Hothouse Climate

Mammals triumph in the Eocene

The Eocene, named after Eos, the Greek goddess of dawn, began about 55 million years ago. The Earth had a hothouse climate with lush vegetation and dense primaeval forests. 

In the warm, tropical Thetis Sea, today's Sahara Desert, the first great ancestors of whales, such as Basilosaurus, evolved. And in the steppes and savannas, mammals began their worldwide triumphal march. The orders of odd-toed ungulates, the ancestors of horses and rhinoceroses, bats, rodents, and primates, from which all apes, and humans themselves evolved, emerged.

In addition, the Eocene produced the largest land mammals of all time. The ancestors of rhinos and horses reached heights of up to 4 metres and weights of up to 15 tonnes.

Throughout the Eocene, both Greenland and Antarctica were ice-free. South America, Australia, and Antarctica were still connected and formed a remnant of the once great southern continent of Gondwana. The polar ocean current that circles Antarctica today did not yet exist. Instead, warm water penetrated southward along the coasts of the continents. As a result, the climate in Antarctica was much milder than today. And there was also a land bridge between Eurasia, Greenland, and the North American continent.

At the end of the Eocene, about 36 million years ago, the climate finally became cooler. The continent of Antarctica shifted south and froze over. The warm seas and humid primaeval forests disappeared.

Scientists know this striking cooling, accompanied by the extinction of many primitive mammal species, as the so called Great Cut. In the Oligocene following the Eocene, the remaining primordial mammals evolved into modern mammals as we know them today.