Mapping the genome of a horse from 700,000 years ago has raised tantalising possibilities
It remains one of the most intriguing premises for a science fiction film. Near the beginning of Steven Spielberg’s classic Jurassic Park, scientists reveal how they have collected the DNA of dinosaurs from mosquitoes trapped in amber more than 65m years ago. These insects had previously fed on dinosaurs’ blood and by extracting these blood cells, and removing their DNA, entrepreneur John Hammond (Richard Attenborough) recreates genomes of velociraptors, triceratops, and other dinosaurs. From these genomes, he clones the terrifying creatures that go on to terrorise the film’s characters.
At the time, the idea – originally outlined by Michael Crichton in his book Jurassic Park – sounded plausible until researchers pointed out that DNA simply cannot exist intact for such lengths of time. In fact, you would be lucky if you could go back more than a few thousand years before DNA becomes hopelessly fragmented, it was argued. Defences that repair DNA in living cells disappear after death. As a result, DNA strands quickly break up. Hence researchers’ limit of a few thousand years for the feasibility of creating an extinct creature’s genome.
But reality has a way of catching up on science fiction. Technical developments in DNA recovery have slowly transformed the business of recovering ancient genetic material. As a result, in 2010, a team led by Svante Pääbo, at the Max Planck Institute for Evolutionary Anthropology in Germany, revealed it had sequenced the entire genome of a Neanderthal based on three specimens at least 38,000 years old from Vindija Cave, Croatia. The work has since provided critical insights into modern humans’ genetic relationships with this ancient hominin species.
Since then Pääbo’s researchers have pushed their genome sequencing power even further into the past and last year announced they had sequenced the genome of a girl belonging to a species of humans called the Denisovans – a close relation to the Neanderthals – who lived about 80,000 years ago. The study showed she had brown eyes, hair and skin.
But now scientists have taken that sequencing age limit even further – and by a considerable margin. In Nature this week, a team led by Ludovic Orlando of Copenhagen University published a paper in which they reveal the entire genetic sequence of a species of horse that lived around 700,000 years ago – almost 10 times the current limit. It is an extraordinary achievement, one that immediately raises the prospect that scientists might soon create the genomes of creatures that died more than a million years ago, possibly several million years. By that reckoning, Crichton and Spielberg would not seem to be so far out.
Of course, Orlando and his colleagues were fortunate in one respect. Their horse genome was recreated from a bone fragment that was found in the Arctic permafrost at Thistle Creek, Canada. Its DNA had, in effect, been kept in cold storage for all that time. Nevertheless, unravelling it involved the use of advanced computing techniques, infomatics, and the ability to study the structures of proteins found in the sample. As Orlando stated: “We were amazed about the quality of the sample. We not only beat the record for [oldest] genome characterisation by almost an order of magnitude… we also discovered that a whole bunch of approaches can be used to characterise the deep evolutionary past.”
The work will certainly help scientists understand the evolution of the horse but it is the wider implications of Orlando’s work that really excites, a point stressed by molecular biologists Craig Millar and David Lambert in an accompanying editorial in Nature. The new research raises “the tantalising proposition that complete genomes several millions of years old may recoverable given the right environmental conditions”, they state. Ancient human forebears such as Homo erectus which first appeared in the fossil record around 1.8m years ago and more recent ancestors, such Homo heidelbergensis, may soon come within range of the genome sequencers, offering all sorts of insights into the evolution of Homo sapiens.
Of course, recreating ancient genomes does not mean we could actually clone such creatures. A host of ethical and practical difficulties would have to be surmounted before that became possible, issues that were ignored by Spielberg and Crichton. Nevertheless, science does indicate they were not quite as far from the mark as was previously supposed.