1.93-1.83 million years ago.
We’re now counting off past time at the rate of 100,000 years a day.
Paleontologists and paleoanthropologists are busy sorting out what was special about the climate and ecology of Africa, especially East Africa, that contributed to various phases of hominin evolution. (“Hominin” is the current label for everyone more closely related to us than to our closest living relatives, i.e. chimpanzees and bonobos. My spell checker wants me to change it to hominid, the old label.) For example, Elizabeth Vrba, a South African paleontologist, argued that about 2.5 million years ago East Africa experienced a “turnover pulse” that affected a number of species. As the climate shifted toward cooler weather, and grasslands expanded, there was a wave of extinction and speciation among antelopes and (arguably) hominins as well.
More recently, paleoanthropologists have argued that not just cooling or aridity per se, but climate variability played a major role in driving hominin evolution, with episodes of greater variability leading to various responses – extinction, habitat tracking, or greater behavioral flexibility – as suggested in the diagram below.
More specifically, it looks like there were particular times when lakes were flickering in and out of existence along the Eastern and Western arms of the East African Rift Valley. Between just under 3 million years ago and just under 1 million three episodes stand out: around 2.7-2.5 mya, 1.9-1.7 mya, and 1.1-0.9 mya. These happen at intervals of 800,000 years, and may be tied to very long cycles in the eccentricity of Earth’s orbit. Within each of these episodes, the Rift Valley was home to large numbers of “amplifier lakes.” These lakes formed briefly during short intervals of high precipitation, and then evaporated quickly, making for a rich (sometimes) but also a particularly challenging environment.
Tellingly, it looks like these episodes were also associated with important events in hominin evolution. of 15 hominin species that evolved in this period, 12 appeared during these episodes.
This story, and much more, are set out in Lewis Dartnell’s very recent Origins: How Earth’s History Shaped Human History.