← Trends in human evolution

8 DNA evidence: mtDNA, nDNA, and molecular clocks

A recurring question asks specifically how DNA evidence is used to reconstruct human evolution — a genuinely distinct line of evidence from fossils, worth its own section.

How DNA evidence is used

  • Mitochondrial DNA (mtDNA) is inherited only from the mother, so it traces maternal lineage directly back through generations without being reshuffled by recombination.
  • Nuclear DNA (nDNA) comes from both parents and is far larger, giving a fuller picture of ancestry and relationships between populations or species.
  • A "molecular clock" uses the roughly constant rate at which mutations accumulate over time to estimate WHEN two lineages diverged from a common ancestor — more differences between two genomes implies a longer time since they split.
  • Comparing mtDNA/nDNA differences between modern human populations is what reveals the SERIAL FOUNDER EFFECT supporting Out of Africa: the further a population's ancestors travelled from Africa, the less genetic diversity it retains.
Exam-style Using DNA divergence to support Out of Africa Explain how new DNA evidence can support the Out of Africa theory of modern human origins. Sequencing mtDNA and nDNA from populations around the world allows researchers to compare the accumulated genetic differences between them. Applying a molecular clock to those differences estimates how long ago each population's lineage diverged from a common ancestor. African populations show the greatest genetic diversity of any region, consistent with modern humans having originated there and had the longest time to accumulate variation — while populations further from Africa show progressively less diversity, consistent with a series of founder effects during migration. ✓ The chain from mutation rate, to divergence time, to a diversity gradient, to Out of Africa is what an Excellence-level DNA-evidence answer traces explicitly. ✕

Treating DNA and fossil evidence as interchangeable

Fossil evidence gives you anatomy and, with dating, a rough timeline, but is patchy and can be ambiguous about species boundaries. DNA evidence gives you relationships and divergence times, but only from recent or exceptionally well-preserved samples, and says nothing directly about anatomy or behaviour. A strong Excellence answer states what EACH line of evidence can and cannot tell you, rather than treating them as the same kind of proof.

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