QnAs with Mary L. Droser

The Ediacaran Period, 635 million to 538 million years ago, saw the first emergence of complex, multicellular life on Earth. This signal moment in Earth’s evolutionary history followed the worldwide glaciation of the Cryogenian Period and immediately preceded an explosion of animal diversity during the Cambrian Period. The soft-bodied animals that lived during the Ediacaran Period, called the Ediacara biota, had tubular, frond-shaped, and other unusual body plans and their relationship to later animals has been debated. Mary L. Droser studies the Ediacara biota and their interactions with the environment. Droser is a geologist at the University of California, Riverside. In recognition of her achievements in advancing knowledge of Precambrian life and its history, Droser was awarded the 2022 Charles Doolittle Walcott Medal by the National Academy of Sciences (1). Droser spoke with PNAS about her research on the Ediacara biota.

Mary L. Droser. Image credit: Emmy Hughes (photographer).

PNAS: How did you become interested in studying paleontology and the Ediacaran Period, in particular?

Droser: When I was 5, I wanted to be a marine biologist. When I was 9, I liked geology, and paleontology is the combination of [the two]. So it wasn’t via dinosaurs, and it wasn’t even living near fossils. I grew up in the New York City area, where there is only igneous rock. I did my PhD looking at Precambrian–Cambrian bioturbation: How did animals first start burrowing? Ever since I was a little kid—I would float around on an inner tube and look with a snorkeling mask at the sea floor—I was always interested in benthic communities and what lives under the sediment, because it’s this mysterious world. For my dissertation, I studied the development of that habitat from the Precambrian to the Cambrian, and the Ediacaran was a natural outcome of that. I was working in South Australia and I just got into the weird world of soft body preservation and, ultimately, the Ediacara biota.

PNAS: Where does the Ediacaran Period fit into the history of life on Earth?

Droser: When people think about the early evolution of animals, most people think in terms of the Cambrian explosion, right? Stephen Jay Gould made that very famous (2) and everybody who thinks about natural history understands the Cambrian explosion as the explosion of animals. The Ediacaran Period, and the Ediacara biota, occurred just before the Cambrian explosion, and it’s not a small amount of time. The Ediacara biota are around for tens of millions of years, and they have an abundant fossil record. Because they’re weird and they don’t easily fit into modern clades, they have long been considered this sort of evolutionary experiment. But, in fact, in working on these fossils—and we have tens of thousands of them, of all different kinds—we are starting to get insight into how at least some of them might fit into early animal evolution. It’s not as big a disconnect between the Ediacaran and the Cambrian as has been previously thought. There’s a whole lot of animal evolution going on in those tens of millions of years leading up to the Cambrian explosion.

PNAS: How do you study soft-bodied organisms that did not leave behind fossilized bones or shells?

Droser: There are a handful of places in the world where you get exquisite, soft-bodied preservation of fossils that are Ediacaran in age. I work in South Australia, where they were first discovered, but there are some in the White Sea, Newfoundland, China, and Namibia. When they occur, they often occur in abundance. In South Australia, we literally have tens of thousands. They are preserved by storm beds. Imagine a sea floor with animals hanging out on top of it, and a storm comes in and smothers those animals. They eventually decay, but the sands of the storm deposit preserve the upper surface of those animals and the seafloor and capture them all as a snapshot. We have bed after bed that represent storm deposits that have smothered these communities. So we excavate these beds and flip them to reveal their lower surface, and we have almost photographs of these Ediacaran communities in life position. It is incredibly rare to get this kind of fossil record of any time. We’re really lucky. It allows us to look at whole communities.

Fossils of a Spriggina and Dickinsonia species on a microbial mat. Image courtesy of Mary L. Droser (University of California, Riverside, CA).

PNAS: How were ecosystems in the Ediacaran similar to or different from present-day ecosystems?

Droser: In the Ediacaran, you had a few things that move[d], but basically everything was sessile. As a result, the sea floor was covered with a microbial mat, the kind of stuff you would never want to put your foot in, literally pond scum. There were no predators, so what you imagine is that you have these lovely communities where nobody’s eating anybody. It’s often been called the Garden of Ediacara. One might imagine that things were small or not abundant but, actually, these communities were sometimes packed with animals. There were little ones, and there were ones that were a meter high off the sea floor. The diversity was certainly not what it is today, but one of the really interesting things we found is the variation between communities. Even without predation, we still had a very heterogeneous sea floor and ecological traits that we consider very modern.

PNAS: What were some of the “firsts” for animal evolution that developed during the Ediacaran?

Droser: A developmental first is the ability to move: muscles. We have the first multicellular organisms to live in the water column. We also have the oldest evidence of sexual reproduction in multicellular organisms. I’m sure there were others that were reproducing sexually but, here, we have size cohorts of densely packed organisms showing synchronous aggregate growth, which is largely a function of sexual reproduction. In the Ediacaran, we see the initiation of a number of different reproductive methods, which is a big deal. We don’t have eyes that we’ve seen, but we do have organisms that scavenge, that eat buried dead things. That’s giving us some sense of sensory abilities.