For example in D&C 107:41-57.
There is a thread about this on the LDS Freedom Forum here.
Notice that one poster tries to rationalize it by suggesting they were on God's time rather than man's time meaning the planet hadn't left Kolob yet I presume. But that backfires since that change in time accounting works in the wrong direction and makes the problem worse.
Another poster suggests that there was a water canopy surrounding the earth that shielded it from harmful radiation. What he doesn't realize is that part of the radiation that such a water canopy would remove would be electromagnetic radiation itself, In other words light. The result would be a frozen wasteland rather than the hot, always summertime environment, that he speculates would prevail as a result. Of course there would need to be some ongoing miracle to hold the water in place as well.
The problem here hopefully is obvious. We have the dead remains of bodies from the time in question and surprise surprise, there is no sign of better DNA back in the good old "antediluvian days" and we have remains of bodies from older than the fall itself. No special long life better DNA to be found anywhere.
One issue by itself presents a problem. That is Telomere shortening as cells divide.
Telomerase counteracts telomere shortening. An enzyme named telomerase adds bases to the ends of telomeres. In young cells, telomerase keeps telomeres from wearing down too much. But as cells divide repeatedly, there is not enough telomerase, so the telomeres grow shorter and the cells age.
So can we finally admit that the church truth claims are false and Joseph Smith did not restore anything but created something out of his imagination?
