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Comment by chrisco255 | Hacker News Reader

I just don't buy that. It's too hand wavey. Natural instinct is to fear the unknown, and to respond quickly to threats. Saber tooth cats roamed the continent too and there is plenty of evidence they hunted humans, along with bears that continue to hunt humans to this day. But there were also dire wolves, giant bison, cave bears, and many other large species that went extinct but for which we have smaller forms of alive today. Something must have occurred environmentally that led to the rapid deaths of large species.

It's also not as if we have evidence of large populations of humans in the glacial era. You would need a modern civilization with at least metallic tools to spread far enough and wide enough to kill off multiple major species.

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They fear the unknown because humans ate all the ones that didn't.

During the age of sail whenever a ship would land on a new island they local animals could be plucked like fruit because they didn't fear humans.

The results were always quite predictable.

Which megafauna went extinct in Africa? Surely some couldn’t evolve in time

Right, well during the age of sail we had already achieved gunpowder, steel and iron working, and landing on small islands with navigation era tech it is quite easy to dominate or change a small ecosystem by cutting down trees, hunting excessively, introducing livestock and other invasive species, etc.

In the ice age the tools were bone and stone. And the lack of agriculture meant that human populations were kept small by available resources by default. The lack of transportation or even domestication of animals in the Americas at the time meant that all travel was on foot. To dominate species like the mastadon, giant ground sloth, cave bear, dire wolf, etc on two continents was just something humans weren't capable of at the time.

Hunting was still risky and labor intensive. Arrows didn't have metal heads. Range was limited. Traveling was risky, both from natural and tribal threats.

It just doesn't add up. The density of humans was probably lower in the Americas than in any other continent (save for Antarctica of course).

> You would need a modern civilization with at least metallic tools to spread far enough and wide enough to kill off multiple major species.

We know humans spread far enough and wide enough in stone age times. Big animals reproduce slowly, and the extinctions happened over centuries or millennia. Metal tools and large populations weren't needed, just stone age hunting techniques and persistence.

The extinctions correspond to human arrival everywhere homonins weren't already established: https://ourworldindata.org/quaternary-megafauna-extinction

I mean dire wolves have just as many pups in a litter as ordinary wolves and go into heat just as often. Same can be said of giant bison, saber tooth cats, etc.

We actually don't know precisely when the extinctions occurred or how long it took for them to occur. All we know is where the gaps in the sparse data set that is the fossil record exist.

It could be that the extinctions actually made it easier for humans to expand and move into those areas after the fact, or that humans uniquely benefitted from the environmental or situational changes that were ongoing.

We know the Younger Dryas was an incredibly intense era of climate change, after the ice age receded it suddenly plunged back into glacial temps for a millenia or two. Very strange and stressful time for species of all types.

> It's too hand wavey. Natural instinct is to fear the unknown, and to respond quickly to threats.

Doesn’t the dodo show that that isn’t a given?

In the book The Sixth Extinction, the author posited that you really don't need very sophisticated or systematic hunting to bring megafauna to extinction. Their strategy for survival is to be large, but at the cost of very long reproductive cycles. Slowly but steadily picking one off every now and then can bring them to extinction over the course of centuries, which at the same time means their slow demise and the cause of it can easily go unnoticed.

Megafauna extinction is closely timed with the arrival of humans on just about every continent or island we happened upon. Even Africa is relatively more depauperate compared to the distant past.

Of course not all of them found their end. There are still moose in North America, and giant tortoises in the Galapagos that literally floated away from the coast of South America, where they later went extinct. But there are fewer everywhere.

Larger creatures have long gestation cycles and tend to be very sensitive to any any changes. Even killing a few pregnant females a year could have been enough in a given region.

> Saber tooth cats roamed the continent too and there is plenty of evidence they hunted humans.

We didn't need to hunt dangerous saber tooth cats to get them extincted. We had only to hunt their less dangerous food. They probably couldn't live off the small prey left after the human wave swept the continent.

Humans are surprising resourceful, especially when survival is on the line. Even before the metal ages it seems we were apex predators.

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