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Back home, Amphicyonidae (bear-dogs) predated Ursidae by only four million years. While the latter still lives in the form of eight species, the former had been extinct for two-and-a-half million years, which made me suspect that the ice ages were what drove the bear-dogs into extinction.

But in an alternate Earth, Ursidae never existed, and Amphicyonidae instead occupied that niche. In order to understand the bear niche, we need to recognize these key factors among bears:

  1. Large, dexterous paws that can articulate to the extent of climbing and swiping, more like cats than dogs
  2. A large brain necessary to learn new things (like the blacks, browns and polars becoming more and more accustomed to urban life)
  3. Omnivory (Blacks and browns, for example, are hypocarnivorous, meaning that meat makes up a maximum of 30% of their caloric intakes)

In an alternate Earth with no ursids, would some species of amphicyonids evolve to look and behave more like them, maybe even survive the Pleistocene ice ages?

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Yes - it is entirely possible

Biological evolution often is influenced by both to fill an environmental niche (ie, evolve to expand into a less competitive environment) or evolve to cater for an existing one (to change to suit an alteration in the existing environment, such as less or more competition in an existing 'biological space').

The evolution from 'Bear Dog' to 'Bear' could be accomplished by both these factors.

It is worth noting the extinction of the Amphicyonidae family is unknown, but many think it is due to increased competition by other carnivores in the same 'space'. By removing this competition, it is conceivable the evolutionary pathway to more similar attributes to Ursidae is open.

The rest would then be via the same environmental factors that make attributes of Ursidae more desirable. In your question there are no alteration to environmental factors, so these traits could naturally evolve, and indeed are likely to, given the same environmental and competitive conditions.

Keep in mind there are other factors to consider - for instance the Ursidae often evolved from Racoon sized beginnings and slowly grew from there. Your Amphicyonidae would need to be similarly placed in their new environment, growing steadily in both size and ability to suit their new evolutionary pathway.

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