Question

Which Buddha thought was the cause of all human suffering

Answer 1

The second truth was the buddha thought for suffering
of the concept of the Four Noble Truths

Answer 2

The Second Truth is that this suffering is caused by selfish craving and personal desire. The Third Truth is that this selfish craving can be overcome. The Fourth Truth is that the way to overcome this misery is through the Eightfold Path. The Four Noble Truths is a fundamental concept taught by the Buddha.