Hippos survived the Ice Age in Europe, new DNA evidence reveals

Central European hippos were thought to have gone extinct around 115,000 years ago, when temperate conditions in the region became frigid with the end of the last interglacial period and the start of the last ice age.

A new study published in Current biology rewrote the timeline of the history of hippos in Central Europe, using radiocarbon dating on fossil remains from Germany’s Upper Rhine Graben, a fossil-rich site that serves as a natural archive of the continent’s climatic past.

Researchers found that these huge mammals did not disappear when the climate cooled 115,000 years ago. The fossils were approximately 47,000 to 31,000 years old, suggesting that these powerful creatures held out much longer than expected, surviving deep into the last ice age.

Central Europe’s hot summers were quickly replaced by frigid days and nights during the last ice age, known as the Weichselian glaciation, which began about 115,000 years ago and lasted until about 11,700 years ago.

Hippos need mild climates, with plenty of vegetation and unfrozen water to survive, which is why they are often seen as a clear indicator of warm interglacial periods. On this basis, scientists have long believed that hippos disappeared from Central Europe at the very beginning of the Ice Age, when colder conditions and sparse vegetation made the region unsuitable for them to settle.

This hypothesis has been further strengthened by the way in which the fossil evidence has been interpreted. Sites such as those in the Upper Rhine Graben were traditionally thought to capture the transition from the warm Eemian, the last interglacial period, to the early, cold Weichselian. As a result, any hippo fossils discovered in these layers were automatically assigned to the Aemian period.

However, the exact timing of the extinction of hippos in Europe remains uncertain, largely because scientists did not have the ancient DNA of these Ice Age giants. Without molecular evidence, they couldn’t even determine whether extinct European populations were genetically related to their modern-day African hippos, leaving key questions about evolution unanswered.

To solve this mystery, the researchers analyzed 19 hippo fossils found in the Upper Rhine Graben and extracted a partial paleogenome from one of the samples. They then compared the ancient genetic data with the genomes of modern hippos to trace the evolutionary relationships between the extinct European population and today’s African hippos.

The results indicated that Ice Age hippos were closely related to today’s common African hippos.

To determine the exact time these animals lived, the team used radiocarbon dating and amino acid geochronology. Calculations showed that instead of disappearing more than 115,000 years ago, as scientists once believed, these hippos were still present 31,000 years ago, at the depths of the ice age. Dating of other Ice Age animals from the same site, such as woolly mammoths and woolly rhinoceroses, confirmed these results.

The hippos probably survived because the ice age was not always harsh. Instead, there were periodic warm phases called interstadials that created localized refugia in the Upper Rhine Graben, with sufficient unfrozen water and vegetation to support them. However, the genome revealed low diversity, indicating that these hippos belonged to a small, isolated population rather than a large interconnected population.

The researchers suggest that the survival of hippos may not represent a population persisting stubbornly throughout the ice age, but rather a series of short-term recolonization events in which the creatures took hold during warmer phases and left when conditions became inhospitable.

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