Rare fossilized algae, discovered unexpectedly, fill in evolutionary
Boulder, Colo., USA: When geobiology graduate student Katie Maloney trekked
into the mountains of Canada’s remote Yukon territory, she was hoping to
find microscopic fossils of early life. Even with detailed field plans, the
odds of finding just the right rocks were low. Far from leaving
empty-handed, though, she hiked back out with some of the most significant
fossils for the time period.
Eukaryotic life (cells with a DNA-containing nucleus) evolved over two
billion years ago, with photosynthetic algae dominating the playing field
for hundreds of millions of years as oxygen accumulated in the Earth’s
atmosphere. Geobiologists think that algae evolved first in freshwater
environments on land, then moved to the oceans. But the timing of that
evolutionary transition remains a mystery, in part because the fossil
record from early Earth is sparse.
Maloney’s findings were published yesterday in Geology. She and
her collaborators found macroscopic fossils of multiple species of algae
that thrived together on the seafloor about 950 million years ago, nestled
between bacterial mounds in a shallow ocean. The discovery partly fills in
the evolutionary gap between algae and more complex life, providing
critical time constraints for eukaryotic evolution.
Although the field site was carefully chosen by Maloney’s field team
leader, sedimentologist Galen Halverson, who has worked in the region for
years, the discovery was an unexpected stroke of luck.
“I was thinking, ‘maybe we’ll find some microfossils,’” Maloney said. The
possibility of finding larger fossils didn’t cross her mind. “So as we
started to find well-preserved specimens, we stopped everything and the
whole team gathered to collect more fossils. Then we started to find these
big, complex slabs with hundreds of specimens. That was really exciting!”
Determining if traces like the ones Maloney found are biogenic (formed by
living organisms) is a necessary step in paleobiology. While that
determination is ultimately made in the lab, a few things tipped her off in
the field. The traces were very curvy, which can be a good indicator of
life, and there were visible structures within them. The fact that there
were hundreds of them twisted together sealed the deal for her.
Few people would likely have noticed the fossils that day.
“We were really lucky that Katie was there to find them because at first
glance, they don't really look like anything,” Maloney’s advisor, Marc
Laflamme, said. “Katie is used to looking at very weird looking fossils, so
she has a bit of an eye for saying, ‘This is something worth checking
Maloney and her colleagues in the field wrestled the heavy slabs into their
helicopter for safe transport back to the lab at the University of
Toronto–Mississauga. She, Laflamme, and their collaborators used microscopy
and geochemical techniques to confirm that the fossils were indeed early
eukaryotes. They then mapped out the specimens’ cellular features in
detail, allowing them to identify multiple species in the community.
While Maloney and her coauthors were writing up their results, they were
confident they had found the first macroscopic specimens from this critical
time period. During the peer review process, though, they received word
from a collaborator that another group in China had made a similar
discovery at about the same time—macrofossils from a similar period. That
did not dissuade them.
“What’s a few hundred million years between friends?” Laflamme laughed. “I
think our fossils have more detail, which makes them easier to interpret…
They're beautiful. They're huge, they're well detailed, there's anatomy.
Your eyes are just drawn to them.”
Ultimately, having two sets of macrofossils from approximately the same
time can only improve the timeline of eukaryotic evolution, serving as
critical calibration points for DNA-based biologic dating techniques. The
new fossils also push back the time when algae were living in marine
environments, indicating that evolution had already occurred in lakes on
land. But for Maloney, an expert in sedimentology, they also raise
questions about what gets preserved in the rock record and why.
“Algae became really important early on because of their role in
oxygenation and biogeochemical cycles,” Maloney said. “So why does it take
them so long to show up reliably in the fossil record? It’s definitely
making us think more about animal ecosystems and whether or not we’re
seeing the whole picture, or if we’re missing quite a bit from a lack of
The whole project has been engaging for Maloney, who pivoted to algae from
more recent biota. “I never expected to be fascinated by algae,” she said.
“But I was pleasantly surprised as I started investigating modern algae,
finding what an important role they play in sustainability and climate
change—all these big issues that we're dealing with today. So it’s been
amazing contributing to algae’s origin story.”
This fieldwork was carried out with permits on traditional lands of the
First Nation of Na-Cho Nyak Dun with their consent.
New multicellular marine macroalgae from the early Tonian of
Katie M. Maloney; Galen P. Halverson; James D. Schiffbauer; Shuhai Xiao;
Timothy M. Gibson; Maxwell A. Lechte; Vivien M. Cumming; Alexie E.G.
Millikin; Jack G. Murphy; Malcolm W. Wallace; David Selby; Marc Laflamme
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