Uracil is really easy to make. All you have to do is get rid of the ‘antlers’ of the methyl group on thymine and replace with a ‘unicorn horn’ of hydrogen for uracil. Actually, scientists believe the evolutionary path was in the other direction, from Uracil in RNA to Thymine in DNA, like so:
Why did this evolutionary transition occur? My scientific theory is that originally, briefly, during the course of evolutionary history, there was a version of DNA with uracil, but one day the methyl fairy came along and blessed it, and that’s how thymine got its antlers. The End.