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Caught between Snowball Earths - the first shelly amoebas

By Martin Leggett - 16 Jun 2011 16:52:0 GMT
Caught between Snowball Earths - the first shelly amoebas

The Earth's climate has been through some pretty extreme scenarios, as it has lurched along a zig-zag path from molten orb to the life-bedecked globe we know today. But one of the most brutal of times must have been during one of the twin 'Snowball Earth' eras, which locked most of the planet into a pair of 'ultimate' Ice Ages - first 710 and then 635 million years ago. However, the period that is bookended by these frigid episodes remains something of a mystery, as far as life goes.

Now, new analyses of limy rocks from Namibia and Mongolia, published in the forthcoming issue of Earth and Planetary Science Letters, has opened a crack into that murky era. Careful probing of the sediments, that were lain at the bottom of the ocean at this time, has revealed that life was thriving, in microscopic form, shortly after the first Snowball Earth had become mush.

The rock's in question - cap-carbonates - are thought to be the first sediments lain down after that ice age. Dissolving the rocks in acid unlocked microscopic remnants of life, which were patiently cataloged by lead author Tanja Bosak, from MIT. ''It's a little bit like looking at clouds, trying to pick out shapes and seeing if anything's consistent,'' she said of the search for the tiny fossil shells.

Amazingly, the shells she found were from soft single-cell creatures, amoebas, which had made the shells themselves - by piecing together minute particles of silica, aluminum and potassium. It is thought the particles were put in place by the amoeba's feet, which stick out of a narrow notch in the shell. The details of the tubes and spheres were revealed by scanning-electron microscopes probing the 10 micron-thick shells, so revealing their 3D shape.

Such behavior is know from today, in creatures called called testate amoeba, which are commonly found in forests, lakes and peaty bogs. But this is the earliest example in the fossil record of shell-building, or agglutination, of this type. It is thought the shell offered protection from the extreme environment, shortly after the planet-wide glaciers had retreated - and also from other predatory single-celled creatures.

That adds some much needed detail to the picture of life between the two glaciations. ''We can now say there really were these robust organisms immediately after the first glaciation,'' Bosak said. ''Having opened this kind of window, we're finding all kinds of organisms related to modern organisms..  we're really starting to realize there's a lot of unexpected life here.''

Top Image Credit: Image: Tanja Bosak