why humans can't live for thousands of years ?

Because God, the Creator, established it when He told Adam and Eve not to touch, and not to eat the fruit that had been forbidden him by God.
In fact, God said: do not touch and do not eat the tree of that fruit, of the knowledge of good and evil, on the day you eat it, for certain, you will die!
Therefore, only the body dies, but the soul of a man-woman lives in eternity!

God created Adam and his seed to live forever, but men abused longevity in order to sin rather than to serve God.


So he recalled eternal life and considerably reduced human lifespans 


Psalm 90:10 — Seventy years are given to us!
Some even live to eighty. But even the best years are filled with pain and trouble; soon they disappear, and we fly away.


Jesus restores eternal life for his faithful souls.

Mostly the sun.

We need it for the production of vitamin D, but its UV rays also slowly start corrupting our DNA so that after a few decades, it gets to the point where it can't do its job well enough to keep up alive.  And some of that corruption is passed on to our offspring.

It appears that before the Great Flood, there was something different about the atmosphere that may have blocked a lot of those UV rays and thus people lived much longer back then.

It is not necessary to evolution for an individual to live for thousands of years. In fact, that would make us slower to adapt to changes in the environment. Because we are the product of evolution, we just pass on our genes to subsequent generations, as long as they are not too disadvantageous. That is all that is required to happen for thousands of years under evolution.

Death came in my sin.  But everybody will live forever.  Here is the truth: The Lord Jesus Christ is God. Nobody can be good enough to avoid hell. The truth is that death leads to immediate heaven or hell, depending only on whether the person believed on the Lord Jesus Christ, or not. The truth is that the Lord Jesus Christ loves you and wants to bless your life freely :) Most of all, Jesus wants you with Him forever, and not in hell. It is too late to be saved, after death.
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Believing on the Lord Jesus Christ means being forgiven all sins past and future and means going to heaven. The truth is that every belief except one will lead to eternal torment in the lake of fire for every person. Because nothing pays for our sins except the death and blood of Jesus, the sacrifice of Jesus that is already accomplished by Him . Jesus loves you! The truth is that the Lord Jesus Christ is God, and Jesus died on the cross to pay for all of our sins in full, and then Jesus was buried, and then Jesus resurrected from the dead. Nothing else pays for our sins, not works, not deeds, not religions. So the only way to heaven and to avoid hell, is by believing on Jesus for eternal life (John 6:47), without adding any of your own works (Romans 4:5). Believe in the Lord Jesus Christ to take you to heaven, and you will be in heaven, no matter what, guaranteed. That easy, thanks to Jesus! Tell Jesus that you thank Him that you will be with Him in heaven

Because of the second law of thermodynamics.
Complicated, organized structures and systems
tend toward disorder, as time passes.
This is because there are so many more ways
that a change can cause disorder,
compared to the so few ways
that a change can improve order.

We consume a lot of energy,
just to approximately maintain our organization,
but eventually, disorder,
that we do not have biological tools to repair,
takes place.
And the disorder accumulates and grows,
until life becomes impossible.
Only very simple life forms can maintain
their much simpler systems, indefinitely.

Complex life's solution is to keep starting over,
by building fresh, new copies.
And this process also involves
a significant number of failures,
like spontaneous abortions and birth defects.

--
Regards,

John Popelish

There are different theories on why humans age and die (none of which require inferring the existence of 'god' or 'sin').

As our cells age, they become less capable of staving off disease - we don't actually 'die of old age' we just submit to one (or more) diseases/disorders.

Some people think that we are genetically 'programmed' (colloquially - not inferring a 'programmer') to only live for a certain amount of time, and think that it might someday be possible to modify our genes to live longer.

Nah, about 80 years or so will do me..
I have a bad back ALREADY, so I'd hate to see it in another 920 years!

Humans originally before sin were going to live for thousands of years. However, Adam and Eve sinned. But eventually when God gives us the new system we will live forever.

Living longer may be connected to chromosomes and telomeres. A telomere is a region of repetitive nucleotide sequences at each end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. In humans, average telomere length declines from about 11 kilobases at birth to less than four kilobases in old age, with average rate of decline being greater in men than in women. Telomere shortening is associated with aging, mortality and aging-related diseases. In 2003, Richard Cawthon discovered that those with longer telomeres lead longer lives than those with short telomeres.


Some laboratories are producing telomerase-activating concoctions to promote anti-aging. Telomerase is found in high levels in germline cells (egg and sperm) and stem cells. In these cells telomere length is maintained after DNA replication and the cells do not show signs of ageing. Telomerase is also found in high levels in cancer cells. This enables cancer cells to be immortal and continue replicating themselves. If telomerase activity was switched off in cancer cells, their telomeres would shorten until they reached a ‘critical length’. This would prevent the cancer cells from dividing uncontrollably to form tumours. Blocking telomerase activity could affect cells where telomerase activity is important, such as sperm, eggs, platelets and immune cells.


I detail all this in answer to your question because of apparent links to shorter life in men, and how semen is an important telomerase center. Then, when I read in the Bible that men were over a hundred years old before beginning to father children, and how they could live for 8-9 hundred years (prior to the Flood), I wondered if there was a connection. Not a spiritual benefit - a long life one. I'm just wondering.