Svante Pääbo: DNA clues to our inner neanderthal

Summary

This insightful presentation explores the fascinating story of human evolution through the lens of our genomes. It covers the basics of DNA structure, mutations, and genetic variation, highlighting how these differences have shaped human history. Modern humans, originating in Africa, spread across the globe and interbred with Neanderthals and Denisovans, leaving traces of their DNA in present-day populations. The talk challenges common misconceptions about racial differences, emphasizing that human diversity is rooted in a shared genetic heritage and ongoing intermingling, ultimately revealing the complex and interconnected nature of human evolution.

Takeaways

• 😀 Genomic material is stored in DNA within chromosomes, and it consists of a sequence of four bases: A, T, C, and G.
• 😀 Mutations occur as a result of errors during DNA replication, and these mutations accumulate over time, providing a way to estimate the age of genetic differences.
• 😀 Modern humans share a common ancestor with chimpanzees around five million years ago, while humans diverged from other hominins about half a million years ago.
• 😀 The genetic variation within Africa is greater than outside of Africa, despite Africa's larger population, because modern humans evolved in Africa and later spread out.
• 😀 Genetic variation is distributed globally in gradients, and while traits like skin color and facial features differ across populations, they are influenced by multiple genetic variants, not a single gene.
• 😀 There are no absolute genetic differences between Africans and non-Africans; however, some genetic variants are more common in certain populations (e.g., 95% of people in Africa having one variant at specific positions).
• 😀 Physical traits like skin color and hair structure have evolved rapidly through selection due to environmental factors, but internal organs like kidneys and livers don’t show regional genetic patterns.
• 😀 When modern humans emerged in Africa around 100,000 years ago, they were not alone—Neanderthals lived in Europe and Western Asia, and Denisovans were present in Asia.
• 😀 Genetic evidence reveals that modern humans interbred with Neanderthals and Denisovans, with people outside Africa carrying traces of Neanderthal DNA (about 2.5%) and people from Melanesia carrying Denisovan DNA (about 5%).
• 😀 The study of ancient genomes, like those of Neanderthals and Denisovans, helps reconstruct the interactions between ancient human populations and how these interactions shaped the modern human genome.
• 😀 The key takeaway from studying human and extinct human genomes is that humans have always mixed—both with each other and with other human species—since the beginning of human history.

Q & A

What is the basic structure of DNA?

DNA is a double-helical molecule composed of two strands, each containing a sequence of four bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair in specific ways: A with T, and C with G, forming the genetic code.

How do mutations in DNA occur?

Mutations in DNA occur when errors are made during the replication of DNA strands. These errors result in incorrect base pairings, leading to genetic variations. Over time, mutations accumulate, offering insights into evolutionary processes.

How much genetic difference exists between two humans?

On average, two humans will differ by approximately 1,200 to 1,300 base pairs in their genomes. These differences help to define individual genetic variation.

What does the genetic difference between humans and chimpanzees tell us?

Humans and chimpanzees share approximately 99% of their DNA, with around 1% difference in the sequence of their genomes. This small variation has significant implications for understanding the evolutionary divergence of our species from our common ancestor with chimpanzees.

Why is there more genetic variation within Africa than outside?

Africa contains the highest genetic diversity because modern humans originated there and evolved in isolation for a long time before migrating out of the continent. As humans spread globally, they carried only a subset of the genetic diversity found in Africa.

What does the absence of absolute genetic differences between Africans and non-Africans suggest?

It suggests that there are no significant genetic barriers separating Africans from non-Africans. While there are differences in the frequency of certain genetic variants, no single genetic position can definitively distinguish a person from Africa versus one from other continents.

How are physical traits like skin color and facial features genetically determined?

Physical traits like skin color and facial features are influenced by many genetic variants, each with small effects. These traits vary in frequency between populations but are not determined by single, large-effect genes.

What role did Neanderthals play in the genetic makeup of modern humans?

Neanderthals interbred with early humans when they met outside Africa, particularly in the Middle East and Europe. As a result, people living outside Africa today have about 2.5% of their DNA derived from Neanderthals.

What are Denisovans, and how did they contribute to human genetics?

Denisovans were a group of ancient humans discovered in the Altai Mountains of Siberia. Modern humans living in Melanesia and nearby Pacific islands carry about 5% Denisovan DNA, indicating interbreeding between Denisovans and human ancestors in Southeast Asia.

What can the study of ancient human genomes teach us about human evolution?

The study of ancient genomes, such as those of Neanderthals and Denisovans, reveals that humans have always mixed with other human species. These interactions shaped our genetic diversity and help trace the movements and encounters of early human populations.