In addition, there has never been a systematic excavation of fossils in the Upper Rhine region. Instead, the bone finds came to light in industrial gravel pits and found their way from there into private collections. Many pieces are damaged or destroyed, and it remains uncertain in which soil layers they originally lay. “For scientific work, this is actually a worst-case scenario,” Arnold says. To make things worse, the fossils are contaminated by vast amounts of foreign genetic material from humans, animals, fungi, or bacteria. Identifying the remains of a hippopotamus beyond any doubt is therefore no simple matter. In the jumble of DNA fragments, even a colossus weighing several tons becomes virtually impossible to detect.
Researchers break with old paradigm
Yet the discovery of hippopotamus fossils alone is no longer enough to throw a paleontologist in Europe for a loop. On the contrary: these chubby, big-mouthed creatures with short legs have left their mark from Southern Europe through France all the way to England. Only mostly at a time when it was generally warmer than in our pre-industrial era. Typical representatives of this Last Interglacial Period were water buffaloes, wild horses, steppe rhinos, and even giant forest elephants. For a long time, hippos were considered the quintessential indicator species of warm periods.
“On the one hand, hippos need large quantities of grass on land; on the other hand, the water must never freeze because they spend most of the day lying in the water,” Arnold says. When his DNA analyses placed the “big guys from the Upper Rhine” – as the weekly newspaper ZEIT put it – in the middle of the Ice Age of all times, there was initially a lot of skepticism among experts. For a long time, hardly anyone had questioned the widespread circular reasoning that regarded the sedate swimmers as a paleontological indicator species: if hippos lived there, the finds could only date from the Last Interglacial Period, about 120,000 years ago. And since these are finds from the Last Interglacial Period, there are also hippos in the Upper Rhine. Or is that not right?
“The fact that colleagues from Mannheim dated the finds using the radiocarbon method at all is almost heretical,” Arnold says. This is because the carbon-based method reaches its physical dating limit at 50,000 years. Many additional tests were required to conclusively rule out potential sources of error and uncertainty, including the carbon in the oil-based varnish with which the collectors had coated the artifacts. Nevertheless, the scientists would likely have been unable to extract usable DNA if the finds were not so young. After all, the DNA molecules survive for a period of 120,000 years, if at all, in Siberian permafrost.
Heat Islands in the Ice Age
And the hippos? They form another piece of the puzzle for a theory that paleontologists have long been pursuing based on soil samples and plant remains: at least in some phases, the Ice Age was not all that cold. Rather, it may well have been quite bearable in parts of Europe, with winters in which at least the rivers did not freeze. “Even today, the Rhine Valley is one of the warmest regions in our country,” Arnold says. “That’s why it is ideal for winegrowing.”
While there is no concrete evidence that humans also shared these warm oases with the hippos, this cannot be completely ruled out – on the contrary. In terms of time and geography, there would certainly have been an overlap between hippos and the genus Homo, and other sites in Germany suggest that mammoths, for example, were hunted by Neanderthals or even modern humans.
How did the hippopotamus cross the Mediterranean?
And another finding has surprised experts: these barrel-shaped pachyderms not only survived in Europe well into the Ice Age, in small groups of just a few individuals, but were also anatomically and genetically identical to today’s hippos in sub-Saharan Africa. This suggests that these animals, which evolve very slowly, may have repeatedly made their way from their African homeland to Europe over many thousands of years.
As is so often the case, new findings also raise new questions: How did a hippopotamus get from Africa to Mannheim? And not just once, but time and time again. A continuous river connection probably did not exist. The route via the Rhône into present-day Switzerland and on to Alsace is considered a possible migration route. Small hippopotamus species also existed in Sicily and Cyprus, though they were genetically different from the hippopotamuses in Europe and Africa.
Phylogenetically, hippos are considered the closest relatives of whales. Nevertheless, they are comparatively poor swimmers and are built more to float in the water like buoys. But crossing the open sea? One thing is clear: with the onset of the Last Glacial Maximum about 30,000 years ago, it finally became too cold even for the hippos on the Rhine. “Presumably, at some point the Rhine also froze over,” Arnold says.
This article appeared in the university magazine Portal - Eins 2026 „Inklusion“.