We knew Neanderthals and Denisovans were mixing. We knew Homo sapiens mixed with both of them too. Family trees of early humans have always looked like messy bushes. Not clean lines. Now we have evidence of another pair getting together. Homo erectus and Denisovans. The proof? Tooth proteins. Specifically, proteins from the teeth of six H. erectus individuals. These folks lived in China 400000 years ago. This study, just published in Nature, gives us the first genetic clue of this specific hookup.
Homo erectus is a heavyweight. They lived for nearly two million years. From 1.9 million years ago until about 100000. While Neanderthals, Denisovans, and H. sapiens were roaming around, erectus was there too. They were the pioneers. The first to leave Africa. They made it all the way to Indonesia. Java, even.
Genetically though? They’ve been ghosts.
We’ve only got DNA data from one specimen. A fossil found in Georgia dating back 1.8 million years. Researchers looked for unique markers there. Found nothing distinct. It looked too similar to everyone else.
Teeth Hold the Secret
China has better odds. Over a dozen sites hold erectus remains of different ages. A goldmine. Qiaomei Fu and her team at the Institute of Vertebrate Palaeontology and Paleoanthropology went for it. They focused on three spots. Zhoukoudian near Beijing (where the famous “Peking man” was found decades ago), plus sites in Hexian and Sunjiadong. The teeth date to roughly 400000-years ago. Middle Pleistocene.
Fu pulled protein from the enamel. Six teeth total. Five from males, one from a female. Why protein? DNA is fragile. It decays. Proteins last longer. Like DNA sequences though, protein sequences tell you who’s related to whom.
They sequenced fragments from nine different proteins. One protein stood out. Ameloblastin. An enamel matrix protein. They found two amino acid variants shared by all six specimens. Variants that set East Asian H. erectus apart from everyone else.
“It’s challenging to get that kind data from ancient enamel so to have a consistency across six teeth was very cool.” — John Hawks, University of Wisconsin–Madison
One variant involves glycine at position 253. In erectus teeth it’s glycine. In sapiens, Neanderthals, Denisovants, and that old Georgian erectus? It’s alanine. So the glycine version might be specific to this group in East Asia.
The Denisovan Link
Here’s the twist. At position 273 they found valine instead of methionone. Researchers knew about this valine variant already. They saw it in Denisovans. A 70000-year-old one from Siberia. And one found near Taiwan. If East Asian erectus had this variant… and Denisovans have it… they probably swapped DNA. Or rather protein signatures that trace back to DNA.
Interbreeding makes sense. Both groups were around in the same time and space. Tanya Smith, an evolutionary at Griffith University, calls it reasonable.
The story gets stronger when you look at older Denisovans. Fu’s team looked at enamel from a 150000-yearold Denisovan found in Harbin, northern China. They also got data for a 200000-yearone from Siberia. These ancient Denisovans had both the methionone and the valine versions. They inherited one from each parent. Heterozygous. Just like we’d expect if their parents mixed differently.
Ghost Species?
Then they looked at us. Modern humans. A tiny fraction of us carry this AMBN variant. For a while genomics researchers have noticed hints of “super-archaic” DNA in modern genomes. Not Neanderthal. Not Denisovan. Something older. Something unknown. A ghost species.
Fu realized the ghost might just be erectus.
“So it was really exciting.”
The rare variant likely moved from erectus to Denisovans. Then from Denisovans to early H. sapiens. A chain reaction of ancient sex.
Is it proven? Not exactly. Hawks points out variants can pop up by chance in different populations. Maybe erectus didn’t give it to Denisovans. Maybe it just appeared independently. That seems unlikely though. Especially given how rare these super-archaic sequences usually are. Finding this link is surprising. The odds had to align perfectly.
Proteins from even older erectus fossils in China might clear this up. But for now? We’re left with this hint. That our oldest relatives didn’t just fade away. They left a mark in our teeth.
Or our genes.
