Horses, Grasses, and Climate

The evolution of the horse family over the last 50 million years is closely related to the spread of grassland ecosystems.

The ancestors of the horses, donkeys, or zebras we know today were small animals, not much larger than a dachshund.

Best known in Germany is the Eurohippus shown here, discovered in the bituminous shale pit of Messel. It was fossilized around 48 million years ago, together with many other species, in the muddy bottom sediments of a deep lake.

Several processes working together have contributed to the reduction of atmospheric CO₂ since the Eocene and the resulting phase of cooler temperatures. An important factor is increased ocean plankton growth. As these plankton cells died, they settled to the ocean bottom and were buried – along with their carbon – in deep-sea sediments. These sediments have held the carbon ever since. The resulting long-term loss of carbon from the system reduced the greenhouse effect of the atmosphere. In turn, this reduced average global temperatures and set the stage for the later development of the ice ages. Today, the reverse process is happening: The burning of fossil fuels like natural gas, mineral oil, or coal releases carbon that had been held in the earth for 100s of millions of years. The result is a sharp increase of CO₂ and global temperatures.

(Image: A modern phytoplankton species, Asterolampra marylandica, J.R. Dolan & G. Hagedorn, CC BY-SA 3.0)

On the long journey from the tiny Eurohippus ...

... to the majestic horses of our human history, climate and grassland ecosystems were important travel companions!

(Horse sculpture from Halikarnassos, 350 B.C., British Museum)