Tuesday, March 16, 2010

'Snowball Earth,' Evolution, and Really Old Rocks

"Snowball Earth: New Evidence Hints at Global Glaciation 716.5 Million Years Ago"
Science Daily (March 5, 2010)

"Geologists have found evidence that sea ice extended to the equator 716.5 million years ago, bringing new precision to a 'snowball Earth' event long suspected to have taken place around that time.

"Led by scientists at Harvard University, the team reports on its work in the journal Science. The new findings -- based on an analysis of ancient tropical rocks that are now found in remote northwestern Canada -- bolster the theory that our planet has, at times in the past, been ice-covered at all latitudes.

" 'This is the first time that the Sturtian glaciation has been shown to have occurred at tropical latitudes, providing direct evidence that this particular glaciation was a "snowball Earth" event,' says lead author Francis A. Macdonald, an assistant professor in the Department of Earth and Planetary Sciences at Harvard. 'Our data also suggests that the Sturtian glaciation lasted a minimum of 5 million years.'

"The survival of eukaryotic life throughout this period indicates sunlight and surface water remained available somewhere on the surface of Earth. The earliest animals arose at roughly the same time, following a major proliferation of eukaryotes...."

The article goes into a little more detail: including how the scientists were able to tell that the rocks they were studying had been near the equator and at (or near, I presume) sea level about three quarters of a billion years ago.

Interestingly, all the major eukaryotic (having cells with a membrane around the nucleus) groups we know today were around before the Sturtian glaciation - with the possible exception of animals.

The lead author for the study, Francis A. Macdonald, said: "...'The questions that arise from this are: If a snowball Earth existed, how did these eukaryotes survive? Moreover, did the Sturtian snowball Earth stimulate evolution and the origin of animals?'

" 'From an evolutionary perspective,' he adds, 'it's not always a bad thing for life on Earth to face severe stress.'..."

No 'final answers,' of course: but this is a pretty good look at one more era in Earth's past.

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Brigid said...

Maybe I'm missing something, or is this actually what you meant to say? "with the possible exception of animals."

The Friendly Neighborhood Proofreader

Brian, aka Nanoc, aka Norski said...


Thanks for checking!

That sentence again, with the first word and the parenthetical remark redacted:

"all the major eukaryotic ... groups we know today were around before the Sturtian glaciation - with the possible exception of animals."

Yes: that's what I meant to say. There's fossil evidence for the major groups or living creatures previous to the Sturtian glaciation. There is not evidence for animals existing before that glacial period. Animals might have been there, but maybe not. We just don't know. Hence the phrase "with the possible exception of animals", to indicate that animals may have to be excepted or not included from pre-Sturtian forms of life: or they may not have to be excepted.

There's a pretty good definition of the verb 'to except' on Princeton's WordNet.

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