Ancient proteins extracted from Homo erectus teeth suggest these early humans interbred with Denisovans, passing genetic material to modern populations. Researchers analyzed enamel from three Chinese sites dating to roughly 400,000 years ago, alongside a Denisovan sample from Harbin. The protein analysis revealed genetic connections that DNA studies couldn't capture due to degradation limits.

DNA survives only in cooler, drier conditions and degrades rapidly over time, making it difficult to study remains older than a few hundred thousand years. Tooth enamel proteins, however, persist much longer and provide a window into deep human ancestry. The Chinese team successfully isolated protein fragments from six Homo erectus individuals and one Denisovan, confirming their method worked by testing animal teeth from the same sites first.

These findings indicate that Denisovans carried DNA from an even earlier human relative before interbreeding with modern humans. Previous genome evidence hinted at this ancestral mixing, but couldn't identify the mysterious donor group. Now, protein analysis fills that gap, showing Homo erectus contributed to the genetic mosaic of both Denisovans and contemporary human populations.

This research opens new possibilities for studying human evolution beyond the DNA preservation barrier. Ancient protein analysis could reveal relationships between other extinct hominins and help map migration patterns across Eurasia. The technique proves especially valuable for understanding our ancestors' complex web of interbreeding events.