What Ancient Bones Can Tell Us: Exploring the Fossilized Past with Julia Tejada

Julia Tejada, a newly arrived assistant professor of geobiology at Caltech, is an explorer of the past. When she is not hunting for fossils in the Amazon, she is pioneering laboratory techniques to study the biochemistry of ancient bones.

Tejada's research aims to understand the flora and fauna that populated the earth millions of years ago, on timescales that paleontologists and geologists call "deep time." Most of the life that has ever existed on Earth is now extinct or has no ecological analogs in modern ecosystems. Sloths, for example, were a major component of ecosystems in the Americas until 10,000 years ago, and their surviving descendants are not representative of the ecological diversity that their ancestors once had. To understand where we and all other living things on Earth came from and how we might change in the future, it is necessary to peer into the past.

We sat down with Tejada to discuss ancient animals, how she explores South American jungles, and how the notion that "you are what you eat" can serve as a powerful scientific tool.

What is it like to work in the rainforest?

It's one of my favorite things to do, to be honest. From the moment you step off the plane, there's a rich scent of fermented wood that transports you to a world of wonder. We fly to the closest city, but then we travel by rivers into the field. We stay in the field for no less than 10 days. Some of the places that we visit are very remote; just reaching those places takes days. For paleontological research, we're limited to the dry season, when the level of the rivers drops enough to expose outcrops where we look for fossils. Otherwise, everything is covered beneath vegetation.

The Amazon has been very understudied in paleontology. There was this long-held idea that because of the low pH of the soil, high rates of weathering, and the thick vegetation, tropical systems and rainforests were not an ideal place for fossil preservation. My colleagues and I have showed that this is actually not true. The Amazon rainforest is full of diverse fossils.

Do you have a favorite memory of discovering a fossil?

It's the best feeling to find something that has been buried for millions and millions of years, and you're the first person who is digging it up. I've been conducting expeditions since I was an undergrad, so I've had the chance to be part of many amazing discoveries. One of the most amazing ones, because it was one of my first digs, was a penguin—a 42-million-year-old penguin with feathers preserved. That penguin had been buried under very rare conditions, with no oxygen and, thus, no microbial degradation, enabling it to be so intact. I've also discovered a huge diversity of fossil crocodiles and many mammal species in the Amazon.

Tell us about your research in the laboratory.

In my lab-based research, I use stable isotope techniques to study the chemistry of ancient bones. One key focus of our work involves understanding the feeding ecology of extinct organisms. Diet plays a pivotal role in driving evolution. It alters genetic expression and influences phenotypic evolution over long timescales. By studying diets and trophic [feeding and nutrition] dynamics, we gain valuable insights into ecosystem composition and function. For example, while fossil plants are rare, analyzing the diets of fossil herbivores allows us to reconstruct the vegetation present in their ancient environments.

Stable isotope methods applied to feeding ecology rely on two fundamental principles. First, isotopic ratios of elements vary across different reservoirs in the biosphere, resulting in distinct isotopic compositions of foods and waters. Second, organisms synthesize their tissues primarily from the food and water they consume, and therefore they are linked isotopically to those sources in a predictable way. This echoes the adage "we are what we eat," as our tissues essentially record our dietary intake. Importantly, digestive processes alter the ratios of isotopes from consumed food to tissue composition. So, while you are indeed what you eat isotopically, your digestive physiology transforms the chemical makeup. My research focuses on understanding how different digestive processes contribute to the isotopic variability that we observe. Only by thoroughly understanding these physiological processes in modern animals can we attempt reconstruct ancient ecosystems using these methods.

What are isotopes?

Isotopes are variations of a chemical element. Elements are defined by their number of protons—that's how they are organized in the periodic table—but they can have different numbers of neutrons, which give them different masses. For example, the isotope carbon-12 is the most common kind of carbon; it has six protons and six neutrons. Carbon-13, on the other hand, is more rare; it has six protons and seven neutrons. The extra neutron makes it heavier, so it behaves slightly differently in chemical reactions. When we talk about "isotope ratios" in a fossil, we mean the amounts of one isotope relative to another.

Different elements tell you different stories. Ratios of oxygen isotopes, for example, are heavily influenced by geographic and environmental factors such as elevation, precipitation, proximity to the ocean, and so on. The oxygen isotope ratio of water in California is very different than the ratio in Florida, for example. Strontium, for example, is linked to the geology of an area. Depending on the types of questions you might be interested in, you can use different isotopic systems to answer them. Your hair is a good example of this. We could reconstruct your travel history by analyzing a shaft of your hair, because the water that you consumed in different places will have different isotopic patterns.

What was your journey like to becoming a professor at Caltech?

It's been a fun journey. While I've relished the experience, I would be lying if I were to say that it was always easy. Pursuing academia demands passion and dedication, often requiring sacrifices along the way. While these sacrifices may not feel burdensome when fueled by enthusiasm, the academic path isn't suited to everyone. It's a road that rewards perseverance and commitment, but it's important to acknowledge that it's not without its trials.

I'm originally from Peru. I did my undergrad in biological sciences there, and I became interested in paleontology and deep-time biology when I was an undergraduate. I started working at the Natural History Museum in Lima, and I met different scientists from all over the world who came to visit the collections in the museum. There was so much opportunity for research in this field, quite a new field in Peru.

I went to the University of Florida to do a master's, and then I went to Columbia University and the Museum of Natural History in New York to do my PhD. I did postdocs at the University of Montpellier and the University of Cambridge, then I came to Caltech in fall 2023.

Why did you want to come to Caltech in particular?

I really wanted to be part of an institution that values basic science and that is not afraid of taking risks, and Caltech is definitely the best place I can imagine for that. Caltech has an amazing history of geochemistry—many people are pushing the boundaries here of what can be measured. I'm able to leverage these strengths that Caltech has to develop new methods to access biological information in really ancient organisms.

But paleontology at Caltech is not new. There is actually an important part of Caltech's history related to paleontology. Chester Stock was a brilliant paleontologist, renowned for his work in Pleistocene faunas of California and Mexico, including the La Brea Tar Pits. After he passed away in 1950, paleontology disappeared from Caltech for several decades.

I like to think that I am a new wave of modern paleontology. I'm interested in deep time, and I still go to the field to do paleontological digs, but I'm taking advantage of these really cutting-edge, modern techniques to move the field to a new direction where it's never been before. And it turns out Chester Stock is my academic great-grandfather. He was the PhD advisor of Malcolm McKenna, who was both my master's and PhD advisor's advisor, so there is this connection that I wasn't aware of.

What's it like working with such ancient history?

It definitely inspires your imagination. I just think about these ancient ecosystems that were so different to those that we see right now, with these majestic creatures that once roamed places where we walk today. It's hard to imagine that where we are in Los Angeles was populated until as late as 10,000 years ago by mammoths, giant ground sloths, and saber-toothed cats. Working at these timescales also instills humility. Our human perspective is limited and biased, representing only a fraction of the immense history of life on Earth.