Complex sentence: Using these techniques, it is estimated that the first mitochondrion evolved, was consumed, or developed around 1.5 billion years ago, as an aerobic prokaryote in a symbiotic relationship within an anaerobic eukaryote.
Split into simpler sentences: Using these techniques, it is estimated that the first mitochondria arose around 1.5 billion years ago. A generally accepted hypothesis is that mitochondria originated as an aerobic prokaryote in a symbiotic relationship within an anaerobic eukaryote.