What was the ear­li­est an­i­mal?

Ge­netic se­quenc­ing is find­ing new an­swers to an­cient ques­tions, writes AN­DREW MASTER­SON.

Cosmos - - Digest -

Just a cou­ple of genes are re­spon­si­ble for one of the long­est run­ning ar­gu­ments in evo­lu­tion­ary bi­ol­ogy: which came first, the sponge or the jelly?

A new study, led by An­to­nis Rokas of Van­der­bilt Univer­sity in Ten­nessee, US, an­a­lysed thou­sands of genes se­quenced from tar­get or­gan­isms to try to re­solve the is­sue.

Rokas and col­leagues set out to de­ter­mine whether marine sponges or ocean preda­tors known as comb jel­lies rep­re­sented the old­est branch of the an­i­mal fam­ily tree.

For the best part of a cen­tury, sponges were held to be the ear­li­est form of an­i­mal life, based on the sim­plic­ity of their struc­tures. In re­cent decades, how­ever, ge­netic se­quenc­ing has thrown up comb jel­lies – or ctenophores – as a bet­ter can­di­date.

And thus the ar­gu­ment has raged. A 2008 study favoured the jel­lies. Ear­lier this year, an­other opted for the sponges.

For Rokas’s team, the ob­ject of the anal­y­sis was not only to try to re­solve the mat­ter but also to un­der­stand how the con­tro­versy arose in the first place. Af­ter all, in 95% of cases ge­netic se­quenc­ing an­swers ques­tions of evo­lu­tion­ary prece­dence with­out am­bi­gu­ity.

To do so, the re­searchers dug into the fine de­tail of genes shared across the two groups. “The trick is to ex­am­ine the gene se­quences from dif­fer­ent or­gan­isms to fig­ure out who they iden­tify as their clos­est rel­a­tives,” ex­plains Rokas. “When you look at a par­tic­u­lar gene in an or­gan­ism, let’s call it A, we ask if it is most closely re­lated to its coun­ter­part in or­gan­ism B? Or to its coun­ter­part in or­gan­ism C? And by how much?”

By ap­ply­ing this method, the team de­ter­mined that comb jel­lies di­verged from a mu­tual ances­tor first.

Phy­lo­ge­netic data cre­ates these oc­ca­sional con­tro­ver­sies, the re­searchers dis­cov­ered, be­cause a hand­ful of “strongly opin­ion­ated” genes some­times con­found the sta­tis­ti­cal analy­ses em­ployed to pro­duce re­sults. By cor­rect­ing for the in­flu­ence of these genes, Rokas and col­leagues hope that fu­ture con­flicts can be re­solved quickly. The study was pub­lished in Na­ture Ecol­ogy and Evo­lu­tion. (See page 52).

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