kind of tissue, and the rapid enlargement of brain volume perhaps outstripped the circulation system in the brain necessary to sustain all the new neurons. Higher oxygen would give some cushion for error, it seems to me. But this is still sheer speculation.

THE END OF HISTORY?

We have now finished our journey through time. The 540-million-year trip is over with this chapter. We have seen evidence of major changes in oxygen levels through time. Will oxygen levels continue to change in the future? That is the subject of the final chapter.



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