TGD view proposes that complex multicellular life evolved in underground oceans and bursted to the surface in Cambrian explosion, which involved a relatively rapid increase of the Earth radius by factor 2 (discrete step in TGD counterpart of cosmic expansion). The underground life must have been able to do photosynthesis and therefore to oxygenate the water. This would solve the oxygenation problem.
Intriguingly, the light spectrum from the Earth's core is in the same range as that from the Sun. Could dark photons (darkness in TGD sense as value heff=nh0>h for Planck constant) have served as energy for underground photosynthesis? One can also imagine solar photons transforming to dark photons at monopole flux tubes making them able to penetrate the Earth's surface.
See the article Expanding Earth Hypothesis and Pre-Cambrian Earth or the chapter Quantum gravitation and quantum biology in TGD Universe.
For a summary of earlier postings see Latest progress in TGD.
For the lists of articles (most of them published in journals founded by Huping Hu) and books about TGD see this.