The East African Rift Valley: Where We Became People

Annamaria on Monday

To geologists, the most interesting place is in East Africa: the largest seismically active rift system on earth.  What’s happening there is that new tectonic plates are being formed by their spreading apart from the landmass.  This the opposite of where tectonic plates have pushed into each other to form mountain ranges.  In this case, when hunks of a tectonic plate separate, it causes chasms.  In the distant past, other such divorces have taken place on our sacred planet, but they are all now underwater or silted in—no longer active.

The separations—and there are two of them—taking place in East Africa have been going on since the onset of the Miocene about 25 million years ago.   The Nubian Plate and the Somali Plate are still moving, now at the rate of six or seven millimeters per year.  In another 10 million years or so, the Somali Plate will be completely broken off, and the sea will surround it.

Earth scientists are fascinated to be able to study this process and to theorize about why and how it is happening in this geologic wonder of the world.   The rift area includes not only parts of Ethiopia and Somalia, but also extends into Kenya, Uganda, Tanzania, and the African Great Lakes.  The rift is deepest north of Nairobi.  Theories vary as to why this process is underway just there and no where else on earth.  It seems that, like many thorny scientific questions, this one is a result of interactions of more than one force, making it difficult to sort out.

We know that the East African Rift Zone is the site of many dormant and active volcanoes, fifty of them in Ethiopia alone.  The Crater Highlands of Tanzania are part of this system, even though they now lie outside the rift area.  One of the active volcanoes is unique: The magma of the Ol Doinyo Lengai volcano contains no silica, so it has extremely low viscosity.  National Geographic described it this way: “Its lava fountains crystallize in midair then shatter like glass.”  Wow, wouldn’t I love to witness that!

Ol Doinyo Lengai

Most fascinating to me is that this is where our race Homo Sapiens evolved.  The bones of our hominid ancestors have emerged from the sediments of the Rift Valley highlands.  These include the famous “Lucy”, the partial australopithecine skeleton from 3 million years ago, discovered by anthropologist Donald Johanson.   The rift in Ethiopia more recently yielded two other 10-million-year-old hominid ancestors: Chororapithecus abyssinicus and Nakalipithecus nakayamai.

Since so many hominid fossils have been found in the Great Rift Valley, scientists have begun to think that the evolving conditions there played a pivotal role in the development of our species.  Current theorists believe that the local situation caused the climate to alternate between wet and arid, which forced our hominid ancestors to adapt by becoming bipedal, increasing their brain power, and developing culturally.  My favorite article on this theory is the 2008 paper by Beth Christensen and Mark Maslin, delightfully titled “Rocking the Cradle of Humanity.”

For me personally, the African wilderness plays in my blood and in my soul in such a way that my response runs far deeper than ordinary joy.   This is where our species evolved, and I believe that, when we human beings go there, regardless of where we were born, on a cellular level, we recognize the place as home.  Ever since I first went, when I am not there, in my soul, I am homesick for it.