Life Changer

A 750-million-year-old fossil discovered by geosciences professor Phoebe Cohen may hold clues to how life has changed the earth and how earth has changed life.

Following up on work done by scientists in the 1970s, Cohen and a fellow Harvard University graduate school student traveled to the Yukon in 2007 to study rock formations and found a veritable gold mine of fossil-containing rock. Advancements in scientific tools and instruments in the last 40 years allowed Cohen to examine the fossils in a new way, by dissolving the carbonate rock in acetic acid rather than looking at a sliver or cross-section of the rock, as had been done in years past.

“After the carbonate dissolves away, what’s left is organic matter,” Cohen says. And what she was able to see in the organic matter amazed her: three-dimensional “bones” of what might be one of the earth’s first eukaryotic single-celled organisms to make hard parts.

Eukaryotic cells have a nucleus and organelles, just like animal and plant cells but unlike bacteria cells. The fossil Cohen discovered measures about one-tenth the width of a human hair. She named it Quadrireticulum palmaspinosum, which loosely translates to “four-sided net with palm tree spine.”

Why did a creature so tiny need bones? Cohen’s theory is that at a certain point in history, single-celled organisms needed to “biomineralize”—or create a type of exoskeleton—in order to protect themselves from being eaten by other single-celled organisms. Later, these biomineralized organisms, weighed down by their hard parts, sank to the bottom of the ocean.

As the organisms became buried in sediment, the carbon they contained was locked away with them, reducing the levels of carbon in the ocean and the atmosphere. Over the next 750 million years they developed into the giant rock in the Yukon that Cohen studied. The process may have caused the earth to cool, perhaps eventually contributing to what Cohen and other scientists call “a snowball earth event,” when ice covered the entire planet for millions of years.

With the help of research assistants Nakita VanBiene ’15 and Kim Kiplagat ’16, Cohen is now studying snowball earth events. She’s in the process of writing a grant proposal to research the time periods before, during and just after the two major snowball earth events in our planet’s history, which took place 717 million and 635 million years ago. With her research Cohen hopes to better understand the relationships between evolutionary events, such as biomineralization, and events in the earth’s system, such as climate change.

“This work helps us understand how life has evolved through time in response to both other living things and to the environment,” Cohen says. “We are working toward a better understanding of the dynamics between evolution and changes in the earth’s climate over geologic time scales.”

Quadrireticulum palmaspinosum

Click to enlarge.

A: Netting

Geosciences professor Phoebe Cohen theorizes that the porous surface of Quadrireticulum palmaspinosum might have weighed less, allowing the organism to float to the surface to collect nutrients or access sunlight. The netting also might have allowed nutrients to get in and waste to get out of the cell.

B: Spine

The long spine increased the effective diameter of the cell, making the organism harder for hungry amoeba to ingest. The spine also might have made the organism more buoyant, helping it to float to the surface.

C: Palm Tree

The spikes probably made the organism harder to ingest. They may also have affected its buoyancy by creating an envelope around the cell, “like little umbrellas,” Cohen says.



–Julia Munemo