Not OP, but I think they are referring to the basic premise of evolution. We still don't have a concrete theory on what causes mutation to occur, only one about which mutations end up surviving and sustaining across generations.

As such the evolution of human intelligence is largely an enormous chain of favorable mutations having been coupled and survived over billions of years. And each mutation that did occur was owing to nothing more than chance.

for a good introduction to order and complexity (including intelligence) arising from nothing, read "the origins of order" by stuart kauffman.