One of the worst nightmares since human evolution is the pain and death due to disease and ageing. It's also one of the most fundamental harsh truths of our life. No matter how virtuous, healthy and streamllined lives you have led, you eventually fall sick and die.

Historically, people believe that diseases were curses from God. Death was considered an essential part of life, so much so that entire religions were formed on explaining and assuring a safe afterlife. Despite lots of magic, divine miracles and countless shady deaths, we are no closer to explain on why organs deteoriate or where do we go when we die.

One of the many possible theories that humans can come up with, one of them is that we were created by Aliens, and though we weren't intended to die, we still die because our creators weren't perfect and left a flaw in our design before "perfecting" their design or by intention.

Another way of realizing this is by considering the human body as a product and gradual weakening of our bodies as a product flaw.

Tying this theory with another pseudo-scientific theory of ancient aliens, we can create the following hypothesis: -

Human DNA was seeded by the Aliens (the Annunaki) in Mesopotamia but due to a civil war (Pagan Gods being one faction and Yahweh being another one for example), they were forced to abandon their project halfway.

And the mortality of humans was not as abrupt as we think. In almost all cultures, we see people living far longer than in reality. Hindu Vedic sages lived for thousands of years, Elves of Norse Mythology lived for thousands of years, Biblical Adam lived for 900+ years etc. That humongous lifespan, say was prevalent just before the Ice Age, gradually decayed to the 70ish years of today.