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May 30, 2007

 The river Hilok and the birth of life on land (Trans-Sib 5)

Heading west into Russia from the Chinese border, the Manchurian branch of the Trans-Siberian railway crosses the Pacific-Arctic watershed in the first night and runs along the river Hilok, of which you've never heard. Google Earth placemark here; rather poor photo here. (They don't wash the windows much on the ageing N19 Vostok train. If it's nice images you want, here's Lake Baikal at sunset, but I've nothing to say about it.)

Natives of a small island, whose longest river is a mere 354 km long, become gawking rubes faced with the great rivers that drain continental masses. The 550 km Hilok runs into the Selenga, which fills Lake Baikal, which feeds the Angara, which flows into the Yenisei and the Arctic Ocean, at 5500 km from the farthest source. Amazingly, the Yenisei system was first navigated along its greatest length in 2001. And it's only one of four Siberian rivers over 4000 km long.

What's the total length of the watercourses of these systems? I tried a crude back-of-the-envelope calculation for the Yenisei. The Hilok has four tributaries of around 100km; it is one of eight confluents of the Selenga; the Selenga/Angara is one of three main branches of the Yenisei, with the eponymous but shorter main stem and the Tunguska. So I approximate the system as a symmetrical tree, with a 2,000 km main stem, 3 branches of 1,000 km, 24 tributaries of 500 km, and 96 sub-tributaries of 100 km, making 30,000 km. The total must be doubled if you count lesser streams, swamps (another placemark for the Barguzin valley) and deltas (placemark). The total length of the system's banks would then be of the order of 100,000 km - or roughly ten times the Arctic shoreline of Asia. The proposition generalizes to the world as a whole; the great river systems - the Mississippi, Nile, Congo, Yangtze, and the 500-pound alpha gorilla Amazon - together have borders with the land an order of magnitude longer than the world's seacoasts.

It must always have been thus, since the first continents stabilised 4 billion years ago. As the tectonic plates broke and merged over the aeons, they must have been drained by shifting river systems of similar scale and geometry to those we have now. The overall ratio between sea and river edges must have stayed within a not very wide range.

Where am I getting to with this? Simply that the popular evolutionary story that life first crawled onto land from the sea must be wrong. It surely came from the rivers. Geometry isn't the only reason; freshwater creatures - plants, bacteria, fungi, arthropods, and finally vertebrates - were pre-adapted to a land environment where the available water was also fresh. Tough luck Aphrodite.

PS: you have to install Google Earth for the placemarks to work. It's free and a great executive toy.

Update 16 June

The theory is also consistent with Ernst Mayr's principle that species typically evolve in small, geographically isolated populations under selective pressure : for unlike oceans, rivers are constantly generating isolated habitats like oxbow lakes, and this would have been even more the case before the ground was covered with a protective layer of vegetation.

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