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The­ory on ori­gin of ani­mals chal­lenged: Ani­mals need only extremely lit­tle oxygen

pub­lished 22 Feb­ru­ary 2014 | mod­i­fied 25 Decem­ber 2014

One of science’s strongest dog­mas is that com­plex life on Earth could only evolve when oxy­gen lev­els in the atmos­phere rose to close to mod­ern lev­els. But now stud­ies of a small sea sponge fished out of a Dan­ish Fjord shows that com­plex life does not need high lev­els of oxy­gen in order to live and grow.

Sponge Halichondria paniceaThe ori­gin of com­plex life is one of science’s great­est mys­ter­ies. How could the first small prim­i­tive cells evolve into the diver­sity of advanced life forms that exists on Earth today? The expla­na­tion in all text­books is: Oxy­gen. Com­plex life evolved because the atmos­pheric lev­els of oxy­gen began to rise approx­i­mately 630635 mil­lion years ago.

How­ever new stud­ies, pub­lished online on 18 Feb­ru­ary in the jour­nal PNAS, of a com­mon sea sponge from Kerte­minde Fjord in Den­mark shows that this expla­na­tion needs to be recon­sid­ered. The sponge stud­ies show that ani­mals can live and grow even with very lim­ited oxy­gen sup­plies. In fact ani­mals can live and grow when the atmos­phere con­tains only 0.5 per cent of the oxy­gen lev­els in today’s atmosphere.

Our stud­ies sug­gest that the ori­gin of ani­mals was not pre­vented by low oxy­gen levels
Daniel Mills, lead author, Nordic Cen­ter for Earth Evo­lu­tion, Uni­ver­sity of South­ern Denmark »

A lit­tle over half a bil­lion years ago, the first forms of com­plex life — ani­mals — evolved on Earth. Bil­lions of years before that life had only con­sisted of sim­ple single-​celled life forms. The emer­gence of ani­mals coin­cided with a sig­nif­i­cant rise in atmos­pheric oxy­gen, and there­fore it seemed obvi­ous to link the two events and con­clude that the increased oxy­gen lev­els had led to the evo­lu­tion of ani­mals. “But nobody has ever tested how much oxy­gen ani­mals need — at least not to my knowl­edge. There­fore we decided to find out”, says Mills.

The liv­ing ani­mals that most closely resem­ble the first ani­mals on Earth are sea sponges. The species Hali­chon­dria pan­icea lives only a few meters from the Uni­ver­sity of South­ern Denmark’s Marine Bio­log­i­cal Research Cen­tre in Kerte­minde, and it was here that Mills fished out indi­vid­u­als for his research.

“When we placed the sponges in our lab, they con­tin­ued to breathe and grow even when the oxy­gen lev­els reached 0.5 per cent of present day atmos­pheric lev­els”, says Mills. This is lower than the oxy­gen lev­els we thought were nec­es­sary for ani­mal life.

Daniel Mills at his lab:

(source: Uni­ver­sity of South­ern Den­mark YouTube channel)

The big ques­tion now is: If low oxy­gen lev­els did not pre­vent ani­mals from evolv­ing — then what did? Why did life con­sist of only prim­i­tive single-​celled bac­te­ria and amoe­bae for bil­lions of years before every­thing sud­denly exploded and com­plex life arose?

“There must have been other eco­log­i­cal and evo­lu­tion­ary mech­a­nisms at play. Maybe life remained micro­bial for so long because it took a while to develop the bio­log­i­cal machin­ery required to con­struct an ani­mal. Per­haps the ancient Earth lacked ani­mals because com­plex, many-​celled bod­ies are sim­ply hard to evolve”, says Mills.

His col­leagues from the Nordic Cen­ter for Earth Evo­lu­tion have pre­vi­ously shown that oxy­gen lev­els have actu­ally risen dra­mat­i­cally at least one time before com­plex life evolved. Although plenty of oxy­gen thus became avail­able it did not lead to the devel­op­ment of com­plex life.

(Source: Uni­ver­sity of South­ern Den­mark Fac­ulty of Sci­ence news, 17.02.2014)

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