One of the most fascinating things about puzzling out our human genome is finding things in it that offer no clue how they got there - no tracks, fingerprints or bread crumb trails through the woods of time immemorial.

Take last October's announcement by a team of genetic sleuths at the University of Texas saying they had found evidence of an unidentified, extinct human species in the DNA of modern day Melanesians living on South Pacific islands northeast of Australia.

"We're missing a population," UT statistical geneticist Ryan Bohlender told Science News at the time. A whole race of humanity came and went, leaving only these few alleles behind as proof they were ever here.

Extinct hominid DNA it's called - scraps of evidence that our Homo sapiens ancestors, who came out of Africa 100,000 to 60,000 years ago, ran into and mingled with other humanoid species that had already wandered out of Africa millennia earlier.

Neanderthals, of course, are the best known of these, not only because they left behind a generous fossil (and DNA) record, but also because both Europeans and Asians carry distinct Neanderthal DNA in their genomes - anywhere from 1.5 to 5 percent.

But there are also traces of interaction with a second species - the mysterious extinct Ice Age Denisovans, who probably shared a common ancestor with Neanderthals way, way back when - but their fossil record has been frustratingly scant (a finger bone and a couple of teeth discovered in a cave in Siberia in 2008 that yielded some DNA).

Until now - well, maybe. A Chinese-US team just unveiled fossilized remnants of two 105,000-to-125,000-year-old skulls from a dig near the city of Xuchang in Henan province. And they may (or may not) be from Neanderthal's distant cousins the Denisovans.

The presentation in the latest edition of Science was a long time coming. Back in 2007, archaeologist Zhan-Yang Li of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing was closing up shop at a dig for the season when his eye caught some quartz tools peeking out of the sediment. He had his team stay an extra two days to extract them. Luckily so.

On the last day they found a yellow piece of skull cap from the muddy floor where the tools had been. For the next six seasons the team extracted 45 more pieces that fit together into the two partial skull domes - no facial or jaw bones were ever found.

But there was enough for the team to show a close resemblance to Neanderthals shape wise, with one skull tipping the brain-volume scale at 1,800 cubic centimeters - the upper end for modern humans as well as Neanderthals - and prominent brow ridges that are more akin to Neanderthals (than to us) but thinner than the Neanderthals from Europe or the Middle East.

"They are not Neanderthals in the full sense," study co-author ErikTrinkaus, a paleoanthropologist at Washington University in St Louis, told Science magazine.

So who are they? Their craniums are too light and too full of brains to be either of two other archaic humans known to have roamed the region - Homo erectus and Homo heidelbergensis, the authors suggest.

But they did notice similarities to another early (100,000-year-old) skull that was found in Xujiayao in China's Nihewan Basin 500 miles to north of the Lingjing site, according to co-author Xiu-Jie Wu, who believes that and the two new ones represent "a kind of unknown or new archaic human that survived on in East Asia 100,000 years ago," she told Science.

Other experts not connected with the study are arguing that the traits are precisely what Denisovans should look like, exactly what the existing DNA predicts.

Analysis of the finger bone and teeth from the Siberian cave done in 2015 suggests that Denisovans co-existed with both humans and Neanderthals in Asia for about 60,000 years.

The key to unlocking the mystery of who these skull plates belonged to and who their cousins were will depend on them giving up some of their DNA, something the team hasn't been able to make happen yet.

In a statement, Trinkhaus said the fossils show, at least, "a pattern of regional population continuity in eastern Eurasia" and "reinforce the unity and dynamic nature of evolution leading up to modern human emergence".

One big happy family.

As University College London paleoanthropologist Maria Martinon-Torres told Science: "China is rewriting the story of human evolution. I find this tremendously exciting."

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