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The oldest surviving proteins have been found in the tooth of Elasmotherium, a prehistoric rhino with one unicorn-esque horn.
These protein sequences are 20 million years old—far older than the previous oldest sequences, from mammoth (1.2 million year old) and camel ancestors (3.7 million years old), respectively.
Elasmotherium was found to be closely related to ancient rhino lineages from Eurasia, even though the tooth was found in Canada.
Elasmotherium sibericum, an ancestor of the rhinoceros, once lumbered across the steppes of what are now parts of Eastern Europe and the extreme north of Canada. This prehistoric beast was covered in shaggy fur (much like its relative, the woolly rhino), and brandished a single horn on its forehead which earned it the name “Siberian Unicorn.” Its genes were hiding another unicorn.
Most ancient DNA (aDNA) over 1 million years old is far too fragmented to reconstruct genetic sequences. However, a record of the evolutionary past was preserved in the tooth of one Elasmotherium, which had been buried in the permafrost of the Haughton Crater in Canada’s High Arctic tundra for over 20 million years. It has seen minimal effects of diagenesis—the physical and chemical changes that occur in sediments as they fossilize—and some of the original proteins had been frozen in time.
The Elasmotherium tooth (discovered by a research team from the University of York) has revealed the oldest known sequence of proteins and amino acids. Before this find, the oldest known DNA sequence pulled directly from a fossil came from the 1.2-million-year-old molar of a mammoth recovered from Greenland. And the protein and amino acid sequences from the bone collagen of camel ancestors (also found in the High Arctic), were 3.7 million years old. Any sequences older than that were degraded beyond understanding.
“Fossils from these sediments are found in a polar landscape, at present characterized by permafrost,” the researchers said in a study recently published in the journal Nature. “Compared with similarly aged material from lower latitudes, this creates a temperature regime favorable for biomolecular preservation, sparing these fossils from the harshest effects of diagenesis.”
Genetic material in labs is usually kept in deep freezers, because DNA is so prone to degrading. This explains why there is little to be found in dry bones, but better preserved sequences—some with soft tissue and even liquid blood—have been seen in specimens that emerged from thick sheets of ice. The York team focused on extracting proteins and amino acids from the enamel of the tooth, since the tightly packed substance protects genetic material. At least seven proteins and 251 amino acids were recovered, and the results did not support what had previously been assumed from the morphological study of rhinocerotid fossils.
The researchers used paleotemperatures from an existing model to see how fluctuations in temperature in the region over millions of years could have affected biomolecules, and their results were consistent with previous findings that determined how well proteins survived through the Miocene (23 to 5.3 million years ago), Pliocene (5.3 to 2.6 million years ago), and Pleistocene (2.6 million to 11,700 years ago) epochs. Comparing the degradation of the proteins to those in other rhinocerotid specimens proved that the new samples were not contaminants, but did in fact belong to the tooth of an Elasmotherium.
Rhinocerotids experienced a burst of diversification before this particular creature was even born. They first thundered onto the scene during the Middle to Late Eocene (47.8 to 34 million years ago) and diversified significantly before the Early Oligocene (34 to 32 million years ago). Analysis of the Elasmotherium protein sequences has determined that this species split from other rhinocerotids some 41-25 million years ago, and that the main rhino clades Elasmothieriinae and Rhinocerrotinae diverged 34-22 million years ago. Despite being found in Canada, Elasmotherium is most closely related to early rhinos across the ocean.
“Morphologically, the Haughton Crater rhinocerotid shares closer affinities with these early-diverging lineages from Eurasia, particularly those in the genus Epiaceratherium,” the researchers said. “Similarly, some other vertebrates in the highly endemic fauna of the Haughton Formation have their closest relatives in Eurasia.”
There are no Elasmotherium de-extinction efforts on the horizon, but this new rhino evolution knowledge could help increase population numbers of endangered rhinos and prevent their extinction.
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