According to eminent scholar Richard Klein, there was a massive genetic mutation that occurred in Africa that SUDDENLY made humans MUCH smarter than they had ever been before.  This mutation did not make the brain any bigger, but it did rewire it, allowing for truly symbolic thought.

According to this article:

To witness the contrast between premodern and modern ways of life, Klein says, sift through the remains from caves along the southern coast of South Africa. Simple Stone Age hunter-gatherers began camping here around 120,000 years ago and stayed on until around 60,000 years ago, when a punishing drought made the region uninhabitable. They developed a useful tool kit featuring carefully chipped knives, choppers, axes and other stone implements. Animal bones from the caves show that they hunted large mammals like eland, a horse-sized antelope. They built fires and buried their dead. These people, along with the Neanderthals then haunting the caves of Europe, were the most technologically adept beings of their time.

However, Klein says, there were just as many things they couldn’t manage, despite their modern-looking bodies and big brains. They didn’t build durable shelters. They almost never hunted dangerous but meaty prey like buffalo, preferring the more docile eland. Fishing was beyond their ken. They rarely crafted tools of bone, and they lacked cultural diversity. Perhaps most important, they left no indisputable signs of art or other symbolic thought.

Later inhabitants of the same caves, who moved in around 20,000 years ago, displayed all these talents and more.

What happened in between?

The burst of modern behavior—like other momentous happenings in our evolution—arose not in South Africa, Klein says, but in East Africa, which was wetter during the drought. Around 45,000 years ago, he believes, a group of simple people in East Africa began to behave in new ways and rapidly expanded in population and range. With better weapons, they broadened their diet to include more challenging and nutritious prey. With their new sense of aesthetic, they made the first clearly identifiable art. And they freed themselves to wander beyond the local watering hole—setting the stage for long-distance trade—with contrivances like canteens and the delicately crafted eggshell beads, which may have functioned as “hostess gifts” to cement goodwill with other clans.

Dramatic evidence of a surge in ingenuity and adaptability comes from a wave of human migration around 40,000 to 35,000 years ago. Fully modern Africans made their way into Europe, Klein says, where they encountered the Neanderthals, cave dwellers who had lived in and around Europe for more than 200,000 years. The lanky Africans, usually called Cro-Magnons once they reached Europe, were more vulnerable to cold than the husky Neanderthals. Yet they came, saw and conquered in short order, and the Neanderthals vanished forever.

Compare that with an earlier migration around 100,000 years ago, in which the Neanderthals eventually prevailed. Physically—but not yet behaviorally—modern Africans took advantage of a long warm spell to expand northward into Neanderthal territory in the Middle East, only to scuttle south again when temperatures later plunged. The critical difference between the two migrations? The earlier settlers apparently lacked the modern ability to respond to change with new survival strategies, such as fitted garments, projectile weapons and well-heated huts.

I’ve done some research and I now believe Homo Erectus had a spatial IQ of 53 and a symbolic IQ of 40, giving it a composite IQ of 41.  Then about 200,000 years ago in East Africa, it mutated into anatomically modern humans and these had a spatial IQ of 75, but a symbolic IQ of still only 40, giving them a composite IQ of 53.

So when they tried to leave Africa, they were brutally killed off by Neanderthals, who in addition to being 2.5 times stronger, had a spatial IQ of 91 and a symbolic IQ of 40, giving them a composite IQ of 62

However sometime after 70,000 years ago, anatomically modern humans mutated again in East Africa into behaviorally modern humans: their spatial IQs stayed 75 but their symbolic IQs suddenly jumped to 75 too, bringing them their composite IQ to 70.

This allowed them to leave Africa without being bullied by the Neanderthals.  The Neanderthals were still 2.5 times stronger, but modern humans were taller, faster, and now 8 points smarter.

Then after evolving to the cold climate of ice age Europe, symbolic IQ improved to 88 and spatial IQ also improved to 88, raising their composite IQs to 87, allowing them to brutally murder all the Neanderthals in record time, despite the huge difference in strength.  The super strong Neanderthals were humiliated to be destroyed by a bunch of scrawny nerdy modern humans

After the Neanderthals were killed off,  the ice age ended, and the malnutrition and disease caused brain size to shrink and composite IQ of modern Europeans to drop to 77.  However with the booming population, new high IQ genes were lifting the composite IQ up to 90.

Then in the 20th century, advances in nutrition, sanitation and vaccines, allowed them to return to pre-agriculture health, and their brains returned to their original size, and with the mutations that occurred during agriculture (see the 10,000 year explosion by Cochran and Harpending), their composite IQ was now 100.