A Record of the Past
DNA: A Record of the Past
All organisms pass copies of their DNA to their offspring. Occasionally, mistakes are made in the copying process, and future generations inherit those mistakes. So human DNA today carries a record of mistakes, or mutations, that happened in our ancestors.
Differences between human DNA and DNA from another species can help biologists estimate when the two species branched apart on the evolutionary tree. And comparing DNA from different groups of living humans reveals the history of ancient human migrations.
You inherit half of your DNA from your mother and half from your father. Before the DNA is passed to the next generation, it gets recombined, or shuffled--that's what makes each person unique. But two chunks of DNA--mitochondrial DNA and the Y chromosome--break the rule. They never get shuffled, so from generation to generation they remain unchanged. As a result, scientists can use them to look millions of years into the past.
Mitochondrial DNA passes from a mother to all her children, but only the daughters pass it to the next generation. Experts can trace female ancestry by studying patterns of mutations in mitochondrial DNA. Some of the mutations happened millions of years ago, others more recently.
The Y chromosome passes only from father to son; females don't have a Y chromosome. Scientists use samples of Y-chromosome DNA to construct a lineage showing relationships among groups of people from all parts of the world.
Looking Back in Time
Scientists use DNA to reconstruct events in human evolution and human migrations. Dates from fossils of human ancestors help to confirm these findings.
Tracing the Human-Chimp Ancestor
By comparing DNA sequences from humans and chimpanzees, experts calculated that the last human-chimp ancestor lived roughly six million years ago. Later, the discovery of a hominid fossil dating back six to seven million years supported this claim.
Ancestors of Us All
To construct a single family tree connecting all humans, researchers analyzed mitochondrial DNA from people all over the world. They traced the root of that tree to a female lineage, nicknamed "mitochondrial Eve," about 150,000 years ago. All other female lineages alive at that time have died out.
Khoisan people in southern Africa carry the most ancient DNA mutations found in humans today.
If the human fossil record goes back millions of years, why don't we have DNA from all extinct hominid species? When animals die and decompose, chemical reactions break down the DNA inside their cells. Minerals gradually replace any remaining bone, and after 100,000 years or so the DNA
Out of Africa
By comparing DNA sequences from humans living all over the world today, researchers learned that modern humans first migrated from Africa into Asia and Australia about 60,000 years ago.
The Last Neanderthals
Scientists have extracted fragments of DNA from several fossils of Neanderthals, the extinct hominid species most closely related to modern humans. DNA comparisons have proven that modern humans did not descend from Neanderthals, nor did the two species interbreed.