This year’s Nobel Prize in Physiology or Medicine was awarded to Svante Pääbo for his discoveries about the genomes of extinct hominins and human evolution.

Pääbo, a Swedish geneticist and director of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, was honored for his groundbreaking research on sequencing the genome of the Neandertals, an extinct relative of humans, and discovering a new hominin species, Denisovans. He also demonstrated that humans—Homo sapiens—interbred with these species after migrating out of Africa.

His work addresses important questions about humanity’s origins, including where we came from, why some species went extinct and what makes us uniquely human.

Homo sapiens arose in Africa about 300,000 years ago, research suggests. Neandertals (also spelled Neanderthals), meanwhile, arose outside Africa and lived in Europe and western Asia from about 400,000 years ago until they went extinct roughly 30,000 years ago. Groups of Homo sapiens left Africa around 70,000 years ago, and spread throughout the world. They co-existed with Neandertals in Eurasia for tens of thousands of years, but little was known about the relationship between the two groups.

Pääbo was born in Stockholm in 1955, and was interested in early human history from a young age. His Nobel-winning research initially focused on extracting ancient DNA from our closest hominin relatives, Neandertals. But ancient DNA is extremely challenging to study because it degrades into tiny fragments and is easily contaminated by DNA from other sources.

First, Pääbo focused on mitochondrial DNA—genetic material found in the energy-producing structures within our cells. He sequenced mitochondrial DNA from a 40,000-year-old piece of bone and showed it was different from the mitochondrial DNA of both modern humans and chimpanzees.

Next, using sophisticated DNA sequencing methods, he and his colleagues went on to sequence the full Neandertal genome, publishing their findings in 2010. The team found that the most recent common ancestor of Homo sapiens and Neandertals lived about 800,000 years ago, and that the two species interbred over thousands of years. About 1 to 4 percent of the genomes of modern humans of European or Asian descent comes from Neandertals.

Pääbo and his colleagues also made the startling discovery of a new hominin species, Denisovans. The researchers sequenced the genome of a 40,000-year-old finger bone fragment from Siberia and showed that it was distinct from both Homo sapiens and Neandertals. Interbreeding also occurred between humans and Denisovans, and up to 6 percent of the DNA of people in Melanesia and parts of Southeast Asia is Denisovan.

This research established paleogenomics as an entirely new scientific discipline. The work revealed much about the influence of ancient hominins on modern humans, including a Denisovan gene that helps modern Tibetans survive at high altitude and a Neandertal gene that affects the immune response to infections.

The findings also reveal information about what makes humans unique. Like humans, Neandertals used tools. But Homo sapiens developed complex cultures and art, and developed the ability to cross open water. Perhaps future research will unravel the mysteries of why these ancestors died out while our species flourished.