Browsing Antelope Drove the Expansion of the African Savannas

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By Michelle Mukonyora
28 September 2016

Savannas, rich grassy landscapes with trees that are spaced such that sufficient light reaches the ground, are more than a backdrop for romanticised safari images. They account for 20% of the earth’s land area, and are home to a rich and complex collection of flora and fauna. In Africa, savannas play a critical role, both socially and economically in rural communities.
The expansion of savannas is believed to have been a key contributor to human evolution. 4.4 million years ago, our tree-dwelling ancestor Ardipithecus ramidus went from navigating through trees, to walking upright on the grasslands. It was at this point in time that hominids diverged from the last common ancestor we shared with modern day chimpanzees. But the question remained, what initially drove the expansion of the African savannas from what was once ancient forests?
Illustration of Ardipithecus ramidus by paleoartist Jay Matternes

Medium-sized antelope destroyed ancient forests
Two researchers from the Universities of Witwatersrand and Cape Town met at a conference and coincidentally found that the abundance of medium and large browsers on the African continent correlated with the density of spiny trees. They then collaborated with another researcher at the University of Johannesburg who reconstructed the evolutionary history of the spiny trees using the DNA sequences of African trees. The results revealed that thorns evolved on 2000 woody tree species 15 million years ago. This was the same time that the medium-sized antelope arrived in Africa from Eurasia. The medium-sized antelopes decimated the ancient young forest trees. They therefore provided a selective pressure on the ancient forests, and the trees with thorns gained an advantage. According to carbon dating of fossil records, it was only later that fire played a role in further increasing the spread of savannas.

Browsing antelope. Credit: Gareth Hempson
Why is this relevant?​
Besides the cool factor of an explanation of how African savannas spread, this study is important because it highlights the importance of conserving our browsing antelope. African antelope are under threat as a direct consequence of climate change. If we fail to protect them from extinction, the savannas will be replaced with vegetation of little economic value, such as scrubby woodlands.
References:
1. Tristan Charles-Dominique, T. et al. Spiny plants, mammal browsers, and the origin of African savannas. PNAS, September 2016 doi:10.1073/pnas.16074931132. University of the Witwatersrand. “Browsing antelope turned ancient African forests into grassy savanna ecosystems: Study of thorn trees in African savanna reveals astonishing comparisons to the evolution of thorns and the arrival of antelope in Africa.” ScienceDaily. www.sciencedaily.com/releases/2016/09/160906103156.htm (accessed September 28, 2016).