A handful of research groups have reported recovering DNA from insects trapped in amber and even dinosaurs entombed in coal—samples as old as 135 million years. But skeptics have shot back that intact DNA from old sources is more likely to be from some modern interloper in the sample, such as bacteria. Now researchers have an independent test of ancient DNA authenticity. On (page 864), an international team reports that a chemical change that converts amino acids in proteins from one mirror-image form to another—a process known as racemization—takes place at virtually the same rate as the degradation of DNA. If amino acids in a sample show this conversion to even a modest degree, original DNA is likely long gone. Any remaining genetic material should be a contaminant. When the researchers used this test on a variety of ancient DNA samples, they found that only those from insects trapped in amber appear to stand the test of time.