While there weren’t humans around 1 billion years ago to measure, we can get at this question through simulations!

The surface temperature wasn’t very different – maybe a few tens of degrees Celsius/Kelvin (and the surface of the Sun is about 5700 K, so this difference is very small). The central temperature at the core (currently about 15 million degrees K) would’ve been about half a million degrees different. But both of these are very small changes!