According to the graph:
a. What caused the increase in the complexity of living organisms is the increase in free oxigen. As we can see in the graph wen free oxigen increases new and more complex organisms start to appear. This is also related to the number of years that have passed.
b.The first land animals appeared around 0.5 billion years before present.
We know this by looking at the graph. In the point were first animals appeared we have to go down in the graph until we get to the X axis and there we can se the year.