Creationist answer: ? (careful - I've heard some doozies)
They evolved from a set of triassic mammals that lived on gondwana. The triassic mammals that remained on land that became asia evolved into the mammals that we see today. The triassic mammals that remained on land that became australia evolved into the marsupials that we see today.
Very simply put, if you change the decay rate, you change the decay energy, and the diameter of the halo changes.
wikipedia writes:
The decay energy is the energy released by a radioactive decay.
If the decay energy is the energy releassed by a "single" radioactive decay, why would the timing of those decays have any effect of the energy of any one decay?
In Psalm 104, it says "Thou coveredst it with the deep as with a garment; the waters stood above the mountains." Like a blanket of water covering the earth.
Couldn't this be referring to the time when the earth was formless and void and the Spirit of God moved over the face of the waters?