IMO, tropical diseases are what decimated Africa zama za babu zetu. Infant mortality rates due to tropical diseases was sky high all over Africa. I am not sure diseases like malaria existed in China and India to the extent they existed in Africa if at all.
As for Europe, the brutal winters, hypothermia and the attendant scarcity of food that came with brutal winters probably decimated Europes population on several occasions.
Not to mention the great plague aka black death which may have killed 60% of Europeans in the 17th century and also made food scarce for those who survived.
And finally, Europeans mass migrated to North America, South America, Africa and Oceania in large numbers since the 16th century. Mass migrations emptied large swathes of Europe such as during the potato famine of Ireland.
In America, Germans are the largest ethnic group, numbering about 50 million.
Chinese & Indians basically did not migrate in such numbers until relatively recently
Na hayo ni maoni yangu