A scientist studying fossils in undisturbed layers of rock identified a species that appeared to not have changed a lot over the years. Which observation would have most likely led him to this conclusion?

After studying the fossil, the person can figure out which animal it might be of the records of animals we know of, by taking bone tests, etc. they can see the DNA of the animal. The bone structure can show the animal looked different, or even could figure that out through the DNA. During the years, it could've mated with other animals creating an updated breed, or simply just changing to fit into new environments aka evolving  

Explanation:

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they had the law tat factories that produces smoke cannot be built near residential buildings.

building of proper sewage system was done to ensure that the waste is carried away from the humans so that they do not get diseases from it.

also they built water systems to provide water in those areas which are drought prone.