mals dur­ing the Cam­bri­an ex­plo­sion and re­main so to­day. 

Artists' con­cept of marine life dur­ing the Cam­bri­an ex­plo­sion. A gi­ant Ano­ma­lo­ca­ris in­ves­ti­gates a tri­lo­bite, while Opa­binia looks on from the right and the "walk­ing cac­tus" Di­a­nia crawls un­der­neath. All of these crea­tures are re­lat­ed to liv­ing arth­ro­pods. (Cred­it: Ka­tri­na Ken­ny & No­bu­mi­chi Ta­mu­ra)


Arthro­pods still ac­count for more than 80 per­cent of all an­i­mal spe­cies. “To a first ap­proxima­t­ion, eve­ry an­i­mal spe­cies is an arthro­pod!” Lee said. “If we are in­ter­est­ed in ma­jor pat­terns in an­i­mal ev­o­lu­tion, arthro­pods are the key group to study.”

With the help of the fos­sil rec­ord and mo­lec­u­lar dat­ing meth­ods, Lee and col­leagues meas­ured the ana­tom­i­cal and ge­net­ic dif­fer­ences be­tween liv­ing arthro­pods to in­fer ev­o­lu­tion­ary rates in the past. Their an­alysis in­di­cat­ed that ev­o­lu­tion­ary change dur­ing the Cam­bri­an oc­curred about four to five times faster than to­day. 

They were sur­prised, they said, to see that ge­net­ic ev­o­lu­tion and ana­tom­i­cal ev­o­lu­tion sped up by al­most the same amount.

Lee said those faster ev­o­lu­tion­ary rates are likely at­tri­bu­table to game-changing in­nova­t­ions—preda­t­ion, vi­sion, and ac­tive swim­ming, for in­stance—that opened up a range of new pos­si­bil­i­ties. That kind of ev­o­lu­tion­ary fast-forward has al­so oc­curred when an­i­mals col­o­nize new en­vi­ron­ments, he added, such as birds or mam­mals on is­lands, or snakes in the sea.


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A “Big Bang” of evolutionary diversity early in the history of animal life is explainable by a reasonable uptick in evolutionary rates, scientists claim. The findings refer to an incredible burst of novelty in animal body forms and habits during the Cambrian explosion, between 540 and 520 million years ago, Using new estimates of early evolutionary rates in the most diverse animals at the time, researchers concluded the event is compatible with evolution as Darwin envisioned it. The findings appear in the journal Current Biology Sept. 12. “This simultaneous burst of life, with few or no precursors, had seemed at odds with Darwin’s idea of gradual evolution through natural selection,” said researcher Mike Lee of the University of Adelaide in Australia. But evolution often speeds up when organisms start to “fill up a newly opened range of environments and niches,” he added. “Our results show that moderately accelerated evolution, sustained over a few tens of millions of years, could have produced this pattern. A five-fold increase in rates of evolution would compress about 100 million years’ worth of change into about 20 million years—a relatively brief period in geological terms.” The researchers considered arthropods—the group including insects, spiders, and crustaceans—because they were the most diverse and successful animals during the Cambrian explosion and remain so today. Arthropods still account for more than 80 percent of all animal species. “To a first approximation, every animal species is an arthropod!” Lee said. “If we are interested in major patterns in animal evolution, arthropods are the key group to study.” With the help of the fossil record and molecular dating methods, Lee and colleagues measured the anatomical and genetic differences between living arthropods to infer evolutionary rates in the past. Their analysis indicated that evolutionary change during the Cambrian occurred about four to five times faster than today. They were surprised to see that genetic evolution and anatomical evolution were sped up by almost the same amount during the Cambrian. Lee explains that those faster evolutionary rates are likely explained by game-changing innovations—predation, vision, and active swimming, for instance—that opened up a range of new possibilities. That kind of evolutionary fast-forward has also occurred when animals colonize new environments, he said. Think birds or mammals on islands, or snakes in the sea.