The greater amount of eumelanin in darker skin protects folate from being broken down by UV radiation and thus increases fitness among populations in high-intensity UV areas. ... However, it does not explain why there is variation in human skin color.
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.
Aqueduct carries water over a long distance.