Answer:
Sulfate, nitrate, and ferric iron are readily available, exist in appreciable amounts, and act as terminal electron acceptors in anaerobic respiratory pathways.
If multiple electron acceptors existed before 575 million years ago, why do we not see evidence of complex multicellularity earlier in Earth's history?
a) The concentration of gaseous oxygen necessary for complex multicellularity did not occur until 575 million years ago.
Explanation:
There are many evolutionary hypotheses of the Earth´s environment billions of years ago, about how the different primitive electron-transport chains, an effective method for synthesizing the crucial individual components: ATP synthase, redox-driven H+ pumps, and photosystems, arose and survived in some very similar up-to-date bacteria, and in geological evidence, but although the first living cells on Earth are guessed to arise more than 3.5 × 109 years ago, the environment lacked oxygen, so the concentration of gaseous oxygen necessary for complex multicellularity did not occur until 575 million years ago:
Redox potential for NO3- -------> N2 = +0.740 Redox potential
It was until then that some bacteria developed H+ pumping electron-transport systems to take advantage of redox energy to maintain their internal pH.
