the biggest pollution happened when agriculture started thousands of years ago. How?
Turning a non-farm (forest, grassland,marsh,brush) into a farm is one of the worst pollutions of all types. What wildlife needs is land where they could rest and feed. Air/water pollution or temperature rising did far less damage than grabbing their home to grow crops.
Industrialization brought us fertilizers, which greatly reduced land per person we need. High buildings also contributed. Per-person-wise, we are taking far less land than before. And this is an improvement.
Of course the human population grew like crazy, but this is another issue.
These other people who kill dolphins where are they when they kill the dolphins?
Can we make objective judgement about their priority? If we are talking about the world ending and they are talking about polluting the water, then yes we can be objective.
East Asian countries (mainly Japanese) hunt dolphins and whales in the high sea. And their hunting was protested and harassed by environmentalism activists.
Taiji dolphin drive hunt - Wikipedia, the free encyclopedia
Whaling in Japan - Wikipedia, the free encyclopedia
On one hand, these activists can say they are doing a good thing to protect dolphins. On the other hand, how many small fishes are killed by the wastes from their boats? How much air pollution did their boats produce? Even in this small case we have a lot of different points of view.
On the grander scale, what are the main pollutions our industry is producing? How much are in developing countries? How many children will starve if the developing countries have to close a lot of factories and reduce their use of energy? It's overall very complicated and we can't just use a "world ending" to argue down everything.
Other world-ending scenarios:
If I'm afraid that the world will end in an artificial intelligence take-over, should we discuss how to ban certain types of computer science research as the top priority, instead of global warming?
If I'm afraid that the world will end in a huge epidemic disease, should we discuss how to focus on drug research as the top priority, instead of global warming?
If I'm afraid that the world will end in a religious war, should we discuss how to try prevent the religious population from growing, instead of global warming?
There can be another 100 such apocalypse scenarios, and there are activists backing each one with solid evidence and sound science. Why should pollution/global warming occupy the top?