You would need to go back a long way, probably about 4-5 million years.
There is little debate that animals in the genus Pan, made up of one species with four subspecies of common chimpanzees (Pan troglodytes of the Western, Nigeria-Cameroon, Central and Eastern varieties, in geographic order, with a substantial gap between Western chimpanzees and the continuum of the other three subspecies, with some zoologists preferring to split Eastern chimpanzees into separate Northern and Southern subspecies rather than lumping them into a single subspecies), and one species of bonobos (Pan paniscus) aka pygmy chimpanzees, are more closely related to modern humans than any other species of animals that still exist today. Common chimpanzees have 23 chromosomes, bonobos have 24 chromosomes and modern humans have 26 chromosomes, but the chromosome divisions are such that particular genes in modern humans usually have corresponding genes, sometimes on a different chromosome, in chimpanzees and bonobos.
There are roughly 170,000-300,000 common chimpanzees and 30,000-50,000 bonobos alive today. The 30,000-50,000 population of each of the chimpanzee populations today is probably a pretty reasonable estimate of the total population of species ancestral to modern humans while they were confined to Africa.
Lots of disasters and misfortunes (volcanos, floods, wildfires, disease outbreaks, droughts) can send a relatively geographically compact population of that size into a tailspin. Indeed, several species of hominins ancestral to us did go extinct over the last several million years.
The divergence between other great apes like chimpanzees and bonobos on one hand and hominins on the other is on the order of 4 to 7 million years ago, with more recent data favoring the older date which is when the earliest hominin fossils are dated. There are several possibly ancestral and possibly sister hominin species in this tie period in Africa.
The six million year old Orrorin is arguably more human-like than Australopithecus (e.g. Lucy) despite being about 3 million years earlier. Sahelanthropus tchadensis, which is about 7 million years old, has been classified variously as a common chimpanzee-human ancestor, as part of the genus Ardipithecus (which is about 4.4 million years old), and as the ancestor not of humans but of gorillas.
An artist's depiction of Ardipithecus.
The oldest member of the genus Homo is Homo Habilis (an artist's depiction of which is below) dates to about 2.33 million years ago.
The first hominin species to leave Africa was Homo Erectus ca. 1.9 million years ago in a single migration wave. Homo Erectus is the hominin species that in terms of technological developments we associate with comic book "cave men", using stone tools and fire and clubbing big game and living in natural caves, but any clothing used was probably pretty primitive because their range extended much less further north than Neanderthals and modern humans who came along later and are derived from a common ancestor well after Homo Erectus.
The effective population size of the Homo Erectus out of Africa migration wave was probably pretty modest and all manner of misadventures could have prevented them from making it out of Africa. One or two encounters with giant crocs in the Nile or sharks while trying to cross the Gate of Tears if they took that route, could probably have done them in if they weren't lucky.
But, once Homo Erectus was out of Africa the window of opportunity for bipedal bear evolution pretty much closed because that ecological niche was now filled.
(Honestly, a scenario with an "inferior" Homo erectus or Ardipithecus species that didn't leave Africa, and a bipedal bear species that only entered Africa ca. 50,000 years ago, might be even more interesting than one in which there is no evolution for hominins at all beyond chimpanzees, or in which later hominin species go extinct.)
Your divergence date pretty much needs to be prior to Homo Erectus, or at least prior to Homo Erectus leaving Africa, as it becomes increasingly hard for Homo Erectus to go extinct at that point (although competition with humanoid bears could conceivably wipe them out), and as it probably takes at least 2-3 million years for bears to evolve from being bears to humanoid bears. Really though, the bears we have today are closer to Ardipithecus in terms of their level of humanoidness relative to the bipedal bears you imagine, so a divergence date more like 4-5 million years ago would probably be closer to the mark.
During the Upper Paleolithic era (ca. 40,000 to 10,000 years ago), before farming and herding were invented, bear migrations and modern human migrations pretty closely coincided, so modern human prehistory can be a pretty reliable benchmark for prehistoric bipedal bear migrations. (The earliest archaeological evidence for modern humans out of Africa is about 130,000 years ago, but genetic evidence suggests that all non-Africans today have a common ancestor ca. 75,000 to 50,000 years ago).
We address several unresolved issues regarding the Late Pleistocene
demography of brown bears: (a) the putative locations of refugia; (b)
the direction of migrations across Eurasia and into North America; and
(c) parallels with other mammals, including humans. We present results
based on more than 200 complete mitochondrial genome sequences from
Eurasian and North-American brown bears. Bayesian phylogenetic
analysis revealed that most individuals belong to a very large
Holarctic clade. The MRCA of this clade lived ca 40 thousand years
ago, most likely in the Altai-Sayan area, a known Late Pleistocene
refugium in Asia. We propose several migration scenarios for bears and
suggest that brown bears and humans underwent a series of parallel
migrations in Eurasia and to North America during the Late
Pleistocene. Moreover, both species exhibited a demographic standstill
in Beringia before colonizing North America. Synchrony in the timing
of past migrations and standstill implies that the ecology of large
mammals includes key limiting factors that can enhance our
understanding of ancient human movements and on large carnivore