The very complete small vertebrate succession from the Guadix-Baza Basin enables tracing of the paleoenvironmental and paleoclimatic evolution of this basin, in relation with the first human dispersal into Western Europe. This analysis includes data from the amphibian, squamate, insectivore, rodent and lagomorph associations. The several analyzed localities have been assembled into six reference levels, which range from the late Pliocene to the early middle Pleistocene. The late Pliocene level of Galera 2 records quite mild conditions. These mild conditions are maintained at the beginning of the Pleistocene, although a trend to drier and, perhaps, colder environmental conditions are already observed at the level of Barranco Conejos, reaching a maximum at the level of Venta Micena. A clear amelioration of the climatic and environmental conditions is recorded at the level of Barranco León D, with mean annual temperatures more than 4°C above the present conditions in the area and mean annual precipitation more than 400. mm over. Coinciding with this climatic change, the first evidence of early human occupation appears abruptly, in the form of thousands of lithic artefacts corresponding to Mode 1 from the sites of Barranco León D and Fuente Nueva 3. The favourable climatic conditions persisted in the following level of Huéscar 1, also with Mode 1 lithic artefacts. This is also the case for the early middle Pleistocene level of Cúllar Baza, although the environment changed to drier conditions, dominated by Mediterranean woodland. Therefore, according to the small vertebrate record in the Guadix-Baza Basin, it seems that the early human occupation of this part of Western Europe was strongly influenced by the climatic and environmental conditions, and that this occupation only took place when the condition were favourable, with high levels of temperature and humidity.