Global warming is just the latest wrinkle: People have been changing the environment since urban settlement began, archaeologists first discovered in Israel. Now it seems our impact began even earlier. Some oddly lush hot spots in modern Africa developed where prehistoric herders and their beasts had camped thousands of years ago, archaeologists report in Nature.
The study by Prof. Fiona Marshall of Washington University in St. Louis and colleagues was done in Kenya, where they found certain areas strangely devoid of trees in the wooded grassland that otherwise thronged with life.
It bears noting that, in contrast to wild Western imaginations of bananas and pawpaws sprouting everywhere one looks, Africa’s grassland savannas tend to be poor in soil nutrients. Hence, sudden green spots here and there, let alone rich in wildlife and thronged with lizards, micro-mammals and insects, beg explanation.
The explanation the archaeologists found is that come the rainy season, spots where nutrient-rich manure emitted by animals piled up over thousands of years sprout wildly.
And why did nutrient-rich manure pile up in these spots? Marshall and the team believe these were prehistoric livestock corrals, and that their location explains patterns existing today, even massive phenomena like animal migration routes.
These green spots today were where prehistoric herders had corralled their quadrupeds to protect them at night as much as 3,000 years ago, they deduced. The soil beneath these ancient brush-walled paddocks, including in the famous Maasai Mara (the Kenyan part of the Serengeti National Park), is still rich in phosphorous, magnesium, calcium and other nutrients, while the surrounding grassland is not.
“Our research suggests that some of these patches may be the result of prehistoric pastoral settlement in African savannas,” Marshall says.
Make no mistake, these green patches are small. They are oval-ish and roughly about 100 meters (about 330 feet) in diameter. They are not sweeping grassland as far as the eye can see.
Another sign that they’re special is their high density of worms and insects. These in turn attract birds, lizards and rodents that feed on them, the team writes.
Wondrously, in 2013 scientists from Yale University showed that a type of dwarf striped gecko that thrives in these Kenyan nutrient hot spots, Lygodactylus keniensis, is extremely rare in the grassland between them. The same team found that trees growing near the edges of these hot spots grew faster and were generally larger than trees elsewhere in the savanna.
Even after the people and their corrals disappeared, the dung left behind would have produced green spots that would reemerge every rainy season. With the corral fencing gone, migrating animals would have learned over the generations to go from one green spot to the next. Makes more sense than ignoring the luscious greenery and going for the nasty brown grass, even to a gnu with a brain the size of a petite papaya.
In the wake of the wildebeest and other grazing quadrupeds, major predators would congregate too. And thus the Serengeti, which stretches from Tanzania to Kenya and which means “unending plain” in Maa, became renowned for its lion population.
The study’s starting point was satellite imaging of tree-bald green patches in East Africa. The team then reached “a surprisingly simple explanation” for how these oval-shaped sites developed in the otherwise nutrient-starved Kenyan savanna: “Manure happens.”
The goat, sheep and whoever else would probably have been let loose to graze by day. But come night, they would have been confined to the oval corrals for protection from predators and thieves. There, they would relieve themselves, concentrating all the nutrients from the surroundings that they had eaten in these corrals.
Nomadic pastoralists in the prehistoric Middle East also shepherded their livestock over the hills during the day and locked them up at night, even if they themselves dwelled in tents. Some Bedouin nomads continue this practice to this very day. Goats and sheep had been domesticated in Mesopotamia over 10,000 years ago, for what that’s worth.
Marshall also notes in her blog that nutrient hot spots were found to have formed in the same way in the otherwise inhospitable Sahel and southern Africa.
Of course, green spots in the brown savanna could have been caused by other things – such as volcanism, which, once the lava cools and starts to break down, creates nutrient-rich earth. However, volcanism doesn’t create round paddocks. Brush fire ditto: It can be very enriching, but it wouldn’t create the sort of patterns the archaeologists could see from the air. Termite mounds could, but they would have had to have been huge.
The Neolithic herding explanation makes more sense, even after all this time. “Our research shows that the positive impacts of increased soil fertility in herder settlement corrals can last for thousands of years,” Marshall explains. “The longevity of these nutrient hot spots demonstrates the surprising long-term legacy of ancient herders whose cattle, goats and sheep helped enrich and diversify the vast savanna landscapes of Africa over three millennia.”
It becomes a non-vicious circle. The green attracts herbivores (including the maligned warthog), which deposit yet more dung, which fertilizes the soil even more, as does their blood when a carnivore happens along and predates on one of these happy campers.
If their animals’ excretions had created such lovely lush spots, why would the Neolithic African herders have moved on? They were nomadic in culture. They would create brush walls around corrals in one spot and then, when the conditions required it, would move on and build another corral somewhere else. And thus over the ages, the cultural practices of mobile herders wound up creating spatially stable fertile environmental niches that today hosts a wild proliferation.
In ancient Israel, archaeologists reported the opposite effect, and slightly earlier than the periods covered by the African study.
A paper published in 2014 by French and Israeli scientists reported on the influence of humans on their environment in ancient Israel. Before people settled in the area that would become the city of Acre (also known as Akko), it featured indigenous forest – mainly oak trees and pine.
After people settled, the trees disappeared and were replaced by the low brush that characterizes the plain. The French-Israeli study concluded that ancient urbanization did not develop in a sustainable way within the environmental constraints of local natural resources. Nor could climate change be blamed for the changes found in the ecosystem of Acre and its environs around 4,000 years ago, the scientists argue: If anything, precipitation had picked up. It was people and their agriculture that caused the ecosystem to change from Mediterranean forest to savanna.