Darwin published the theory of evolution in 1859, but scientists haven't been able to watch the molecular gears of evolution actually turn until recently.
We can trace the ancestry of related species by counting discrete and quantifiable changes in their DNA. These changes are caused by mutations, or mistakes, that occur during reproduction.
After millions of generations, these mutations accumulate, resulting in significantly different genomes across species. But which mutations persist and which lead to evolutionary dead ends? Thirty years ago, one scientist started an experiment to answer this question. And it's still going.