What is a basic reason for human activity to be so harmful to the Earth's environment and ecology?
For hundreds of thousands of years, human activity has altered and changed the environment to be more hospitable. Like any other creature, humans are not specifically evolved for any single environment, and because of human intelligence (compared to animal intelligence) humans are more likely to spread to areas where they must have significant effect on the environment in order to live there.
One good reason for the increase in human-caused pollution is the sheer numbers of humans living on the Earth. In earlier millennia, human populations were small, as humans could not fight disease or protect against animal attacks and natural disasters. As science progressed and humans became more able to protect themselves, the human lifespan increased, and so did the number of surviving babies. This meant more humans and so more space on the planet was needed to house them all.
This expansion in human life also meant more large-scale human activity. Where a single family farm would encompass fifty or one hundred acres, now that same farmland is worked by machines and feeds a thousand people or more. This also means that more natural resources needed to be harvested, from forest wood and clean river water to oil, metals, and animals. Increased human activity means increased stress on natural resources; strip-mines, for example, destroy the ecology in the area, which may take decades to reform.
In recent years, steps have been taken to avoid harming the environment more than necessary. With time comes wisdom, and it is clear that the Earth will not be able to sustain the human population if it continues to increase. More efficient farming methods, recycling, and extra-planetary movement are all needed to avoid what might be termed a "resource cliff."