I am suggesting people took calmly and confidently the idea of life after death. Nothing in that description contradicts the idea that we all have instincts for self-preservation, of course.
What sense does it make sense that early man was afraid of dying? It is modern man that fears death, and this has been projected backwards into explanations for some human behavior. I think rather than religion being a subterfuge from reality, our first parents took it's statements confidently. There were no hesitation making doubts issuing from mixed motivations in their adherence to the tribal dicta.
After all, flowers return every spring. Birds fly far away, and then come back. Constellatins rotate in the sky. And if ancient men feared their own mortality, then why did they put the best jewels into caskets? There are many reasons to assume ancient men confidently assumed they would survive their own demises. And considering the options available to these men, this was a sensible confidence.
What if though, what people feared, deeply, unavoidably, was -- change? Such motivations would have useful functions in the larger machinery of humanity. And also, modern man's fear of death, may be even more, a concern for his own ego psychological constructs. Those illusions are fungible. But they figure more largely in modern intellectual man.