Question: Okay, I just asked a [question](https://worldbuilding.stackexchange.com/questions/82436/would-humankind-remain-the-dominat-species-in-a-world-with-twice-as-big-animals), in which mother nature transformed all animals (or brain containing living beings, if that fits better for you) into a being twice as big in order for them to kill of humankind, recovering the ecosystem.

Well, with twice the heigth, length and width, it's actually eight times the size ($2^3$).

The question was, if humankind could remain as the dominant species and the answers made it clear: Yes, easily.

But it got me thinking. Would the result be the same, if all animals would have turned twice as big at the year 0?

The rules are the same: 

* This transformation happened worldwide at the same time and only took about a minute.
* The physical abilities (strength, speed etc.) scale accordingly to the size and the animals are able to handle the gained mass just as good as their reallife counterparts.
* Mother Nature DO provide the earth with enough food to feed the bigger animals. After all, she wants to keep her children alive, killing only humankind. 
Well, carnivore don't need changes, as their prey scale accordingly. Let's just say the vegetable food grows faster, there is more of it or it provides more of whatever keeps the animals alive. (Doesn't really matter, the question is about the fight between humankind and animals)
* The earth was EXACTLY ours until the year 0.

With all that: Would humankind still be able to become the dominant species in the year 2000 (or earlier), or wouldn't they be able to handle the changes?

Answer: 
Unless the animals are magically guided to specifically attack human beings, no way.

Specifically, humans already did this once, about 12 thousand years ago. The giant ground sloths, giant rodents, regular lions and tigers on the American continent were systematically wiped out for practical or culinary reasons in the centuries following human arrival.

Humans needed flint, sticks, fire, and language. They still have all these things in the year 0, but now they also have armies, written language, boats, and metals.

If you would like a specific example: The [polar bear hunters of Zhokov island](https://polarbearscience.com/2013/02/18/the-ancient-polar-bear-hunters-of-zhokhov-island-siberia/). 397 polar bear skulls, hunted with bone and stone tools.
Are animals that are 2x bigger actually more dangerous? (not really)

(Blatantly copying my answer to a similar question) 

This analysis assumes that all the organs of the animals are scaled evenly in all directions. 

**There is something called the square cube law.**
There are many good things that grow with the square of the of the scal like muscle strength (which is proportional to the cross section area of the muscle) so 2x in each dimension is 4x strength. 

There are many bad things that grow with the cube of size like weight which is proportional to the volume of the creature. So 2x in each dimension is 8x weight. Since the creature is caring 8x the weight it must consume 8x the calories to not starve. The required strength of lungs and heart which now must supply oxygen to 8x muscle, and push 8x the blood volume 2x higher.

Assuming that running speed is proportional to strength / weight we would expect large creatures to be twice as slow.

A larger creature has a harder time with stealth.

**What does this mean**

* Some species would die out.

Some species would no longer fit in their ecological niches, bees would be to large to drink pollen from flowers monkeys could no longer climb trees water striders will no longer walk on the surface of the water.
Some could not consume enough calories
 Cows eat most of the day to get enough calories, there is not time for them to eat 8x more. They would either have to switch to a more calorie dense food or starve. 

* The large animals would be very venerable to bows

The creatures are very large targets that are significantly slower than the ones we have today. They have 2x higher blood pressure so they will bleed out much faster if wounded. They are huge targets and will trouble sneaking up on anything. They must eat much more so they cannot afford to stalk or wait long they must attack. 

* They would be much more deadly at close range.

A giant lion would be capable of impressive destruction, but if a lion reaches you, you are dead anyway whether or not it is giant or normal.