The genomic sequencing of Mammoth teeth preserved in Eastern Siberian permafrost have produced the oldest ancient DNA on record. Genomic DNA extracted from a trio of tooth specimens from the 1970s has identified a new type of mammoth that gave rise to a known North American species. Here we summarise two recent articles (1, 2), published in Nature.
Accessing Ancient DNA
Researchers had suspected that ancient DNA may be able to survive beyond 1 million years. However, until now it had only been a hypothesis. When an organism dies, its chromosomes shatter into pieces that become shorter over time. Eventually the DNA strands become so small that even if they can be extracted, their information content is lost. However, preservation within constant permafrost slows DNA fragmentation. The sample in this study was discovered within optimal conditions to stay preserved.
Dating the Remains
Thanks to advances in sequencing technologies and bioinformatics, the team were able to genetically sequence the remains. The oldest sample produced 49 million base pairs of nuclear DNA. Analysis of this genetic material suggested it was 1.65 million years old.
The authors dated the mammoths using a combination of approaches. They used biostratigraphy, in which faunal remains at the site were compared to existing fauna for which absolute dates are available. They also conducted molecular dating of the mitochondrial DNA, as a higher percentage of the mitochondrial genome was able to be sequenced than that of nuclear DNA.
A New Species?
Based on their shape, the two oldest teeth looked like they belonged to steppe mammoths. These are a European species that researchers believe pre-date woolly mammoths and Columbian Mammoths (a North American species). However, their genomes painted a more complex picture. The oldest sample was believed to belong to an entirely new species, named Krestovka.
Krestova was not from a population ancestral to woolly mammoths. Instead, the researchers propose that Krestovka was ancestral to mammoths which entered North America about 1.5 million years ago and gave rise to the Columbian mammoths. While researchers can’t say for sure that it’s a different species, the data certainly supports this hypothesis.
Their findings also supported the hypothesis that new species can form via mixing, and not just splitting from a single parent species. The Columbian mammoths trace half their ancestry to woolly mammoths, and the other half to the Krestovka lineage. The data suggests these two lineages mixed more than 420,000 years ago. Therefore, this is the first evidence for ‘hybrid speciation’ from ancient DNA.
The Future of Ancient DNA
While the chances of finding million-year-old remains of ancient human relatives in the permafrost are very low, with the right environment, such as a deep cave, we may discover samples of that age. Currently, the oldest ancient human relative discovered so far is dated to 430,000 years ago.
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