Svante Pääbo: Talking to the Past

For a collection of numbers on a screen, the World Population Clock stirs a lot of emotions (1). Watch it tick on, recording a life, another life, a death, then more lives. The number — well past 8 billion now — reflects the extent of Homo sapiens’ conquest over the world. Evidence of our culture, with its complex language, society and infrastructure, is everywhere. But we seem to be the only earthly species to live in such a way, the only species to track our own numbers on a digital clock. We swarm the planet, all its continents and yet we are, essentially, alone.

To challenge this isolation, scientists reach out in all directions, hoping for some kind of reflection that might shed light on who we are. Astronomers look to space; they probe the depths of the universe in search of life like our own. Others, like Svante Pääbo, look to the past.

300,000 years ago, when Homo sapiens first evolved, there was no paper, no writing, no human-like language with which to record stories, cultures, or day to day recounts. Scant traces of our ancestors are all that are left to tease us: fossilised footprints, makeshift tools, bones, grave sites. These markers are indecipherable whispers, slipping through in a hazy, broken form from a past era to our own.

With a time machine or resurrection tool perhaps we could converse with the dead, but while these remain foreign to our current reality, how can we talk to the past?

For Pääbo, the language of genetics is the key. Using the information carried in Palaeolithic bones, Pääbo has discovered links between present-day humans and prehistoric hominids that tell the story of our evolution and current condition. These incredible findings have earnt Pääbo the Nobel Prize for Physiology or Medicine in 2022 (2).

Some of his most important achievements establishing the field of Paleogenomics include the full sequencing of the Neanderthal genome and the discovery of a whole new hominin species: the Denisovan (3, 4). But what fascinates me is his discovery of genetic interrelations between these prehistoric species and Homo sapiens themselves.

Pääbo compared Neanderthal and Denisovan genetics to those of modern humans across the world. He discovered similarities and patterns that suggest a flow of genes took place between our ancestors and these hominid species: in other words, our predecessors mingled sexually with Neanderthals and Denisovans at some point in history, passing their genetics onto us as encoded evidence of this fact (5).

Human genomes from Europe and Asia were most closely related to Neanderthal genomes, and Pääbo has shown 1-2% of modern non-African Homo sapiens genes are Neanderthal in origin (3). Similar patterns were observed for Denisovans, with the closest relation with modern humans from Pacific islands (6). This data exposes an intimacy between prehistoric hominids that challenges our idea of humans as a species confined to solitude.

This conversation between genomes is not without implications for modern human physiology. When Homo sapiens moved into Eurasia, Denisovan and Neanderthal locals had already adapted to places in which Homo sapiens were mere tourists (7). Transfer of certain genes from local populations into the Homo sapiens line may have assisted in their survival. One example is a gene found in Denisovans that is important for survival at high altitudes and has been inherited by modern day Tibetans (8). Researching the discrepancies between modern and prehistoric genetics can thereby allow us to show the function and significance of these shared genes.

It is hard to visualise the world in which Neanderthals and Homo sapiens first met. Did the scene play out as a peaceful interaction between two groups of equals? Perhaps it was more akin to the pattern of colonisation with which we are familiar in modern history. As the last species of our evolutionary branch, the Homo genus, we cannot now recreate such a meeting.

However these prehistoric meetings played out, we now have evidence that Homo sapiens and local species of hominids in Eurasia communicated on the most intimate of levels. An optimist might argue that these groups of pre-humans shared a harmonious understanding that could be reproduced if humans find an analogous life form elsewhere in the future. Communication is a powerful tool after all, traversing species and millennia. Perhaps genetic insights into the past can remind us that we are not really as isolated as we might think. REFERENCES

Current world population [Internet]. Worldometer. 2023 [cited 2023Mar7]. Available from: https://www.worldometers.info/world-population/

Hedestam GK, Wedell A. The Nobel Prize in Physiology or Medicine 2022 [Internet]. NobelPrize.org. The Nobel Foundation; 2022 [cited 2023Mar7]. Available from: https://www.nobelprize.org/prizes/medicine/2022/advanced-information/

Green RE, Krause J, Briggs AW, Maricic T, Stenzel U, Kircher M, et al. A draft sequence of the Neandertal genome. Science. 2010May7;328(5979):710–22.

Krause J, Fu Q, Good JM, Viola B, Shunkov MV, Derevianko AP, et al. The complete mitochondrial DNA genome of an unknown hominin from southern Siberia. Nature. 2010Mar24;464(7290):894–7.

Villanea FA, Schraiber JG. Multiple episodes of interbreeding between Neanderthal and modern humans. Nature Ecology & Evolution. 2018May26;3(1):39–44.

Reich D, Patterson N, Kircher M, Delfin F, Nandineni MR, Pugach I, et al. Denisova admixture and the first modern human dispersals into Southeast Asia and Oceania. The American Journal of Human Genetics. 2011Oct11;89(4):516–28.

Rogers AR, Bohlender RJ, Huff CD. Early history of neanderthals and Denisovans. Proceedings of the National Academy of Sciences. 2017Jul7;114(37):9859–63.

Huerta-Sánchez E, Jin X, Asan, Bianba Z, Peter BM, Vinckenbosch N, et al. Altitude adaptation in Tibetans caused by introgression of Denisovan-like DNA. Nature. 2014;512(7513):194–7.