as a teenager in the Dutch city of Rotterdam, Adriaan Kortlandt would often get on his bicycle after school and pedal out to a nature park in the suburb of Lekkerkerk.

He spent long hours alone there, watching and photographing a colony of cormorants, a pelican-like bird, as they built their nests and dove into the water to catch fish.

[4] However, when Kortlandt suggested his findings could inform human psychology and medicine, it brought him into conflict with contemporaries like Niko Tinbergen.

In 1940, both Kortlandt[5] and Tinbergen[6] independently identified[7] the behavioural phenomenon that is now called displacement activity (Dutch: overspronggedrag)and the hierarchy of instincts.

These observations, combined with the behavior of wild chimpanzees, indicated to him that early humans might have used objects like sticks and rocks for defense and attacked predators collectively.

Kortlandt concluded even simple thorn branches could have helped early humans intimidate predators on the dangerous African savannahs.