A bigger organism for a bigger brain in itself also comes with a limitation. You need a diet that provides enough energy for both the body and the brain, both which drain a lot. So after a while you'll have to enhance your diet or hit a barrier.

Apparently for humanity that change was the addition of roasted meat to the diet. Before we were primarily herbivores with occasionnally scavenged meat, which wouldn't be a lot (unless lucky), raw, and of poor quality.

With actively hunting and roasting the fresh meat, the brain had the right energy source to advance further (a lot). Development of the hunt itself alone has been a good stimulance for growth. It requires teamwork, planning and courage (ie. the ability to overcome natural fear). All this requires an advanced 'train of thought' which in itself is a good way to develop the brain. Ofcourse this requires knowledge of fire.

Apparently humanity has known and used fire for a long time, but it took quite a while before we were actually able to create it ourselves. Before that, fire from a lightning blast, or other natural sources if available, would be used and fed to keep it going. If Iunna existed in those days, she would be very important, cause 'Guardian of the Fire' was a highly important job within a tribe/clan :).