Icy waters of 'Snowball Earth' may have spurred early organisms to grow
bigger

Date:
July 29, 2021
Source:
University of Colorado at Boulder
Summary:
A new study tackles one of the oldest questions in the history of
the planet: How did living organisms get so big?


FULL STORY ==========================================================================
A new study from CU Boulder finds that hundreds of millions of years ago,
small single-celled organisms may have evolved into larger multicellular
life forms to better propel themselves through icy waters.


==========================================================================
The research was led by paleobiologist Carl Simpson and appears today
in the journal The American Naturalist. It hones in on a question
that's central to the history of the planet: How did life on Earth,
which started off teeny-tiny, get so big? "Once organisms get big,
they have a clear ecological advantage because the physics around how
they capture food become totally different," said Simpson, assistant
professor in the Department of Geological Sciences at CU Boulder and the
CU Museum of Natural History. "But the hard part for researchers has
been explaining how they got big in the first place." In his latest
study, Simpson draws on a series of mathematical equations to argue
that this all-important shift may have come down to hydrodynamics --
or the pursuit of a more efficient backstroke.

Roughly 750 million years ago, and for reasons that scientists are still debating, the planet became suddenly and dramatically colder -- a period
of time called "Snowball Earth." To adapt to these frigid conditions,
which can make swimming more difficult, small organisms like bacteria
may have begun to glom together to form larger and more complex life.

Simpson still has a lot of work to do before he can prove his theory. But,
the geologist said, the results could help to reveal how the ancestors of
all modern multicellular life, from flowers to elephants and even people,
first arose on Earth.



==========================================================================
"By swimming together, these cells could remain small on an individual
level but still produce more power," Simpson said. "They become both
bigger and faster as a group." Snowball Earth Those successes took
place at a seemingly inhospitable time in the planet's past.

During "Snowball Earth," the globe may have been all but recognizable. Ice sheets a half-mile thick or more may have blanketed the planet for as
much as 70 million years, while temperatures in the oceans plummeted to
less than 32 degrees Fahrenheit.

But even amid those frigid conditions, something spectacular happened:
The first organisms made up of many different cells, not just one,
began to emerge around the planet. Scientists still aren't sure what
those ancient multicellular organisms might have looked like. One theory suggests that they resembled Volvox, a type of algae that are common in
oceans today and are shaped like a hollow sphere or snow globe.



========================================================================== "That's something that has lodged in my mind for years," Simpson
said. "How do Snowball Earth and the rise of multicellular organisms
go together?" The answer to that counterintuitive problem may hinge on
a little-known property of water.

Simpson explained that when saltwater gets colder, it also becomes
several times thicker, or more viscous. Humans are too big to notice the change. But for organisms the size of modern-day bacteria, the difference
can be huge.

"When you're small, you're stuck," he said. "The water moves you."
Taking a swim The geologist ran a series of calculations to gauge how
organisms of various shapes and sizes might fare in the oceans of Snowball Earth. And, in this case, bigger might be better.

Simpson said that modern-day bacteria and other single-celled organisms
move around in aquatic environments using two different sets of tools:
There are cilia -- which are wavy, hair-like projections -- and flagella
-- think the "tails" on sperm cells. Both of these tools would have been painfully slow in frigid ocean conditions, his results show.

If individual cells joined forces to make a bigger organism, in contrast,
they could produce a lot more swimming power while keeping the energy
needs of each cell low.

"The advantage of the multicellular strategy is that each cell stays small
and has low metabolic requirements, but these cells can swim together,"
Simpson said.

He's currently testing the theory using experiments with modern algae
in a lab and by digging deeper into Earth's fossil record. One thing
is clear, Simpson said: Once life forms got big, a whole new world of possibilities became available to them. Primitive animals like sponges,
for example, survive not by floating in the ocean but by actively pumping
water through their bodies.

"When you're big, you now can move the water rather than the other way
around," Simpson said.

========================================================================== Story Source: Materials provided by
University_of_Colorado_at_Boulder. Original written by Daniel
Strain. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Carl Simpson. Adaptation to a viscous Snowball Earth Ocean as a
path to
complex multicellularity. The American Naturalist, 2021; DOI:
10.1086/ 716634 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210729122058.htm

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