Denisovan Proteins in Tibet

The study of human origins has historically relied on finding bones and, more recently, extracting ancient DNA. However, DNA is fragile and often degrades in harsh climates. A breakthrough in the Himalayas has changed the way scientists look at the past. By analyzing ancient proteins instead of DNA, researchers successfully identified a Denisovan jawbone on the Tibetan Plateau. This discovery proves these ancient humans lived at high altitudes long before modern humans arrived.

The Xiahe Mandible Discovery

For decades, the only physical evidence of Denisovans—a sister group to Neanderthals—came from a single cave in Siberia called the Denisova Cave. That changed with the analysis of the Xiahe mandible.

A Buddhist monk originally found this fossilized jawbone in 1980 in the Baishiya Karst Cave, located on the Tibetan Plateau at an altitude of roughly 3,280 meters (10,700 feet). The bone eventually made its way to Lanzhou University. For years, its origins were a mystery. The bone was massive, with teeth that were exceptionally large compared to modern humans.

When researchers finally attempted to sequence DNA from the fossil to identify it, they hit a wall. The jawbone was at least 160,000 years old. Due to the passage of time and the cave environment, the genetic material had completely degraded. In traditional archaeology, this would have been a dead end.

Proteomics: The Key to Identification

Since DNA was unavailable, a team led by Jean-Jacques Hublin from the Max Planck Institute for Evolutionary Anthropology turned to a newer technique called paleoproteomics. Specifically, they analyzed the proteins found in the collagen of the jawbone’s teeth.

Proteins are more durable than DNA. While DNA acts like a fragile instruction manual that tears easily, proteins are the structural bricks that persist much longer in the fossil record.

How the Analysis Worked

The researchers used a method often referred to as ZooMS (Zooarchaeology by Mass Spectrometry) or simply protein sequencing.

1. Extraction: The team drilled a tiny sample of dentine from one of the molars in the jawbone.
2. Sequencing: They looked at the amino acid sequences within the collagen.
3. Comparison: They compared the protein structure of the Xiahe mandible against known databases of modern humans, Neanderthals, and the few DNA-based protein predictions we have for Denisovans.

The results were conclusive. The amino acid sequence did not match modern humans (Homo sapiens) or Neanderthals. It was a match for Denisovans. This marked the first time a Denisovan fossil was identified based solely on protein analysis rather than DNA or physical shape alone.

High-Altitude Adaptation and the EPAS1 Gene

This discovery solved a major genetic puzzle regarding modern Tibetans. Geneticists had previously discovered that many Sherpas and Tibetans carry a specific gene variant known as EPAS1. This gene helps the body manage hemoglobin production, allowing people to survive in low-oxygen environments without developing thick blood or altitude sickness.

Scientists knew this gene variant originated from Denisovans, but it was unclear where or how the interbreeding happened. The Xiahe mandible proves that Denisovans were already adapted to the Tibetan Plateau 160,000 years ago.

When modern humans eventually migrated into the region, they interbred with the local Denisovan population. The modern humans who inherited the EPAS1 gene had a massive survival advantage in the thin Himalayan air, which is why the gene persists in the population today.

Implications for Future Archaeology

The success of protein analysis in Tibet has opened the door for identifying other mysterious fossils. Museums around the world are filled with bones that look human but do not yield DNA.

• Warmer Climates: DNA breaks down rapidly in hot, humid environments like Southeast Asia or Africa. Proteins survive longer in these conditions.
• Older Samples: While DNA sequencing is currently limited to about 400,000 to 700,000 years, proteins can potentially survive for millions of years.
• Wider Range: This specific finding suggests Denisovans were not limited to Siberia. They likely occupied a vast area of Asia, ranging from the freezing north to the high-altitude Himalayas and potentially into tropical Southeast Asia.

Confirming the Findings

Following the protein analysis, further excavations at the Baishiya Karst Cave provided more evidence to support the initial findings. In 2020, researchers analyzed sediment samples from the cave floor.

Unlike the bone itself, the dirt layers in the cave actually preserved traces of mitochondrial DNA. This sediment DNA matched the Denisovan profile, confirming that these ancient humans occupied the cave essentially continuously for tens of thousands of years. This served as a vital cross-check, validating that the protein analysis of the jawbone was accurate.

Frequently Asked Questions

What is the Xiahe mandible? The Xiahe mandible is a fossilized jawbone found in 1980 on the Tibetan Plateau. It is the first Denisovan fossil discovered outside of Siberia and was identified using protein analysis.

Why did scientists use proteins instead of DNA? The jawbone is approximately 160,000 years old. DNA degrades over time, especially in certain environments. The DNA in this specific fossil was too damaged to sequence, but the structural proteins (collagen) in the teeth were still intact.

What is the connection between Denisovans and modern Tibetans? Modern Tibetans inherited the EPAS1 gene from Denisovans. This gene allows them to live at high altitudes with low oxygen levels. The discovery of the jawbone proves Denisovans lived in these high altitudes long before modern humans arrived.

Where else have Denisovans been found? Physical remains have only been definitively identified in the Denisova Cave in Siberia and the Baishiya Karst Cave in Tibet. However, genetic traces of Denisovans are found in modern populations across Asia, Melanesia, and Australia.

How does mass spectrometry identify ancient humans? Mass spectrometry measures the mass of different molecules. By analyzing the collagen in a bone, scientists can map out the sequence of amino acids. Different human species (Neanderthals, Denisovans, Homo sapiens) have slight but distinct variations in these sequences.