Why was the 'cradle of mankind' located at Lake Turkana in Africa? Earth scientist José Joordens is well aware:'Lake Turkana was an inexhaustible source of water, even during dry periods.'
It is hot and dry, in the vast desert plain that forms the border between Kenya and Ethiopia. A trip to Lake Turkana, which is located in the middle of this plain, is therefore a real undertaking. 'Going out without a guide is life-threatening!', the tourist is repeatedly warned. But the reward is great. Here life begins again, albeit not in its sweetest form. Crocodiles swim in the lake, scorpions and poisonous snakes are hiding in the rocks around. But there are also flamingos, zebras and topis. And people live there – the Dasanetch, the Turkana and the Gabbra – even if they are few.
Hundreds of fossils
That used to be different. From three to two million years ago, the area around Lake Turkana was very busy. ‘Hundreds of hominin fossils have been found around this lake since 1968,’ explains earth scientist José Joordens, who obtained his PhD from the Free University in Amsterdam in 2011 for an NWO study into the link between primordial life in Africa and the development of the climate. The region is therefore also known as the cradle of humanity. But why actually? What were our predecessors, the hominins, looking for in this sweltering desert?
Always water
Joordens does have an idea what it was all about:water. Over the past four million years, East Africa has experienced periods of relatively high rainfall alternating with periods of severe drought. This alternation is caused by the rotation of the direction of the Earth's axis of rotation, Joordens discovered, which constantly changes East Africa's position relative to the sun. Joordens:'However, the Turkana basin has remained relatively wet all this time, and as a result has been the refuge to which the hominids returned every dry period.'
The Turkana basin was the drain of the drainage system of the Ethiopian highlands, she explains, being at the end of the funnel, as it were. This means that there was a permanent river link between Turkana and the Ethiopian Highlands, which allowed water to flow into the basin even in drier periods. Still is, by the way. This was not the case for other basins in the East African Rift.
Strontium in fish fossils
Joordens came to her conclusion by reconstructing the rainfall intensity in East Africa and comparing it with the habitat and occurrence of hominins in the lake area. Together with her Amsterdam colleague Hubert Vonhof, geochemist and isotope expert, she determined the ratio between the strontium isotopes 87 Sr and 86 Sr in fish fossils from ancient Lake Turkana deposits. In river water, this ratio is determined by that of the rock over which the water flows. In the lake, which consists of a mixture of the water from the different rivers that flow into it, you can calculate the proportion of each river from the 87 Sr/ 86 Deduce Sr ratio.
Two million years ago, the water in Lake Turkana was mainly supplied from the north by the Omo River, which flows over flood basalts with a relatively low 87 Sr/ 86 Sr ratio to the lake flowed. In the meantime, a number of much smaller rivers from the southwest reached the lake via a relatively 87 Sr-rich metamorphic rock. Whenever the proportion of Omo water in the lake increased, the isotope ratio went down. ‘And the share of Omo water went up when more rain fell in the mountain area in the north,’ says Joordens. Ultimately, the 87 Sr/ 86 Sr ratio as a measure of the intensity of the monsoon in the Ethiopian Highlands.
Refuge
Joordens' data provided unprecedented accuracy in climate data in the area, indicating that monsoon intensity was primarily driven by the precession cycle:the spinning of the Earth's axis of rotation, like the axis of a swinging hum. , with a period of about 21,000 years. But most striking was the fact that hominin fossils were found to occur in deposits from both the very dry and the very wet periods.
'We think that the Turkana basin was a so-called refugium (refuge)', says Joordens, 'thanks to the fact that the lake never ran dry.' It seems a plausible explanation, after all, water is a basic necessity of life. To quench thirst, to enable plant growth, but also as a habitat for fish and shellfish.
Enthusiastic
Henry Hooghiemstra, professor of paleoecology and landscape ecology at the University of Amsterdam, is extremely enthusiastic about the strontium isotope method that the researchers have developed. ‘It is a completely new and very smart way to indirectly measure the amount of precipitation. And it is a nice and independent addition to other methods that have been used so far, such as traditional pollen research.”
Weir lake
And the current inhabitants of the Turkana Basin? They have other problems on their mind. After being an oasis in the hot desert for four million years, thanks to the water supply from the mighty Omo River, Lake Turkana is likely to dry up soon. The Ethiopian government has far-reaching plans to build dams in the Omo, to create reservoirs and generate electricity. Construction has definitely started. It is expected that the supply of Omo water to Lake Turkana will stagnate, especially during the flooding of the reservoirs, says Joordens. In that case, the modern hominin will have to look elsewhere for the first time in four million years.
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