Ancient DNA refers to preserved DNA from subfossilized (not fully fossilized) remains of ancient animals or humans. For instance, ancient DNA has been successfully extracted from the bones of Neanderthals, extinct human relatives; mammoths, large elephant-like beasts that roamed the Earth throughout the last Ice Age, and Egyptian priests that lived 4000 years ago. Fossil evidence offers comparatively little information relative to what can be learned from sequencing ancient DNA, so it is much in demand and there are many geneticists who specialize in ancient DNA.
The concept of ancient DNA was popularized by the Steven Spielberg film Jurassic Park, one of the most popular movies of the 1990s. In Jurassic Park, dinosaurs are revived by extracting their DNA from mosquitoes preserved in amber that bit these dinosaurs shortly before becoming trapped in tree resin. Indeed, at one point it was thought that dinosaur DNA has been recovered, but subsequent analyses have shown that this is not the case, and the DNA in question is from contamination. Studies suggest that the oldest legitimate ancient DNA is about a million years in age. There is some disagreement as to whether DNA older than this may be preserved in amber.
Ancient DNA means that the genomes of some extinct species, such as Neanderthals, may be sequenced. The Max Planck Institute for Evolutionary Anthropology fully sequenced the Neanderthal genome in 2008, making it the second hominid genome (besides humans) to be fully sequenced. Ancient DNA for the Neanderthal Genome Project was taken from a 38,000 year old specimen found in a cave in Croatia.
Some experts on technological risk were taken aback when in 2007 the genetic code of the 1918 Spanish flu virus was determined based on DNA recovered from remains of humans who had succumbed to the disease frozen in pack ice. The 1918 Spanish flu was the worst global pandemic since the Black Death of 15th century Europe, killing 100 million people. This is more people than died in both World Wars combined. The genetic code of this Spanish flu virus was put openly online, for anyone to theoretically use. That is the downside of ancient DNA studies.