The first females in our lineage, to become sexually receptive year round, changed everything (probably while we were still in the trees).

A single dominant male fathering most of the offspring, produces a genetically homogeneous population with a small gene pool and is wasteful of non-reproducing males genes and resources.

Once males no longer had to waste their energy competing for scarce females in heat, they could start cooperating with each other to help provide for their individual mates, producing more child care, survivability and higher birth rates.

Because the females were now competing for the best available males, this sexual selection by each female for intelligence, strength, industriousness, hunting and running ability etc. rapidly produced a large heterogeneous gene pool. A large variety of sustainable traits in the population then made rapid evolution possible. Female sexual selection made it inevitable.

As long as life is easy, evolution has little to do. When the African climate dried out and the trees disappeared from the jungle the apes were in trouble. They had no choice but to live on the ground. The few successful surviving species of this challenge rapidly evolved to energy efficient upright bipeds freeing the hands for fighting hunting tool making etc. Like chimps we already lived in large social groups which require larger brains. This helped contribute the extra brainpower for the transition, along with the rapid evolutionary power of female sexual selection.
Similarly, Bonobos also evolved female sexual receptivity when they also split off from the chimps long after we did. They however, still live successfully in the trees like the chimps and are more peaceful and cooperative.

Once the Hominids were successful hunter-gatherers on the ground their evolution slowed while they spread out of Africa to Eurasia. Still without language and not yet human, the real evolutionary challenges lay ahead. A couple of long decimating ice ages necessitated further rapid evolution to avoid extinction. With the power of sexual selection in this harsh environment, we evolved larger modern brains capable of creating new technologies on the coast of south Africa by first beginning the difficult exploitation of the sea with shellfish. After the ice ages the new Homo sapiens displaced the old hominid populations around the world and continued to rapidly evolve language and culture to this day, still emphasizing the importance of proper mate selection.

Few animals other than chimps can see themselves in a mirror like we can i.e. from another chimps point of view (two degrees removed) looking back at himself and know the image is himself instead of another animal. This objective thinking, from an artificial point of view outside of ourself, allows us to substitute artificial mental symbols for the real thing and manipulate them by age three. With language we can think five degrees removed from, chimps two-degree objective point of view.

We can’t turn our brains off either. They are always doing something. I like to make use of this by asking questions for it to work on while I am busy. I am always amazed by what it finds. It is the source of my most creative non-linear ideas. We owe it all to female sexual selection and hard times.