MIT researchers have found hundreds of tiny fossils of the first known ciliates. The ciliates, named tintinnids, resided in hard, flask-shaped shells with bubbled exteriors that likely help them float. Image: Tanja Bosak |
Anyone
who has taken high school biology has likely come into contact with a ciliate.
The much-studied paramecium is one of 7,000 species of ciliates, a vast group
of microorganisms that share a common morphology: single-celled blobs covered
in tiny hairs, or cilia. These cilia—Greek for “eyelash”—are used to propel a
microbe through water and catch prey.
Today
these hairy microbes are ubiquitous in marine environments. However, it’s
unclear how long ciliates have inhabited Earth: After they die, members of most
species simply disintegrate in their watery environs, leaving behind no
fossilized remains.
Now,
geologists at the Massachusetts Institute of Technology (MIT) and Harvard University have unearthed rare,
flask-shaped microfossils dating back 635 to 715 million years, representing
the oldest known ciliates in the fossil record. The remains are more than 100
million years older than any previously identified ciliate fossils, and the
researchers say the discovery suggests early life on Earth may have been more
complex than previously thought. What’s more, they say such prehistoric
microbes may have helped trigger multicellular life, and the evolution of the
first animals.
“These
massive changes in biology and chemistry during this time led to the evolution
of animals,” says Tanja Bosak, the Cecil and Ida Green Career Development
Assistant Professor in MIT’s Department of Earth, Atmospheric and Planetary
Sciences. “We don’t know how fast these changes occurred, and now we are
finding evidence of an increase in complexity.”
Bosak
and her colleagues have reported their findings in a paper posted online in Geology.
Life’s rocky road
The group discovered the fossils in rocks from southwestern Mongolia. In
2008, Francis Macdonald, an assistant professor of geology at Harvard and the
paper’s coauthor, hiked through the Tsagaan Oloom Formation, a rocky terrain
full of glacial deposits. These deposits are remnants from the two most severe
ice ages, or “Snowball Earth” events, during the Cryogenian period, 635 to 715
million years ago. Relatively few fossils have been found from this time,
making it difficult for geologists to pinpoint exactly what lived during this
period.
Macdonald
brought rock samples back to Cambridge,
where Bosak and her colleagues began a meticulous hunt for tiny fossils. The
team dissolved sections of rock in acid, then combed through the residue,
looking for interesting shapes under the microscope.
The
team soon uncovered hundreds of “beautifully preserved” fossils resembling
miniature flasks, Bosak says, with constricted necks and flaring collars. Each
fossil was covered in bubble-like structures. Bosak compared the fossils with
modern organisms, finding a nearly perfect match in a group of ciliates called
tintinnids.
Shell life
Unlike most ciliates, tintinnids have a tough, vase-like shell that’s both
resistant and flexible. A tintinnid lives within this shell, reaching out
through the opening with hair-like appendages to draw food in. The bubbles on
the shell’s surface serve as flotation devices, keeping the microbe afloat as
its cilia propel it through water.
Because
of their thick shells, tintinnids are rare ciliates that fossilize. While most
unprotected ciliates simply dissolve away, the resistant organic shells of
tintinnids can sink to the ocean bottom. Bosak says it’s this deposition of
carbon that may have contributed to the evolution of the first animals.
“You
have this resistant material that sinks to anaerobic oceans, where it takes
longer to degrade,” Bosak says. “As a result, you could sequester more carbon …
that in turn releases more oxygen.”
More
oxygen in the atmosphere would foster complex, oxygen-breathing life. According
to Bosak, the geologic timing is consistent with this theory: The ciliate
fossils date to the period between the two ice ages; soon after the second ice
age, fossils of the first animal embryos were identified.
The
appearance of tintinnids as early as the Cryogenian period suggests other
organisms may have existed as well, possibly setting the stage for animal
evolution.
“Having
found this, we know other things should have been there, possibly not leaving a
fossil record,” Bosak says. “And this really shows there could have been much more
going on than we thought.”
Nicholas
Butterfield, a lecturer in paleobiology at the University
of Cambridge in the U.K., says the
group’s findings provide convincing evidence for ancient organisms that are “significantly similar” to modern ciliates. However, in his view, the fossils
mark a minimum date for the evolutionary appearance of tintinnids—the hairy
organisms could have been floating about hundreds of millions of years earlier.
“It’s
conceivable that they only evolved, or became ecologically important, at this
time,” says Butterfield, who was not involved in the research. “Ciliates
probably do play an important role in how the oceans work, but there’s no
reason to believe that that role wasn’t defined much earlier.”
The
team plans to examine the shells of tintinnids more closely, and will perform
chemical analyses to understand what kinds of conditions might have prompted
such shells to evolve. The researchers will also measure the carbon composition
of individual fossils from different strata to identify exactly how carbon
might have cycled, and what changes might have occurred leading up to the first
animals.
“This
provides some hope that we can actually start looking at biological changes,”
Bosak says. “There is a record of these changes, and that’s what we’re showing
by finding these fossils.”
