A fossil finger of Denisovan indicates the moment of evolution of the Neanderthal man

A group of anthropologists finally pieced together a Denisovan finger bone uncovered in 2009, thus revealing something astonishing. Denisovan's fingers looked more like ours than Neanderthals, although DNA shows that Denisovans are closer to Neanderthals. This suggests that Neanderthals evolved into subtle differences in the shape of their fingers (phalanges) sometime after their separation from Denisovans some 410,000 years ago.

DNA can tell us a lot about the relationship between species, but we still need to look at the bones themselves to understand how and when particular traits have changed. The combination of DNA and skeletal evidence can help us understand the details that differentiate modern humans from our close hominins – and the environmental and other forces that shaped these differences.

The fickle fate of a finger

In 2010, the DNA of a fragment of this finger bone (the proximal or closest end of the body) revealed the existence of another species of hominin that we had been missing for all that time. The Denisovans were named for Denisova's cave in Siberia, where anthropologists dug up the bone. This is the tip of the little right finger of a 13-year-old girl Denisovan, who died 50,000 years ago. His DNA sequence has become the source of most of what we now know about his enigmatic people, as fossil discoveries have been surprisingly rare for such a vast and living species.

Shortly after the exhumation of the finger bone, anthropologists who discovered it cut it in half and sent the proximal end to the Institute Max Planck in Germany and the distal end (the very end of the finger) at the University of California at Berkeley. Over the next decade, someone lost the only pictures of the complete bone, leaving the researchers without any idea what the finger was like.

The molecular biologist E. Andrew Bennett of the Jacques Monod Institute in France and his colleagues have now used photos of the distal piece and digital scans of the proximal piece to bring the two fragments together.

With a digital (the get? figure-When rebuilding the bone of the finger, Bennett and his colleagues had enough evidence to say that the bone was coming from the right hand and to conclude that the girl now called Denisova 3 had between 13 and 14 years old when she passed away. The bone plate located at the end of the bone of the finger, called the epiphysis, was fusing with the bone of the stem when she died. In most modern human girls (and in Neanderthals, we have finger bones and estimated age), this occurs between about 13 and 14 years of age.

A question of proportion

The authors carefully measured the proportions of the finger bone, the size of the important features and the distance between the key marks on the bone surface. They used these measurements to compare the shape and proportions (not the absolute size) of the bone to the finger bones of a Neanderthal sample and Homo sapiens remains. Neanderthals and Homo sapiens sorted into two distinct groups, Denisova 3 fits well in Homo sapiens and not in his closest Neanderthal cousins.

Neanderthal finger bones are quite easy to distinguish from Homo sapiens finger bones – for paleoanthropologists, anyway. Most Neanderthals had proportionally longer finger bones, with larger ends (called tufts). Bennett and his colleagues say that "seems to be related to functional adaptations rather than cold weather", unlike many other anatomical differences between us and the Neanderthals. The finger bone of Denisova 3 was no different from that of a Homo sapiens… but quite different from that of a Neanderthal.

These differences between Neanderthal's fingers must have evolved sometime after Neanderthals and Denisovans separated from their last common ancestor 410,000 years ago. Bennett and his colleagues suggest that this must have happened relatively late in Neanderthal history. A fingertip of a Neanderthal who lived on the site of Moula-Guercy in France (he hangs up a tale) about 100,000 years ago, looked more like Homo sapiens later than the Neanderthals – but it's the only one in the sample that did it. Therefore, at one point or another, something about the demands of Neanderthal life must push him to develop longer and wider finger bones.

A jaw of Denisovan from the Xiahe site in China, 160 000 years old, tells a different story of this evolutionary story, but that corresponds well to what Denisova's little finger tells us. Some features of the jaw of Denisovan look like many to Neanderthals, suggesting that both species inherited these characteristics from their last common ancestor. However, some features of the Neanderthal jaws do not appear in the Denisovan version, suggesting that these features, such as differences in the shape of the fingers, probably emerged later in response to different evolutionary pressures.

These resemble subtle differences, but they provide clues to the types of evolutionary pressures that have shaped Neanderthals, Denisovans, and our ancestors in a modest but important way over the past few tens of thousands of years of sharing the planet. One day, they might even reveal why we are still there when Neanderthals and Denisovans are not.

Progress of science, 2019. DOI: 10.1126 / sciadv.aaw3950 (About DOIs).