The appearance of the sun, moon and stars on day 4, the purpose of which was to act as "signs" for seasons, day and years, was essential for those who needed the signs--animals and ultimately mankind. The second great oxygenation event about 585 million years made the appearance of the sun, moon and stars possible, just prior to the introduction of many animal species. This scientific observation fits in perfectly with the Biblical account recording days 4 and 5.
