Could DNA Have Survived 6 Million Years Inside a Turtle Shell?

Could DNA Have Survived 6 Million Years Inside a Turtle Shell?

A team of paleontologists studying the fossilised shell of a sea turtle from the Miocene Epoch found something surprising: preserved bone cells that they believe may contain ancient DNA, the molecule that holds the genetic information of living things.

In an ancient turtle shell found on Panama’s Piña Beach, northwest of Panama City, the team identified osteocytes, or bone cells. They then used a type of stain called DAPI to attempt to label the DNA in the fossilized cell structures. Their findings were published last week in the Journal of Vertebrate Paleontology.

“Within the entire vertebrate fossil record on the planet, this had only been previously reported in two dinosaur fossils, including one of Tyrannosaurus rex,” said study lead author Edwin Cadena, a paleontologist at the Universidad del Rosario in Bogotá, Colombia, in a Smithsonian release.

Cadena is referring to reports of osteocytes preserved in a T. rex and Brachylophosaurus canadensis, a hadrosaur, made in 2013. In the new paper, the researchers also referenced the discovery of fossilized cartilage in a roughly 120-million-year-old Caudipteryx, which some paleontologists believed contained evidence of fossilized biomolecules, though that is controversial.

The sea turtle fossil belonged to a member of the genus Lepidochelys, which is represented today by the olive ridley and Kemp’s ridley sea turtles. The researchers believe the ancient member of Lepidochelys may be a new species, but they withheld from making that determination due to a lack of skeletal evidence; no skull, plastron, or limb bones were found. The partially preserved carapace—the oldest recorded member of the genus—is now housed at the Museo Paleontológico de Panamá in Panama City.

When organisms fossilise, they fossilise all the way down to a molecular level. Under a microscope, ancient bones can yield information about T. rex growth rate through the fossilization of their blood vessels and details of sauropod biology thanks to preservation of their proteins. The problem is that DNA rapidly degrades, meaning that ideal conditions are needed for the molecule to preserve on thousand-year (and indeed, million-year) timescales.

In scrutinizing the shell under a stereomicroscope, the researchers found bone microstructures were exceptionally preserved. Blood vessels, collagen, and osteocytes were all identifiable. The team also saw bundles of material in the osteocytes they described in the paper as “nucleus-like”—the nucleus being the organelle in cells that contains DNA.

Clockwise from top-left: an overlay photo of the DAPI stain and the brightfield photos; another overlay showing the location of a stained nucleus-like structure; a group of osteocytes with nucleus-like structures but no DAPI stain reaction; a group of osteocytes with no nucleus-like structures and no DAPI stain reaction.

The team stained the osteocytes with DAPI, a solution that glows blue when it interacts with DNA. Lo and behold, some of the nucleus-like regions turned blue once the stain was applied.

“No reaction to DAPI was observed outside the ‘nucleus-like’ internal structures of the osteocytes, supporting the potential endogenous origin of these traces,” the researchers wrote. “However, not all osteocytes that have this ‘nucleus-like’ internal structure show reaction to DAPI…It is also possible that DNA remains are not preserved in all cells or have been highly degraded, preventing them from reacting with DAPI.”

In other words, it could be evidence of ancient fossilized DNA, but it’s no hard proof. The oldest DNA yet found and sequenced was discovered in million-year-old mammoth teeth from Siberia in 2021. That DNA was preserved in permafrost, the until-recently permanently frozen soil that preserves Pleistocene creatures like cave lions, bears, and mammoths to a stunning degree, often saving the animals’ soft tissues and fur. And back in 2019, a different team of researchers managed to recover genetic information from a 1.7-million-year-old rhinoceros tooth, though no DNA was recovered from the specimen.

“On the positive side, at least this specimen is on the same order of magnitude as the oldest, non-controversial DNA sequences—but it is still several million years older and from a much harsher environment for DNA preservation,” Evan Saitta, a researcher at the University of Chicago, told Gizmodo in an email. “On the negative side, the conclusions are still reached through limited, problematic methods, such as these histochemical stains. If these indicators of DNA are true positives in all of these ancient fossils, then sequence it!”

Saitta and his colleagues attempted to sequence ancient DNA from the bone of a Centrosaurus in 2020, and they found a modern microbiome but no ancient DNA. “There comes a point where claims must meet reality, and this is the process of science,” Saitta said.

“When researchers study mammoths, molecular data is often used to help reconstruct their evolutionary relationships,” he added. “When researchers study much older organisms, such as dinosaurs, I see no one utilizing DNA or protein sequences, despite all the excitement and ‘belief’ in their existence over the decades.”

It’s a tall order to prove that something that reacts in a fossilized nucleus-like structure is truly the fast-degrading molecule DNA. According to researchers who spoke to Gizmodo about the Caudipteryx fossil in 2021, microbes in the dinosaur remains (like those found in Saitta’s Centrosaurus) or mineral infill of the space that once housed dinosaur DNA may be reacting to the dyes and stains applied.

That may be the case here, but it may not. What’s clear is that such ancient DNA has not yet been identified for certain, and not in any way that’s sequenceable.

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