Looking at a jellyfish is like looking into the ancient past. Survivors from the late Precambrian Era, these organisms lived in an environment completely alien to the wide swath of modern Earth. They thrived during a time when the waters of our planet were largely anoxic, the lack of oxygen making them inhospitable to most animals existing today.  

Though the Earth changed drastically over the next hundreds of millions of years, jellyfish persisted. 

“One reason that I love jellyfish is they’re living members of one of the oldest groups of animals,” said David Gold, an associate professor in the Department of Earth and Planetary Sciences at the College of Letters and Science. “If you want to understand what those first animals looked like, what those animals were capable of, what their bodies were like, what genes they had, then looking at jellyfish can tell you a lot about those very first creatures that were out there.” 

Gold specializes in molecular paleontology, an area of study that combines geological, genetic and developmental tools to study the early evolution of animal life. A biologist by training, he’s fascinated by the development of life systems over long time scales. 

How has the Earth shaped lifeforms? And conversely, how have lifeforms shaped the Earth?

“That can involve anything from looking at jellyfish, how their bodies are built, how they shape ecological systems around them, to what kinds of biological information we can get out of fossils and rocks, to using genetics — looking at the DNA of living organisms — to test hypotheses about things that may have happened a long time ago, and to better interpret the fossil record,” Gold said. 

A fascination with jellyfish 

Jellyfish are deceptively simple organisms. While they may lack the complexity of other creatures, they’ve optimized their biology, making the most of their limited cell types.

In the lab, Gold and his colleagues primarily use the moon jellyfish (Aurelia aurita) to explore developmental and evolutionary questions. 

“Their cells are very flexible, much more flexible than most mammals or insects or creatures that we tend to study,” Gold said. “Part of what’s fascinating about them is you can cut them in half and the two halves swim away and regrow into new animals.” 

Due to their skeleton-less nature, jellyfish don’t have a great fossil record. Gold and his team have turned to genetics to study the development of these organisms and more broadly the Cnidaria, which also includes corals and sea anemones.

“These animals are known for their stingers and there are many different kinds of stinging cells, which are adapted to what kind of prey you eat and how you defend yourself,” Gold said. 

In a study appearing this year in Evolution & Development, Gold and graduate student Noémie Sierra examined the diversity of these stinging cells, also known as cnidocytes. They examined how different cnidocyte types evolved and how that impacted the feeding habits of these early carnivores. 

“By looking at modern-day animals with these different stinging cell types, we can look at their DNA to see how closely they’re related to each other and from those differences in DNA, we can make inferences about how long ago these different groups separated from each other,” Gold said. “Through that process, you can reconstruct the history of the cells and how they evolved.”

Based on the genetic detective work, Gold and Sierra found that around 600 million years ago a rapid diversification in the cell type occurred, evolving to be specific to distinct feeding styles and prey. 

Unexpected finds in the lab

Gold consistently finds himself surprised by jellyfish behavior. Studying one aspect of their nature tends to lead to unexpected observations.  

Previous research has shown that jellyfish play an important role in ocean mixing, their up and down movements stirring the water column. 

“I suspect they may have played an important role in the early history of the world when animals were first evolving,” Gold said. “We have evidence from the geological record that the oceans were not well mixed. It was low oxygen, there was a lot of sulfur and it was an unpleasant ocean. Around the time animals show up, it becomes more oxygen-rich.” 

Gold suspected that jellyfish played a vital role in shifting the ancient oceans’ chemistry to a more habitable one. He decided to test their water mixing capabilities in the lab. 

“I added this compound to make the water more viscous and these tiny jellyfish I was studying, they just sucked up the chemical and inflated like little balloons,” Gold recalled. 

Instead of swimming up and down in the water column, the jellyfish floated to the surface.

“This got me interested in this possibility that instead of just eating the way that we normally do, maybe they are also really good at sucking up organic matter,” Gold said. “That might be a big part of why they do so well in these dead zones in the oceans where other animals have a hard time living.” 

Molecular fossils and paleontology 

As Gold puts it, he’s always had “a foot in the fossil record.” His fascination with the ancient past stems from his childhood. In high school, he worked at the La Brea Tar Pits and Museum in Los Angeles. 

Today, the fossils Gold studies are molecular in size.

“Molecular fossils are just organic compounds — things created by living beings" he said. "They can be proteins or fats and they get preserved in fossils and rocks, and we can actually recover those and learn things about the lifeforms they come from.” 

Gold and his colleagues are not just focused on the first animals, they're using molecular paleontology techniques on a variety of projects, including trying to pinpoint when warm-bloodedness evolved in the dinosaur lineage. 

“Birds evolved from dinosaurs, and so at some point, those metabolic rates are increasing,” he said. “There is a biomarker that gets preserved in bones that can give you insight into metabolic rates.” 

In another intriguing project, Gold and his colleagues are analyzing a curious compound from the earliest animal fossils. 

“People have recovered a compound that is thought be a signature of human waste products. It’s a very human-specific product, but it’s also sometimes found in other mammals and apes” Gold said. “But we’re finding it in these ancient, ancient fossils of some of the first creatures that are really not human or vertebrate. They kind of look like a worm if you smashed it into a pancake.” 

The discovery introduces an interesting conundrum. And it’s these curiosities that motivate Gold’s pursuit of knowledge.  

“I think in some deep sense, we’re all fascinated with these questions, why are we here? Why is the world the way it is?" he said. "I’ve always been drawn to the sciences, but I’m also really fascinated with thinking about those human aspects.” 

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