How exactly did animals from places like Africa, the Middle East, etc. survive crossing the Bering land bridge during the Ice Age as wouldn’t a good amount of these animals come from much warmer climates?

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How exactly did animals from places like Africa, the Middle East, etc. survive crossing the Bering land bridge during the Ice Age as wouldn’t a good amount of these animals come from much warmer climates?

In: Earth Science

The Ice Age wasn’t *that* much colder. The scary thing is how little the temperature has to change for everything to get wonky. [This article](https://www.weforum.org/agenda/2020/09/last-ice-age-global-temperature-scientist-predict/) argues the global average temperature during this period was about 7.8C (46F). For comparison, the current global average seems to be 12.7C (54F). So the window hasn’t changed a lot.

The article seems to suggest an explanation, too:

> “What is interesting is that Alaska was not entirely covered with ice,” Tierney said. “There was an ice-free corridor that allowed humans to travel across the Bering Strait, into Alaska. Central Alaska was actually not that much colder than today, so for Ice Age humans it might have been a relatively nice place to settle.

I’m sure there’s a more technical explanation, but it seems like we believe for whatever reason that area was relatively warm in that time period.

It was a gradual process over thousands to millions of years. No single animal went from Africa to America. They went a little ways from where they were born, and their offspring went a little further, and then the next generations went further, etc, giving them plenty of time to evolve for those conditions.

They didn’t *walk* across the bridge, they *evolved* across it. Populations exist in particular areas. The members at the edges of those areas will be under slightly different environmental conditions to the average of the species, where they will gain a survival benefit from slightly different traits. These new traits allow them to move slightly further afield still. Over many generations, you end up with a spectrum of populations within the species: Members at the north tend to stick to the north and reproduce in the north, and so retain northerly traits like thick fur and increased size, and members in the south don’t like the cold so they mostly stick to the south and reproduce with southerners and retain southern traits. Although still capable of reproducing, these populations only reproduce in the middle of the population spread, where it’s still warm enough for the southerners and not too warm for the northerners. Given enough time, the differences between populations may become so great that the southerners and northerners can no longer breed with each other (but may both still be able to breed with those in the middle).

For an interesting sort of example of this, there’s a concept called a ring species. Ring species occur when a species encounters a large obstacle, such as a mountain, and part of the original population spreads out around both sides of the obstacle. Eventually, the populations spread out enough that those who went left and those who went right meet again at the other side of the obstacle. However, because of differences in environments and limitations on movement, the population that went left and the population that went right can not interbreed with each other, but they can interbreed with their immediate neighbours on either side. Imagine it in terms of colours: at one end you have blue and the other red. Blue and red can’t breed with each other, but blue can breed with green, green can breed with yellow, yellow can breed with orange and orange can breed with red. So, by the colloquial idea of “It’s a different species if they can’t make fertile offspring”, these populations are both the same species and different species simultaneously, depending on which members of the species you pair up. This illustrates just how much a species’ gene pools and traits can vary just by partial geographic isolation.


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