Once upon a time, before India knew Asia, when alligators sunned themselves on shores north of the Arctic Circle, a small, timid, dog-like creature tentatively waded into a river. Fifty million years passed. The continents wandered and crashed, and the ocean reconfigured itself.Now, where there were once Arctic alligators, there was ice. As for the creature who once dipped its toes into the tepid river, it now swam the frigid seas. The intervening age had transformed it into the largest animal in the history of life on Earth. . . .
Starting about three million years ago, after a long decline from the high-CO2 greenhouse of the dinosaurs, the earth descended into a waxing and waning low-CO2 ice age—one that continues to this day (albeit precariously). In this ongoing ice age, the planet has swung back and forth between more wintry climes when there was a half-mile of ice crushing Boston and sea levels were 400 feet lower—to warm, but brief, interglacials like today, when the ice sheets temporarily retreat to the poles. And back and forth and back and forth and back again, as the northern hemisphere wobbles in and out of the sunshine.From The Atlantic.
The story in The Atlantic magazine story goes on to explain that this ice age three million years ago caused small whales and the mega sharks that ate them to go extinct. This happened to the small whales because the shallow oceans over coastal shelves that the thrived in were gone and they couldn't store enough food to travel the long distances necessary to find them. Mega sharks and smaller predatory whales died out because their prey did. Large whales survived and grew larger because their size made them unmanageable prey and allowed them to live off their food stores on trips from the Arctic to Hawaii and back.
Beyond the scope of that article is the evolutionary path of primates.
Primates came into their own around the time of that 53 million year old peak warmth period along with other mammals filling ecological niches left open after the dinosaurs went extinct in the wake of the catastrophic impact that hit the Gulf of Mexico about 60 million years ago. Many Asian primates then went extinct around 34 million years ago when India crashed into Asia resulting in climate change in the region that destroyed their jungle habitats. The common ancestor of Monkeys and great apes also arose around this time.
All of species in subtribe Hominina, Sahelanthropus tchadensis, Orrorin tugenensis, Pan troglodytes and Pan paniscus share a common ancestor who was probably also intermediate behaviorally between modern chimpanzees and modern bonobos.
The low end estimate of the time frame of the most recent common ancestor of chimpanzees and modern humans is 4.5 million years ago, the high end is about 9.0 million years ago, and the better data favors a date of about 8 million years ago.
Some of the genus and species intermediate between genus Homo and primates ancestral to the large primates of today such as Sahelanthropus tchadensis and Orrorin tugenensis date to 6 to 7 million years ago. Genus Ardipithecus, which has transitioning between life in the trees and bipedal locomotion is about 4.4 million years old.
Australopithecus afarensis (Lucy and her kin) to the genus Homo took flourished from ca. 3.7 million to 3 million years ago. While Lucy and some of her kind were quite small, others were arguably as large as medium sized modern humans (the "arguably" comes from uncertainty regarding species identification, not from uncertainty regarding body size of some Australopithecus or Homo species at that time). Footprints in Tanzania from members of the species A. afarensis and possible also from other early archaic species date to 3.66 million years ago. This is right around the time of the global ice age that turned the biggest whales into the largest animals that have ever lived, and caused other aquatic megafauna to go extinct.
H. naledi arose an estimated 2.5 to 9 million years ago. The earliest reliably dated remains of Homo habilis, the earliest member of the genus Homo, are dated to 2.33 million years ago. There are other early Homo species dating to between 2.5 million and 1.5 million years ago (SN: 10/3/15, p. 6). Homo erectus dates to 2.0-2.1 million years ago in Africa.
Thus, the transition to bipedalism in human ancestor species arose around the time of a global ice age that probably turned a great deal of previously jungle habitat into loosely forested savannah and grassland areas.