While the Plio-Pleistocene saw the radiation of multiple hominin species adapted to a variety of environments across Africa and beyond, the origin of our clade is firmly rooted in the Miocene. In fact, the distribution and composition of the living African fauna today owe much to the events of the late Miocene and earliest Pliocene. This period featured momentous climatic, glacial, and eustatic shifts on a global scale, in contrast to the preceding warm and relatively stable, middle Miocene (Zachos et al., 2001). Decades of paleoclimatic research – primarily based on ocean, ice, and lake cores (e.g., Hodell et al., 1986; deMenocal, 2004; Cohen et al., 2009; Bonnefille, 2010; Feakins et al., 2013) have enabled the reconstruction of the major changes that occurred during this period. We know that the Earth’s temperature fell precipitously, and that escalation of Antarctic glaciation created knock-on effects in ocean water temperatures, dramatic sea-level drops, and atmospheric circulation changes leading to widespread aridification.

The Homa Peninsula has been known to science since 1911, and fossil specimens from the area comprise many type specimens for common African mammalian paleospecies. Here we discuss the fauna and the paleoenvironmental information from the Homa Peninsula. The Homa Peninsula is a 200 km2 area in Homa Bay County, situated on the southern margin of the Winam Gulf of Lake Victoria in Kenya (Figure 29.1). Lake Victoria is estimated to be the third largest lake in the world, with a surface area of 68,900 km2 and a maximum length of approximately 616 km. Although its catchment is extensive, it is relatively shallow compared to any other lake of similar size, with a maximum depth of 84 m. Lake Victoria is located in a depression formed by the western and eastern branches of the East African Rift System (EARS), and is at an average elevation of 1135 m a.s.l. (Database for Hydrological Time Series of Inland Waters, 2017).

The Baringo Basin first came to the attention of European explorers during a geological survey by Joseph Thomson in 1884. The Cambridge Expedition to the East African Lakes discovered fossils in 1930 and 1931, but descriptions were delayed until a report by Vivian E. Fuchs (1950). During the 1950s, Robert M. Shackleton worked in the Baringo Basin and was followed by geologists from the Kenya Geological Survey, who also contributed to the basic geological mapping and framework, particularly in the southern portion of the Tugen Hills that is the focus of this chapter (McCall et al., 1967; Walsh, 1969).