What was the Percent of the Earth's Oxygen in the year 2000?

Since the beginning of the Cambrian period, O2 levels have fluctuated between 15% and 30% of atmospheric volume. Towards the end of the Carboniferous period (about 300 million years ago) atmospheric O2 levels reached a maximum of 35% by volume, which may have contributed to the large size of insects and amphibians at this time.Whilst human activities, such as the burning of fossil fuels, have an impact on relative carbon dioxide concentrations, their impact on oxygen levels is less significant.