Buried in the Denisova Cave in southern Siberia sat the tip of a finger bone – a pinky finger to be exact.

The bone had been there for 30,000 to 50,000 years until it was unearthed by researchers in 2008. Too small to give any clues to its ancient origins, the researchers carefully packaged the bone and shipped it to the laboratory of Svante Pääbo. Pääbo was the world’s expert on ancient human DNA and in 14 short years would go on to win the Nobel Prize in Medicine, in part for what he was about to find in that unassuming pinky finger.

In a clean room built to capture any rogue DNA flying around, Pääbo drilled into the finger bone and extracted the ancient DNA.

He expected to see the hallmarks of ancient Homo sapiens DNA or the glimmers of one of our close relatives, Neanderthals. But the DNA was different. It was human for sure, but not human like us nor was it human like Neanderthal. Nor Homo erectus. Nor Homo floresiensis. Nor any other of the ancient human species (collectively known as “hominins”) thought to live at that time.

From ancient DNA alone, Svante Pääbo had discovered a brand new ancient human species: the Denisovans.

In this Century of Science theme, The Human Story, Science News travels back in time almost 7 million years, recounting how researchers discovered all that we know of how Homo sapiens – or “modern humans” – came to be. Starting with Sahelanthropus tchadensis as the first hominin ancestor to branch from other apes, the story of human evolution is … complicated.

Early anthropologists and archaeologists originally believed that human evolution proceeded in a discrete, step-wise fashion. But, as researchers discovered more and more human fossils, the story of human evolution started to look less like a straight ascension of our North Entry stairs and more like a jaunt through M.C. Escher’s “Relativity.” A tangled web of twists, turns, reversals and even loop-de-loops, human evolution is never quite what it seems.

Svante Pääbo started to untangle the web as a graduate student at Uppsala University in Sweden. Fascinated by what made modern humans distinct from the other hominin species, Pääbo wanted to use DNA sequencing – a fairly new tool in the early 1980s – to read ancient DNA from Neanderthal fossils. Quickly, Pääbo discovered that the limits of early sequencing technology weren’t his only foe.

DNA doesn’t last forever. After it’s been locked in a fossilized bone, sitting under rocks and dirt for tens of thousands of years, the string of A’s, T’s, G’s and C’s that makes up DNA starts to fall apart. This leaves very little well-preserved DNA from which to get a sequence.

Luckily in the late 1980s, another researcher fascinated with modern genetic tools developed technology that fundamentally changed DNA-based research: polymerase chain reaction (PCR). What PCR does is take a tiny bit of ancient DNA that’s still in good shape and make more of it – a lot more of it.

“The invention of PCR overcame many problems,” Pääbo said in 2014. With so many more copies, reading its sequence is easier than navigating our 14 acres of Museum without a map!

With PCR, Pääbo started tackling a variety of ancient DNAs, honing his methods to one day track down the elusive Neanderthal. From ancient sloths to ancient mammoths, Pääbo recovered and read snippets of DNA for ancient species after ancient species. Pääbo even got bits and pieces of Neanderthal DNA from PCR, but not enough to understand what made modern humans different from our ancient comrade.

Finally in the mid-2000s came a breakthrough in DNA technology: high-throughput sequencing. High-throughput sequencing is like PCR on steroids. Imagine a person’s genetic blueprint as a book. Whereas PCR can make copies of a one or two pages from the book, high-throughput sequencing can make copies of every single page of the book all at the same time.

Using this technology, Pääbo finally read the first Neanderthal genetic blueprint in May 2010. And the treasures it held were well worth the 25-year wait.

Pääbo and his team compared the string of A’s, T’s, G’s and Cs in the Neanderthal blueprint to the blueprints of five living, breathing humans from around the globe. Stunningly, they found that 1-4% of modern humans’ DNA from Europe and Asia descends from Neanderthals, but Neanderthal DNA was noticeably missing in modern humans of African descent.

That left only one conclusion: Neanderthals and modern humans interbed during their time together.

Fast forward to December 2010. The DNA sequence from the Denisovan pinky bone blinks across Pääbo’s computer screen. He’s looking for similarities and differences with other hominins, trying to find just where in the tangled web this new human species fits … then, bingo!

The Denisovans are not only their own species, but they also interbred with modern ancient humans and with Neanderthals. And with that, the web of human evolution got a little more complicated and a little more clear.