Scanning electron microscopy images reveal an microscopic, oval-shaped shell with tapered ends, from which an organism’s feet may have extended. The surface of the shell are made up of tiny bits of silica, aluminum and potassium, which the organism likely collected from the environment and glued to form armor. Image: Tanja Bosak |
The
first organisms to emerge after an ancient worldwide glaciation likely evolved
hardy survival skills, arming themselves with tough exteriors to weather a
frozen climate.
Researchers
at MIT, Harvard Univ.,
and Smith College have discovered hundreds of
microscopic fossils in rocks dating back nearly 710 million years, around the
time when the planet emerged from a global glaciation, or “Snowball Earth,”
event. The fossils are remnants of tiny, amoeba-like organisms that likely
survived the harsh post-glacial environment by building armor and reaching out
with microscopic “feet” to grab minerals from the environment, cobbling
particles together to make protective shells.
The
discovery is the earliest evidence of shell building, or agglutination, in the
fossil record. The team found a diversity of fossils, suggesting life may have
recovered relatively quickly following the first major Snowball Earth event.
The researchers report their findings in Earth and Planetary Science Letters.
The
widely held Snowball Earth theory maintains that massive ice sheets covered the
planet from pole to pole hundreds of millions of years ago. Geologists have
found evidence of two major snowball periods—at 710 and 635 million years ago—in
glacial deposits that formed close to the modern equator. Fossil records
illustrate an explosion of complex, multicellular life following the more
recent ice age. However, not much is known about life between the two major
glaciations—a period of about 75 million years that, until now, exhibited few
signs of life.
“We
know quite well what happened before the first Snowball, but we have no idea
what happened in between,” says Tanja Bosak, assistant professor of geobiology
at MIT, and the paper’s lead author. “Now we’re really starting to realize
there’s a lot of unexpected life here.”
Ice Age armor
Bosak’s colleagues, Francis Macdonald of Harvard and Sara Pruss of Smith,
trekked to northern Namibia
and Mongolia
to sample cap-carbonate rocks. The team hauled the samples back to Cambridge, where Bosak
dissolved the rocks in acid. She plated the residue on slides and looked for
signs of fossilized life. “It’s a little bit like looking at clouds, trying to
pick out shapes and seeing if anything’s consistent,” Bosak says.
Peering
at the sludge through a microscope, she discovered a sea of tiny dark ovals,
each with a single notch at its edge. To get a closer look, Bosak used scanning
electron microscopy to create high-resolution, three-dimensional images,
revealing hollow, 10-micron-thick shells. Fossils from Namibia were mostly round; those from Mongolia, more
tube-like. Most fossils contained a slit or neck at one end, from which the
organism’s pseudopodia, or feet, may have protruded.
Bosak
analyzed the shells’ composition using x-ray spectroscopy, finding a rough
patchwork of silica, aluminum, and potassium particles that the organism likely
plucked from the environment and glued to its surface.
Bosak
says these single-celled microbes may have evolved the ability to build shells
to protect against an extreme deep-ocean environment, as well as a potentially
growing population of single-celled species, some of which may have preyed on
other organisms.
A Snowball window
“We
can now say there really were these robust organisms immediately after the
first glaciation,” Bosak says. “Having opened this kind of window, we’re
finding all kinds of organisms related to modern organisms.”
The
closest modern relative may be testate amoebae, single-celled microbes found in
forests, lakes, and peat bogs. These tiny organisms have been known to collect
particles of silica, clay minerals, fungi, and pollen, cementing them into a
hard cloak or shell. Bosak says testate amoebae were extremely abundant before
the first Snowball Earth, although there is no evidence that the plentiful
protist evolved its shell-building mechanism until after that ice age.
Bosak’s
guess is that the post-glacial environment was a “brine” teeming with organisms
and newly evolved traits. She says the group plans to return to Mongolia to
sample more rocks from the same time period, and hopes other researchers will
start to investigate rates of evolutionary change in similar rocks.
