Early humans were afraid of lots of things which includes natural disasters, fire, unknown aspects of their environment and dangerous animals.
Early humans also developed fears that were related to starvation, death and disease as well as the fear of the dark and potential dangers that lurked in the dark.
Basically anything that early humans were not familiar with the early humans were afraid of these things.
Early humans were very vulnerable to predators such as snakes, large birds of prey, wolves, bears and lions and they also had to contend with prehistoric beasts such as the saber toothed cat also known as Smilodon and Megalania.
Early humans also faced many dangers from thunderstorms, volcanoes, wildfires and lightning which they were also afraid of.
And the competition for resources as well as territory could also lead to conflict and violence and made other humans a source of fear.
Early humans were also afraid of some insects and some pests that could cause harm through bites or stings or transmit disease which lead to a fear of them.
Fear of death was likely to be a powerful motivator to early humans as it represented the ultimate end to the early humans existence.
And food scarcity and the inability to find sustenance were also constant treats to early humans which lead to fear of hunger and famine.
Early humans were also afraid of disease as exposure to unknown pathogens and the possibility of illness and death from the disease were also scary to early humans as there were no medical advancements like there is today.
Also early humans also attributed natural events and misfortune to spirits which lead to rituals and practices that were aimed at appeasing or controlling of the forces.
And the fear of unease or revulsion when encountering something which is almost, but not quite human or familiar may also have roots in a fear of disease or contagion in early humans.