This is a completely mind blowing idea that humans were down to a count of about 1000 adults. Look at us now at 7 billion, and we just can't stop warring with our fellow relatives.
No sign that this is not just a temporary stop, in fact the opposite. There were periods of relative peace in world history for longer than that.
In the last 100 years we had 2 world wars, 100s of millions killed, and tons of peripheral wars during the "Cold War".
Besides for a lot of modern history wars didn't involve much of the citizenry (they had mercenary armies). Now civilians are almost fair game. And where they had arrows, spears and swords for most of history, we now have airforces and atomic bombs.
Those numbers are highly discredited.
Historians such as Charles Patrick Fitzgerald argue that a claim of 36 million deaths is incompatible with contemporary accounts of the war. They point out that the numbers recorded on the postwar registers reflect not only population loss, but also a breakdown of the census system as well as the removal from the census figures of various classes of untaxed persons, such as those in religious orders, foreigners and merchants. For these reasons, census numbers for the post-rebellion Tang are considered unreliable. Another consideration is the fact that the territory controlled by Tang central authority was diminished by the equivalent of several of the northern provinces, so that something like a quarter of the surviving population were no longer within the area subject to the imperial revenue system.
So, in all, even less that 5%...
If the earth had just 200 people, 20 of them killed is 10%, but it's nothing that a small determined fight between two tribes can't easily achieve, without even weapons.
Whereas the 3-4% of the 3 billion during WWII is a much bigger affair -- took tons of bombs, battles, war-caused famines, an extermination program, and even nuclear bombs...
That is definitely not what I have read¹. What era are you thinking of?
¹ One quickly googled up article: https://medium.com/future-crunch/the-decline-of-war-8760f9a5...
https://en.wikipedia.org/wiki/Pax_Romana
From the article: "As the world becomes more interconnected, the powerful have ever more incentives to avoid the catastrophic economic consequences of going to war too. Conflict isn’t good for your economy in a world of dense trade networks and digital flows."
The exact same argument was put forward just before WWI. Then all hell erupted.
https://en.wikipedia.org/wiki/The_Great_Illusion
In historical terms the decline is insignificant in duration. It just marks the point from the end of the Cold War, when there remained one sole global superpower (and most of the wars in the last 20+ years, from the Gulf, to Yugoslavia, Iraq, Afghanistan, etc are wars where it fought or helped fuel). The article mistakes a decline in war during the end of the Cold War (when a global player emerged) for some long term trend.
Obviously there are more and more signs that this is not to be so -- with other players emerging like China, tension rising in different areas, even neo-fascists coming back into play in various countries governments.