Final answer:
Hominoid diversity declined between forty thousand and fifteen thousand years ago, primarily due to the spread of Homo sapiens from Africa, which resulted in the replacement of other human species.
Step-by-step explanation:
The decline in hominoid diversity took place between forty thousand and fifteen thousand years ago. The most prevailing theory explaining this phenomenon is the "out of Africa" model, which posits that modern humans (Homo sapiens) first evolved in Africa about 200,000 years ago and then spread out of Africa, replacing other human species about 100,000 years ago. There is also a "multiregional evolution model" which suggests the simultaneous evolution of Homo sapiens from Homo erectus in multiple regions, although this model is not as widely supported due to its reliance on fossil and archaeological records instead of genetic data.
Hominins first became bipedal during the late Miocene, 10 to 5 million years ago (MYA), and this adaptation continued to be refined in the early Homo species, such as Homo erectus, around 2.8 million years ago. The genus Homo, including species like Homo habilis and Homo gautengensis, exhibits significant biological and cultural diversity, which has been essential for the survival and success of Homo sapiens in a wide range of environments. Throughout the Pliocene epoch and beyond, hominins faced various climatic and environmental challenges, yet only Homo sapiens managed to spread across the entire globe.