The genome of a 700,000-year-old fossil horse bone has been mapped, breaking the record for ancient DNA reading. The question is which species we will soon be able to read from long extinct species.
Until this week, the oldest genome fully mapped came from a 110,000-year-old polar bear jaw. But after the research of the Danes Eske Willerslev and Ludovic Orlando (University of Copenhagen), the record goes much further back in time. The three billion letters of DNA they elucidated come from a horse that lived about 700,000 years ago.
Since the foot bone was discovered in 2003 in Thistle Creek, Canada, Willerslev and Orlando have worked with a large international team to read the ancient horse genome. They calculated that it was that old by using the age of volcanic ash from the same layer of the earth. The researchers also mapped the DNA of five modern horses, a horse species from 43,000 years ago, a donkey and the Przewalski's horse.
Pure pure
By comparing the DNA maps, the scientists discovered that the Przewalksi horse is the closest living relative of our modern horses. Moreover, it is a pure 'wild' breed, at least no traces of the DNA of domestic horses were found in the genome map. It also appears that the last common ancestor of donkeys and horses lived between 4 and 4.5 million years ago - before the last ice age - about twice as long ago as previously thought. Horse population size has varied greatly over the past two million years, mainly during periods of strong climate change.
DNA in decay
The work is special because DNA decays slowly. When cells die, the DNA also slowly breaks down. The repair mechanisms of the cell stop working and the enzymes present, called nucleases, break down the bonds between the nucleotides. Bacteria take over that work. If water and oxygen are present, the process goes even faster. And if it's too hot, you can completely forget the hope for ancient DNA. Last year, scientists from the same university discovered that DNA has a half-life of 521 years. That means that after 521 years, half of the nucleotide bonds have been broken. Another 521 years later, half of the remaining connections are still left, and so on. The researchers then estimated the absolute limit of DNA survival should be around 6.8 million years, provided the DNA has been preserved in an optimal cold, dry environment. But because these are such small quantities, in practice the limit will be a few million years less.
Ancient Human Card?
The question is which ancient DNA maps we can expect in the coming years, now that the techniques for reading this DNA are also being further refined. Scientists eagerly await a DNA map of our ancestors, such as Homo heidelbergensis and Homo erectus . But whether these species ventured into northern cold regions where temperatures were optimal for DNA preservation is not certain. Until then, we'll have to make do with the Neanderthal genome, which was read from 44,000-year-old bones, and that of its relative, the Homo denisova . Incidentally, the permafrost is also melting hard, so if there are remains they must be found quickly.