Answer:
Due to presence at early days of the planet.
Explanation:
We only find the remains of single celled organisms in the oldest fossils because the single celled organisms are the organisms that were present in the early days of this planet. These single celled microbes were present about 3.7 billion years ago while on the other hand, the oldest rocks are only a little older i. e. 3.8 billion years so these single celled organisms are considered as oldest fossils on this planet.
A. Decreasing amount of energy needed to start a reaction.
It depends on where the fuel source is coming from and also how much fuel is there.
In order to make the proton gradient, protons are pumped across the mitochondrial membrane at several points in the electron transport chain. However, that proton gradient is only used to make ATP by the ATP synthase, which is technically separate from the electron transport chain.
