Rusinga Island, with an elongated shape approximately 10 miles (16 km) from end to end and 3 miles (5 km) at its widest point, lies in the eastern part ofLake Victoria at the mouth of theWinam Gulf. Part ofKenya, it is linked toMbita Point on the mainland by the new Rusinga-Mbita bridge which replaced the old causeway.[1]
The local language isLuo, although the ancestors of the current inhabitants wereSuba people who came in boats several hundred years ago from Uganda as refugees from a dynastic war. Many Rusinga place names portray Suba[2] origins, including the island's name itself and its central peak,Lunene. There was an extinct language of Uganda called Singa, alternatives Lusinga and Lisinga, spoken only on Rusinga Island (which, of course is in Kenya).[3] As of 2006, estimates of Rusinga's population ranged between 20,000 and 30,000. The entire island is part of theHoma Bay County.
Most residents of Rusinga make their living from subsistence agriculture (maize and millet), as well as fishing. The nativetilapia is still caught, though this species (like all others native to the lake) has been decimated by the voraciousNile perch that was introduced into the lake in 1954. Constant onshore winds cool the lakeward side of the island and provide clean beaches with ideal swimming and boating conditions, but poor roads between Rusinga and the nearest town,Homa Bay, inhibit trade and tourism. The brightly glittering black sands of the beaches are made of crystals of melanitegarnet, barkevikitehornblende, andmagnetite eroded from theuncompahgrite lava fragments in theagglomerates that overlie the fossil beds.
Theisland is also notable as the family home and burial site ofTom Mboya, who before his assassination in 1969 was widely pegged asJomo Kenyatta's successor as President of the new nation of Kenya.
Rusinga is widely known for its extraordinarily rich and important fossil beds of extinctMiocene mammals, dated to 18 million years.[4] The island had been only cursorily explored until the Leakey expedition of 1947-1948 began systematic searches and excavations, which have continued sporadically since then. The end of 1948 saw the collection of about 15,000 fossils from theMiocene, including 64primates called byLouis Leakey "Miocene apes."
All the species ofProconsul were among the 64 and all were given the nameafricanus, although many were reclassified intonyanzae,major andheseloni later.Mary Leakey discovered the first complete skull of Proconsul, then considered a "stem hominoid", in 1948. Excavation of the fossil was completed by Louis' assistant, Heselon Mukiri (whence Walker's 1993 name heseloni). Many thousands of fossils are now known from five major sites, with abundant hominoids including an almost complete skeleton of a second species ofProconsul, as well asNyanzapithecus,Limnopithecus,Dendropithecus andMicropithecus,[5] all of which show arboreal rather than terrestrial adaptations. The first truemonkeys do not appear until around 15 million years ago, so it is widely supposed that the diverse Early Miocene African catarrhines like those found on Rusinga filled that adaptive niche. The phylogenetic position of these primates has been debated. It has been theorized thatProconsul is a stem catarrhine and therefore ancestral to bothCercopithecids (Old World monkeys) andhominids (great apes and humans), rather than a stem hominoid.[6]
Pleistocene mammal fossils, including an extinctantelope genus,Rusingoryx, notable for its nasal dome hypothesised to produce loud calls, known nowhere else, are also common in former shoreline deposits around the edges of the island, left behind as Lake Victoria has slowly subsided over the centuries due to erosion in its outlet.
The fossil beds are layers of volcanic ash produced by a succession of explosive eruptions during the earliest stages of a volcano that eventually covered an area 75 miles in diameter. The volcano is now eroded down to the frozen magma in its vent that makes up theKisingiri hills on the mainland opposite Rusinga, and the surrounding remnants of the cone: the semicircularRangwa mountain range, and the islands of Rusinga and neighboringMfangano Island. Thisrift valley volcano on the southern flank of the now-inactive Winam Gulf tapped much deeper in the mantle than oceanic orsubduction zone volcanos, and its lavas and explosive ash clouds thus contained much more carbonate and alkali than normal. This meant that even though the Miocene environment was a tropical rainforest, the chemistry of the successive ash beds was that of a desert dry lake, preserving everything from caterpillars and berries to apes and elephants in an unusual situation found only in a few other East African volcanos, notablyMenengai andHoma Mountain in western Kenya,Napak andMount Elgon in Uganda, and the much youngerOl Doinyo Lengai in Tanzania, which created the fossil beds ofOlduvai Gorge.
