How did life exist with about 30-40% less Solar Heat?

If is very resilient and tough. There have been several mass extinctions on Earth because of asteroid impacts, global glaciation, global volcanism and a number of other things. These have all had a radical impacts on the Earth's climate. Life as always bounced back.

Life has demonstrated it's ability to adapt to a wide variety of environments and conditions. A ten percent increase in the Sun's output over a billion years is a long time. Life could easily adapt to that given the time frame.

I remember the oldest signs of life appeared about a billion and a half years after solar system's formation. The life forms were much adapted to very harsh conditions from which the earth is.

The earth didn't become stable enough to support life until about a billion years ago. Dinosaurs existed like 76 million years back.

Forms of life exist under the ice in the Arctic, and on the bottom of the ocean near "toxic" hot thermal vents.

Life is far more resilient that you think. Some bacteria can withstand radiation levels hundreds of thousands of times greater than would kill humans. They can survive in space, and being fossilised.